Coordination Action FP Seventh Framework Programme Theme 7: Transport D04. Report on state-of-the-art of test methods

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1 Tyre and Road Surface Optimisation for Skid Resistance and Further Effects Coordination Action FP Seventh Framework Programme Theme 7: Transport D04 Report on state-of-the-art of test methods The research leading to these results has received funding from the European Community s Seventh Framework Programme (FP7/ ) under grant agreement n Main Editor(s) Minh-Tan Do, Peter G Roe Due Date 1 st December 2008 Delivery Date 5 th December 2008 Work Package Dissemination Level WP2 Harmonisation of skid-resistance methods and choice of reference surfaces Public (PU) Project Coordinator Mrs. Damaris OMASITS, arsenal research, Austria phone: , damaris.omasits(at)arsenal.ac.at internet: This project is part of the FEHRL Strategic Research Programme SERRP IV (

2 Contributor(s) Main Contributor(s) Minh-Tan Do, LCPC, France Phone: , minh-tan.do@lcpc.fr Peter Roe, TRL, UK Phone , proe@trl.co.uk Contributor(s) (alphabetical order) Erik Vos, RWS, Netherlands Phone: , erik.vos@rws.nl Jacob Groenendijk, KOAC-NPC, Netherlands Phone: , groenendijk@koac-npc.com Review Reviewer(s) Peter Saleh, Arsenal, Austria Helen Viner, TRL, UK Date: 05/12/2008, Version: 4 2 (89)

3 Control Sheet Version History Version Date Editor Summary of Modifications 1 09/09/2008 Minh-Tan Do Summary 2 07/11/2008 Minh-Tan Do Draft for partner comments before reviewing 3 02/12/2008 Peter Roe Revised and extended draft, to take account of peer reviewer comments and improve English usage. Final Version (v4) released by Circulated to Name Date Recipient Date Minh-Tan Do Work Package Leader 04/12/2008 Coordinator 04/12/2008 Damaris Omasits Coordinator 04/12/2008 Consortium 05/12/2008 Johanna Bohrn Quality Manager 05/12/2008 European Commission 05/12/2008 Date: 05/12/2008, Version: 4 3 (89)

4 Table of Contents 1 Introduction Skid resistance measurement principles Longitudinal friction principle Transverse friction principle Static or slow-moving devices Factors influencing skid resistance measurements and device operation Test speed Test tyre Load on test wheel Water Temperature Identifying current devices used to measure skid resistance Skid-resistance devices identified by CEN/TC227/WG ADHERA BV-11 and SFT (Saab Friction Tester) GripTester RoadSTAR ROAR DK ROAR NL RWS NL Skid Resistance Trailer SCRIM Skiddometer BV SKM SRM TRT Other devices DFT Dynamic Friction Tester IMAG Mu-Meter Mk-5 and Mk Odoliograph OSCAR PFT (TRL) SALTAR friction meter VTI Skiddometer BV VTI Skiddometer BV SRT Pendulum T2GO VTI Portable Friction Tester (PFT) VTT Friction Lorry Discussion Conclusions...69 Date: 05/12/2008, Version: 4 4 (89)

5 9 Appendix 1 Calibration procedures ADHERA GRIPTESTER ROADSTAR ROAR DK ROAR NL RWS NL Skid Resistance Trailer SCRIM Skiddometer BV SKM SRM Tatra Runway Tester References...88 Date: 05/12/2008, Version: 4 5 (89)

6 Abbreviations Abbreviation CEN OSCAR PFT RoadSTAR ROAR SCRIM SKM SFT SRM TC TRT WG Meaning Comité Européen de Normalisation (European Committee for Standardization) Optimum Surface Contamination Analyser and Recorder Pavement Friction Tester Road Surface Tester of Arsenal Research Road Analyser and Recorder manufactured by Norsemeter Sideway-force Coefficient Routine Investigation Machine Seitenkraftmessung SAAB Friction Tester Stuttgarter Reibungsmesser Technical Committee Tatra Runway Tester Working Group Definitions Term Airfield operational testing Bound surface Braking force coefficient Calibration Contact area Fixed slip Fixed-slip friction Friction Definition Measurement of the skid resistance of a surface on an airfield in response to an operational need and in whatever conditions exist at the time of the test, which may include contamination by ice, snow, slush or water. Top layer or surface course of a road with the aggregates secured permanently in place Ratio between the longitudinal frictional force and the load on the test tyre, the test tyre mass and the rim mass. This coefficient is without dimension. Periodic adjustment of the offset, the gain and the linearity of the output of a measurement method so that all the calibrated devices of a particular type deliver the same value within a known and accepted range of uncertainty, when measuring under identical conditions within given boundaries or parameters. Overall area of the road surface instantaneously in contact with a tyre. Condition in which a braking system forces the test wheel to roll at a fixed reduction of its operating speed. Friction between a test tyre and a road surface when the wheel is controlled to move at a fixed proportion of its natural speed. Resistance to relative motion between two bodies in contact. The Date: 05/12/2008, Version: 4 6 (89)

7 Horizontal force (drag) Horizontal force (side force) Longitudinal friction coefficient (LFC) Macrotexture Mean profile depth Megatexture Microtexture Nearside wheel path Operating speed Pedestrian slip resistance Push mode Repeatability r Reproducibility R Routine testing Sampling length/interval Side force coefficient Skid resistance frictional force is the force which acts tangentially in the contact area. Horizontal force acting tangentially on the test wheel in line with the direction of travel. Horizontal force acting perpendicular to a freely-rotating, angled test wheel. Ratio between horizontal force (drag) and vertical force (load) for a braked wheel in controlled conditions. This is normally a decimal number quoted to two significant figures. Deviation of a pavement from a true planar pavement with characteristic dimensions along the pavement of 0.5 mm to 50 mm, corresponding to texture wavelengths with one-third-octave bands including the range 0.63 mm to 50 mm centre wavelengths. Descriptor of macro texture, obtained from a texture profile measurement as defined in EN ISO and EN ISO Roughness elements with a horizontal length of 50 to 500 mm. Roughness of this magnitude can influence accumulations of water on the pavement surface (for instance, in unevenness). Deviation of a pavement from a true planar pavement with characteristic dimensions along the pavement of less than 0.5 mm, corresponding to texture wavelengths with one-third-octave bands and up to 0,5 mm centre wavelengths. Wheel path that is closest to the edge of the road in the normal direction of travel. For countries that normally drive on the right, this is the righthand side and for countries that normally drive on the left, this is the lefthand side. Speed at which the device traverses the test surface. The property of the trafficked surface to maintain the adhesion of a pedestrian shoe sole. When the device is pushed by a pedestrian The maximum difference expected between two measurements made by the same machine, with the same tyre, operated by the same crew on the same section of road in a short space of time, with a probability of 95 %. The maximum difference expected between two measurements made by different machines with different tyres using different crews on the same section of road in a short space of time, with a probability of 95 %. Measurement of the skid resistance of a surface in standardized test conditions, which normally include a defined water flow rate. The distance over which responses of the sensors are sampled to determine a single measurement of the recorded variables. Ratio between the vertical force (load) and horizontal force (side force) in controlled conditions. This is normally a decimal number quoted to two significant figures. Characterisation of the friction of a road surface when measured in accordance with a standardised method. Date: 05/12/2008, Version: 4 7 (89)

8 Slip angle Slip ratio Slip speed Subsection Test section Theoretical water film thickness Tow mode Vertical force Water delivery system Water flow rate Wet road skid resistance Wheel paths The angle between the mid-plane of the test tyre contact surface and the direction of travel. Slip speed divided by the operating speed. Relative speed between the test tyre and the travelled surface in the contact area. Defined length of surface for which one set of the measured variables is reported by the device. Length of road between defined points (e.g. location references, specific features, or measured distances) comprising a number of subsections over which a continuous sequence of measurements is made. Theoretical thickness of a water film deposited on the surface in front of the measuring tyre, assuming the surface has zero texture depth. When the device is towed by a vehicle Force applied by the wheel assembly (the static and dynamic force on the test tyre, the test tyre weight and the rim weight) on the contact area. System for depositing a given amount of water in front of the test tyre so that it then passes between the tyre and the surface being measured. Rate (litres/second) at which water is deposited on the surface to be measured in front of the test tyre. Property of a trafficked surface that limits relative movement between the surface and the part of a vehicle tyre in contact with the surface, when lubricated with a film of water. Parts of the pavement surface where the majority of vehicle wheel passes are concentrated. List of Figures Figure 1.1 Harmonisation scheme and actions carried out in WP Figure 2.1 Illustration of LFC G curve...17 Figure 2.2 Illustration of slip angle...18 Figure 2.3 Illustration of SFC δ curve...19 Figure 3.1 Effect of vehicle speed and percentage slip on friction...21 Figure 5.1 ADHERA: General view (left) and trailer (right) with cover opened...28 Figure 5.2 BV11 trailer (left) and SFT (right)...30 Figure 5.3 GripTester...32 Figure 5.4 RoadSTAR general view (left) and close-up of measuring wheel (right)...34 Figure 5.5 A general view of ROAR DK (left) and close-up of the test wheel mechanism (right)...37 Figure 5.6 A general view of ROAR NL (left) and close-up of the measuring system mounted at the rear in the right wheel track (right) Figure 5.7 RWS NL skid resistance trailer...40 Figure 5.8 SCRIM wheel assembly for a left-side test wheel (wheel in its raised position)...42 Figure 5.9 Skiddometer BB-8 (example used in Switzerland)...44 Figure 5.10 SKM test wheel assembly (this example is on the machine operated by BASt).46 Date: 05/12/2008, Version: 4 8 (89)

9 Figure 5.11 The SRM showing the test wheels mounted on the rear of the vehicle (this example is from Switzerland)...48 Figure 5.12 The TRT...50 Figure 6.1 Dynamic Friction Tester...53 Figure 6.2 The IMAG...54 Figure 6.3 Diagram of the Mu-meter...55 Figure 6.4 Odoliographs following water tankers devices from MET (left) and CRRB (right)...56 Figure 6.5 OSCAR (the test wheel and water feed nozzle can be seen on the left)...57 Figure 6.6 The PFT...58 Figure 6.7 Griffigkeitsmesseinrichtung SALTAR...60 Figure 6.8 The VTI Skiddometer BV Figure 6.9 VTI Skiddometer BV Figure 6.10 The Pendulum Tester...63 Figure 6.11 T2GO...64 Figure 6.12 The VTI Portable Friction Tester...65 List of Tables Table 1.1 Overview of the major outcomes of the individual Tasks of WP Table 5.1 Precision indicators for ADHERA measurements...28 Table 5.2 Standard test conditions for ADHERA...29 Table 5.3 Precision indicators for BV11/SFT measurements...30 Table 5.4 Standard test conditions for BV-11 and SFT...31 Table 5.5 Precision indicators for GripTester measurements...32 Table 5.6 Standard test conditions for GripTester...33 Table 5.7 Precision indicators for RoadSTAR measurements...35 Table 5.8 Standard test conditions for RoadSTAR...36 Table 5.9 Precision indicators for ROAR DK measurements...37 Table 5.10 Standard test conditions for ROAR DK...38 Table 5.11 Precision indicators for ROAR NL measurements...39 Table 5.12 Standard test conditions for ROAR NL...40 Table 5.13 Precision indicators for RWS Skid Resistance Trailer measurements...41 Table 5.14 Standard test conditions for the RWS Skid ResistanceTrailer...41 Table 5.15 Precision indicators for SCRIM measurements...43 Table 5.16 Standard test conditions for SCRIM...43 Table 5.17 Precision indicators for Skiddometer BV-8 measurements...44 Table 5.18 Standard test conditions for Skiddometer BV Table 5.19 Precision indicators for SKM measurements...46 Table 5.20 Standard test conditions for SKM...47 Table 5.21 Precision indicators for SRM measurements...48 Table 5.22 Standard test conditions for SRM...49 Table 5.23 Precision indicators for TRT measurements...50 Table 5.24 Standard test conditions for TRT...51 Table 7.1 Main characteristics of the principal devices identified...68 Table 9.1 Frequency of periodic calibrations of GripTester...71 Table 9.2 Criteria for periodic calibrations of RoadSTAR...73 Table 9.3 Criteria for monthly repeated measurements with RoadSTAR...73 Table 9.4 Criteria for repeated measurements with ROAD DK...74 Table 9.5 Pre-running length against interval time between testing with ROAR NL...75 Table 9.6 Pre-running length against interval time between testing with RWS trailer...76 Table 9.7 Frequency of periodic calibrations of SCRIM...77 Table 9.8 Static calibration of SCRIM...78 Date: 05/12/2008, Version: 4 9 (89)

10 Executive Summary Road safety is closely related to road skid-resistance: accident statistics show that low skid resistance leads to increased risk of accidents. Countries have developed skid policies for the monitoring of road networks and the acceptance of new works and the policies typically rely on specialised equipment to measure the key properties of skid resistance and texture depth. For the sake of simplicity, the output of the test devices is typically reduced to one descriptor for the property measured. For skid resistance, this is usually a measurement of friction under conditions specific to the particular device. However, this can obscure the influence of many factors involved in the tyre/road friction process during vehicle manoeuvres. The results from the measurements are used in various ways in different countries, sometimes as direct measurements, sometimes processed into some kind of index for comparison against standards where these have been set. There are no direct comparisons between skid-resistance indexes from one country to another, or even from device to device in the same country. Consequently, if a greater consistency of approach to the provision of skid resistance is to be encouraged across Europe, there is a need for a common scale against which comparisons between standards and the different types of measurement can be made. One of the main objectives of the TYROSAFE project, dealt with in the work-package 2, is to set out a strategy for moving towards a standard related to the measurement and calculation of such a common scale. One of the possible ways to achieve this, as suggested by the CEN committee dealing with test methods related to road surface characteristics (TC227 Working Group 5), is to use a reference device. Task 2.1 of TYROSAFE will investigate possible specifications for this reference device. The main objective of this report, however, is to compile as exhaustive a list as possible of skid-resistance measuring devices operated throughout Europe. This list is confined to devices specifically designed and used to assess road surface condition. It does not cover accelerometer devices such as those used by police forces in braking tests for collision investigation purposes. The report explains the basic principles on which skid resistance measuring devices operate longitudinal friction, transverse friction and slider techniques before describing the essential features of those devices that have been identified as currently operating within Europe. Twenty-three devices were identified from CEN TC227 WG5 and project partner sources. For each device, the measurement principle and test method are described. For those devices having explicitly defined calibration procedures set out in CEN draft Technical Specifications, these procedures have been included in an Appendix. Date: 05/12/2008, Version: 4 10 (89)

11 This first overview has shown a wide variety of measurement configurations and test conditions, ranging from static spot-check devices through large-scale routine investigation tools to research equipment for specialised purposes. This emphasises both the difficulty of making comparisons between the results from different devices and the need for a rigorous harmonisation technique if this is to be achieved. Previous experience has shown the difficulty of using simple mathematical models in the process of harmonising devices with widely varying operating characteristics and a number of aspects will need to be considered in future stages of this part of the TYROSAFE project as the work moves towards its objectives. Further investigations are required to establish more details relating to some of the devices listed here so that wider comparisons can be made. This should include identifying practices for calibration so that any future recommendations can be based on all aspects that could potentially contribute to the accuracy of any future harmonised skid resistance index. Date: 05/12/2008, Version: 4 11 (89)

12 1 Introduction The safe passage of road traffic needs a certain amount of grip (friction) between the tyres of the vehicles and the road surface. The frictional forces are necessary for the vehicle to accelerate, decelerate or safely change direction. The level of frictional forces that can be built up depends on the properties of both the road surface and the tyres. Much research has shown that the limiting frictional forces for a given road surface and tyre combination depend on many factors, including tyre load, tyre tread compound and depth, road surface characteristics, the presence of water, ice or other contaminants in the tyre/road interface and vehicle speed. In order to characterise road surfaces with respect to friction, for decades many countries have derived their own test methods. These are, of necessity, very much simplified in order to assess specifically the condition of the road surface. They all measure in some way the frictional force developed between a moving tyre or slider and the road surface (which is usually wetted) and record the quotient of the measured force with the applied vertical load (a friction coefficient). However, for each test method the effects of many of the potential influencing factors are controlled by standardising the measuring conditions. The standard conditions chosen reflect the practicalities of carrying out the particular test and are assumed to be relevant for characterizing the complex reality of friction in the tyre/road interface. Usually the measurement is called the skid resistance and is represented by a single value. Because the test methods and the chosen conditions vary, the actual numbers recorded can differ widely for the same road surface, Several European countries have investigated the link between skid resistance level and accident rates. The result of this research is that with a sufficiently high value of skid resistance the safety of roads can be improved by reducing the risk of skidding and hence the number or severity of accidents. Many European countries have developed their own skid policies for the road networks for which they are responsible. The approaches vary between countries but they often contain elements such as periodic routine monitoring of skid resistance of in service road and comparing the results with pre-determined values. In some countries the measurements are also used for comparison with acceptance levels for new works. As has been explained, the available standardized test methods all simplify the reality of the complex friction process in the tyre/road interface during vehicle manoeuvres and they do that in different ways. It therefore should be no surprise that a direct comparison of skid values from country to country is not an easy task. Also the relevance of the different test methods with respect to safety will be different since the techniques and standardised test conditions reflect different aspects of the tyre/road friction mechanism. For example, at one extreme, some methods simulate conditions close to those experienced by a tyre braking under the control of an Anti-lock braking system while, at the other, some devices use a Date: 05/12/2008, Version: 4 12 (89)

13 skidding locked wheel. It is clear that a common scale for characterizing road surfaces with respect to skid resistance properties is lacking. This can be seen as a serious hindrance for making skid policy for making the European road network safer. The TYROSAFE Project is a Coordination and Support Action (CSA) in the Seventh EU Framework Programme and aims at coordinating and preparing for European harmonisation and optimisation of the assessment and management of essential tyre/road interaction parameters to increase safety and support the greening of European road transport. This work is being carried out in the following six work packages (WP): WP1: Policies of EU countries for skid resistance / rolling resistance / noise emissions; WP2: Harmonisation of skid-resistance test methods and choice of reference; surfaces WP3: Road surfaces properties skid resistance / rolling resistance / noise emissions; WP4: Environmental effects and impact of climatic change skid resistance / rolling resistance / noise emissions; WP5: Dissemination and raising awareness; WP6: Management. The objective of Work Package 2 of TYROSAFE is to end up with a widely supported road map towards future skid-resistance harmonisation policy in 2020, including aspects such as testing equipment, quality assurance and implementation strategy. The major field of application in mind is for monitoring the skid resistance quality of the European road network and for new work acceptance control. Basically the lines being followed are those formulated in 2005 by the CEN working group on Surface Characteristics (CEN/TC227 WG5), to prepare in the longer term (over 10 years) a harmonised standard based on the measurement of a friction index with a common and single European friction measuring equipment. The harmonisation process is illustrated in Figure 1.1 along with actions to be carried out in WP2 of TYROSAFE. To reach its objective, WP2 is split into four Tasks: In Task 2.1 knowledge of current national skid resistance test methods will be collated, together with findings of previous harmonisation research projects, which will be collected and analysed. Based on the outcomes of these exercises, proposals will be formulated for possible options for the specification of a Standard European Skid Resistance Device (SESRD). In Task 2.2 the focus will be on the use and harmonisation of reference surfaces in the Quality Assurance part of the harmonisation policy as was suggested by the HERMES project. In Task 2.3, based on the results of Task 2.1 and 2.2, a road map or implementation plan will be developed to point the way towards a harmonised approach to wet skid resistance test methods in Special attention will be paid to intermediate stages Date: 05/12/2008, Version: 4 13 (89)

14 (2010, 2015) to allow for the need for individual countries to make a smooth transition to the new approach. The focus in this transition period will be to maintain consistency with existing historical data and to maximize the possible use of the present fleet of testing devices till the end of their technical working lives. This Task will also initiate promoting activities for finding a number of pilot countries for early implementation in their national monitoring programmes. To obtain constructive input from stakeholders and experts and to mobilize support for the road map/implementation plan, several workshops will be organised in Task 2.4. Local (national) test methods 2008 Local friction devices Local ref. surfaces Correlation State of the art local/reference (current practices, previous projects, standards) - research needs - QA procedure Specifications Development 2.4 Partners Specifications + Experts + Road Alternatives authorities SESRD Reference surfaces Pilot tests State of the art - existing standards EU test method 2020 Figure 1.1 Harmonisation scheme and actions carried out in WP 2 Table 1.1 gives an overview of the major outcomes planned for the individual Tasks of WP 2. Table 1.1 Overview of the major outcomes of the individual Tasks of WP 2 Task Deliverable Name Month 2.1 D04 Report on state-of-the-art test methods M5 2.1 D05 Report on analysis and findings of previous M8 skid resistance harmonisation research projects 2.2 D07 Report on state-of-the-art of test surfaces M8 for skid resistance 2.3 D09 Road map and implementation plan to M12 future harmonised test methods and reference surfaces Two dedicated workshops M5 and M10 This report is part of Task 2.1 and constitutes the deliverable D04. Its main aim is to identify and bring together relevant information relating to devices currently used in Europe for skidresistance measurement purposes that can then act as a platform for subsequent stages of Date: 05/12/2008, Version: 4 14 (89)

15 the work. Although the original project proposal suggested that this particular output (D04) would include coverage of skid resistance policies, the TYROSAFE Management Group has decided that to avoid unnecessary repetition or duplication of effort, the topic of skid resistance policies and how supporting measurements are obtained would be dealt with specifically in Work Package 1. Sections 2 and 3 of the report provide some brief initial background explanation of general principles relating to road/tyre friction and how these are applied to skid resistance measurement. The bulk of the rest of the report describes the individual devices. The individual descriptions include three topics: what they look like, what they measure and how they operate. Where the information is available, details of how individual devices are calibrated are included in Appendix 1. Discussion of the ways in which data from the individual devices are used is outside the immediate scope of this report. Some of the devices covered here provide data to support skid resistance policies in individual countries (and in some cases, more than one country) while others are confined to research or localised road surface investigations. Date: 05/12/2008, Version: 4 15 (89)

16 2 Skid resistance measurement principles Before describing the various individual devices identified, this and the next chapter provide some background information about important physical effects and concepts that influence the design and operation of skid resistance measuring equipment. Current measurement devices can usually be classified into one of three groups depending on the principle used. The first two of these longitudinal and transverse friction utilise a test wheel that slides over the road surface to generate a frictional force that is then measured and used to calculate a value representing the skid resistance of the road. The test wheel typically has a pneumatic tyre and is mounted on a vehicle that is operated at or near normal traffic speeds. Devices in the third group are smaller and are either stationary or very slow moving when they are used. 2.1 Longitudinal friction principle For a vehicle travelling in a straight line, when the driver applies the brake, a torque is applied to the vehicle wheels via the braking system. A reacting force develops in the tyre/road contact area. Provided that grip is maintained, the angular (or rotational) speed of the wheels decreases and the vehicle slows down as kinetic energy is absorbed in the braking system. However, as the braking torque increases, the wheel speed may reduce below the vehicle speed and consequently the tyre slips on the road, generating friction forces in the contact area (due to adhesion and deformation processes) to slow down the vehicle. In the extreme, the wheel may cease to rotate (known as the locked condition), and one area of the tyre slides or skids over the road surface. Longitudinal friction measuring devices try to simulate part of this process, typically by controlling the rate at which the wheel rotates relative to the road speed. This leads to the idea of the slip ratio and it is important to appreciate how the longitudinal friction coefficient varies with the slip ratio Slip ratio The tyre slip ratio G is defined by the formula (1): (1) G = V Rω V where ω: angular speed of the wheel; R: wheel radius; V: vehicle speed. G varies between 0 and 1. For skid-resistance measuring devices, G is generally expressed as a percentage. Thus, for G = 0%, the tyre speed is equal to the vehicle speed and the wheel is freely rotating: for G = 100%, there is no rotation and the wheel is locked. Date: 05/12/2008, Version: 4 16 (89)

17 2.1.2 Friction slip curve The longitudinal friction coefficient LFC varies with the tyre slip ratio as illustrated in Figure 2.1. LFC LFC max LFC locked ~ 0,01 15% < G max < 20% 100% G Figure 2.1 Illustration of LFC G curve It can be seen that, initially, friction increases as the slip ratio increases but it reaches a maximum value before decreasing as the slip ratio continues to increase until the lockedwheel state is reached. This variation can be explained by the movement of the tyre treads in the tyre/road contact area changing from a largely shear phase to a mainly slipping phase. The maximum value of LFC denoted by G max, (sometimes known as peak friction ) typically occurs at a slip ratio between 15% and 20% Using the longitudinal friction principle for skid resistance measurements Skid-resistance measurement devices that measure longitudinal friction operate with a slip ratio that is either set by means of a fixed mechanical linkage or by means of a controlled braking system that adjusts the brake to maintain a constant ratio between the vehicle speed and the test wheel speed. The slip ratio is generally chosen to lie between G max and 100%. Some devices may offer a choice of the slip ratio that can be used but this is usually fixed at that ratio once selected. Mechanical systems automatically have a fixed slip ratio. Servosystems may suffer from a slight delay as the braking forces are adjusted to reflect changes vehicle speed or in response to a sudden change in friction (if the friction suddenly reduces, the brake force may slow the test wheel down too much and if friction suddenly increases, the brake force may need to increase). Other LFC devices have a variable slip ratio that gradually increases the braking force until the wheel locks, enabling them to plot the whole friction slip curve during the test. Some locked-wheel devices may also record the frictional forces during whole the braking cycle, thus allowing the friction slip curve to be inferred even though the main reported value is in the locked-wheel condition. Date: 05/12/2008, Version: 4 17 (89)

18 The fixed slip ratio approach is more suitable for general monitoring purposes since the wheel continues to rotate during the test and can therefore be used continuously. Lockedwheel and variable-slip systems can only sample a short length of road during one test and so are better-suited to research use. 2.2 Transverse friction principle In a bend, the driver uses the steering system to turn the vehicle s front-wheels so that there is a difference between the vehicle direction and the wheel rotation-plane. The induced angular difference is known as the slip angle. It induces tyre/road friction, which in turn generates a centripetal force opposing the centrifugal force exerted on the vehicle in the bend, allowing the vehicle to follow round the curve. Just as with longitudinal friction, when as the braking force increases the wheel starts to slip over the road surface, so in the transverse friction situation if the centrifugal force exceeds the friction force available, the tyre will slip sideways, even though it continues to rotate. Transverse-friction (also known as side-force ) skid resistance measuring devices try to simulate this process. This leads to the concept of the slip angle and it is important to appreciate how the transverse, or sideway, friction coefficient varies with the slip angle Slip angle The slip angle is the angle formed by the wheel s plane of rotation and the tangent to the wheel s path (Figure 2.2). On a skid resistance test device the wheel s path normally follows the direction of travel of the test vehicle. Figure 2.2 Illustration of slip angle Date: 05/12/2008, Version: 4 18 (89)

19 2.2.2 Friction slip angle curve The sideways friction coefficient SFC varies with the tyre slip-angle as illustrated in Figure 2.3. SFC SFC max 4 < δ max < 7 Figure 2.3 Illustration of SFC δ curve δ It can be seen that the friction increases at first as the slip angle increases, reaching a maximum before decreasing as the slip angle continues to increase. This process is analogous to the variation observed in longitudinal braking, as the tyre tread in the tyre/road contact area moves from a shear phase to a slipping phase. Typically, the maximum value of SFC occurs at a slip angle, denoted by δ max, between 4 and 7 for a light vehicle, and between 6 and 10 for a truck Using the transverse friction principle for skid resistance measurements Skid resistance measurement devices operating on the angled wheel principle normally operate at a fixed slip angle which is typically set to be well beyond δ max. The force developed along the axle of the test wheel is measured and used to compute a friction value to represent skid resistance that is known as sideway-force coefficient (also abbreviated to SFC). In this case the abbreviation refers explicitly to the special case of the value measured with a skid-resistance device operating on angled-wheel principle under controlled conditions. The side-force method for measuring skid resistance allows continuous measurement and such devices are often used for routine monitoring purposes. Some devices can vary the slip angle through the test but, as with variable-slip longitudinal systems, these are normally confined to research work. Date: 05/12/2008, Version: 4 19 (89)

20 2.3 Static or slow-moving devices The previous section has dealt with the main principles that apply to devices that use wheels fitted with rubber tyres to measure skid resistance. There is, however, a further group of devices which are designed primarily to be easily portable and suitable for laboratory or localised use. These typically utilise rubber sliders to make contact with the road surface, with a mechanism that initiates relative motion between the slider and the road. Two methods have been used. The first of these is a pendulum arm that swings under gravity with the rubber slider mounted beneath the foot of the pendulum so that the pendulum slows down as a result of friction between the slider and the road surface. The work done to decelerate the pendulum is related to the skid resistance and devices of this type usually have a pointer that is pushed up a simple calibrated scale to indicate this and serve as the measured value. The second method is to attach sliders beneath a rotating head that is lowered on to the road so that friction between the sliders and the road causes the head to slow down. These devices typically measure the rotational speed and reaction torque as the head slows down. They derive a friction coefficient from this and the vertical load, usually taking the value at a pre-defined tangential slider speed to represent the skid resistance. Some devices do not remain stationary on the road during a test but are pushed manually along the road surface at a walking speed or slower. These have been designed primarily for use in confined areas or for specialised purposes such as measuring grip on footways or on road markings. They can utilise any of the main principles but in a form suitable for lowspeed use. Date: 05/12/2008, Version: 4 20 (89)

21 3 Factors influencing skid resistance measurements and device operation As well as the various operating principles described above, there are a number of factors that influence skid resistance measurements and that therefore need to be controlled in some way while measurements are made for particular purposes. This chapter summarises the main issues in relation to four major factors. 3.1 Test speed Sections and described the variation in friction that occurs with slip ratio or slip angle (percentage slip), and hence the need to standardise on the values used in a measurement. As well as these factors, skid resistance is also influenced by the speed at which the contact area is passing over the surface the slip speed. For longitudinal friction systems, the slip speed is influenced by the slip ratio, which determines the relative slip between the tyre contact patch, and the test vehicle speed. For transverse friction systems the slip speed as a proportion of the vehicle speed is governed by the cosine of the slip angle of the test wheel. The general principle relating to the speed of the test vehicle is that changing the vehicle speed alters the slip speed and, as slip speed increases, skid resistance decreases. Theoretical models have been developed that represent these influences and Figure 3.1 is a three-dimensional figure illustrating visually how the percentage slip and vehicle speed interact Friction Vehicle speed / km/h Percentage slip Figure 3.1 Effect of vehicle speed and percentage slip on friction The friction values in the graph in Figure 3.1 are purely illustrative. The actual underlying level of friction or skid resistance depends on the condition of the road at the time of Date: 05/12/2008, Version: 4 21 (89)

22 measurement which, of course, is what is being assessed. The way in which measured skid resistance changes with speed also depends on characteristics of the road, in particular the macrotexure or texture depth of the surface. Currently, the influence of texture depth on the measurement process is not fully understood. Different devices respond differently to it depending on factors such as the type of tyre and the slip ratio. They may respond differently on different types of surfacing. This has been found to be one of the major factors influencing the process of harmonising measurements made on different principles and under different conditions. Although theoretical models have been developed to represent the influence of macrotexure, attempts to use these as a means of harmonising different types of measurement device have not been entirely successful and will be an important aspect to be considered by the TYROSAFE project. Clearly, even when using one type of device, it is important to control the speed at which measurements are made. 3.2 Test tyre All vehicle-based systems use pneumatic tyres on their test wheels. The properties of the test tyre are also an important aspect of the process of generating the frictional forces that are measured by skid-resistance devices. For this reason, test tyres for any particular type of device are usually standardised. Tyres vary widely in terms of their size, profile, tread pattern and depth as well as rubber properties. For this reason, most skid resistance devices use tyres specifically designed for skid resistance measurement. Some are device-specific while others are made to a defined specification and are used by more than one device. An important aspect of the test tyre is its tread profile. The principle that an individual device uses to make the measurements reflects a view taken by its designers as to what aspect of road/tyre friction is to be measured: similarly, the tread pattern chosen for the tyre will also reflect the purpose of the measurements. This is because the presence of tread on the tyre has an influence on the measurements, particularly in relation to speed, in a manner analogous to the macrotexure on the road. One or two devices use ordinary vehicle patterned tyres (albeit of one specific size and type) and some use standardised ribbed tyres, but most devices use smooth tyres. The properties of the rubber vary and whereas a vehicle tyre is typically designed to maximise wet grip, skid resistance test tyre properties are often deliberately designed to be sensitive to the condition of the road, especially at low grip levels. Measurements from slider devices may also be influenced markedly by the properties of the rubber from which the sliders are made. Date: 05/12/2008, Version: 4 22 (89)

23 3.3 Load on test wheel All vehicle-based skid resistance measurements rely on a measurement of the coefficient of friction between a sliding test wheel and the road surface. To compute this value, both the frictional force reacting against the braked or angled wheel (which is normally directly measured by the device) and the vertical load acting on the wheel are needed. The vertical load is normally achieved either by a static load acting on the test wheel or by application of a downward force through a controlled loading system. In a classical physical situation, the coefficient of friction between two surfaces under a given set of conditions is a constant and any changes in load should be balanced by a change in the reaction force. However, the response of a test tyre sliding over a textured road is not classical and, although the principle is broadly true, there will be some variations. The load is likely to vary during the test depending on the design of the system and the way in which it responds to factors such as unevenness in the road surface. Clearly, therefore, vertical load must be controlled in some way. Some devices using a static load assume that the load is constant on average for the period over which a single measurement is made. Other devices measure the vertical load directly, simultaneously with the frictional force, smoothing out short-term variations by averaging results over a defined length of road or time interval. Analogous principles apply to slider systems which use springs or static weight to apply the load. 3.4 Water Skid resistance measurements are normally made on a wetted road surface. The amount of water on the road can have an influence on the measurements depending on the nature of the surface and other test conditions (such as speed). Too little water may lead to localised dry conditions developing in the tyre contact area, resulting in higher than expected measurements. Too much water could lead to hydrostatic pressure building and influencing the frictional force or even, in an extreme case, leading to aquaplaning and markedly reduced friction. In the latter case, of course, the test tyre would no longer be in contact with the road. It is important that there is some control over the water on the road. Vehicle-based systems generally carry their own water supply tank and water which is fed at a controlled rate, either through gravity or by a pump, through a special nozzle to wet the road just in front of the test tyre. The water depth is often specified, usually in terms of an average depth above a smooth texture. In practice, even with a closely-controlled delivery system, the actual depth of the water film on the road surface varies widely, particularly depending on the nature and shape Date: 05/12/2008, Version: 4 23 (89)

24 of the surface itself and other factors such as the time between the water hitting the road and the test tyre reaching it, as well as water already being carried on the test tyre itself. Slider systems are also influenced by the amount of water, and normally copious amounts are required. 3.5 Temperature Skid resistance measurements can be influenced by temperature, particularly the temperature of the test tyre which can influence the properties of the tyre rubber, especially at extreme levels. Across the range of European countries, the temperatures that can occur both in the air and on the road can vary widely. There may be restrictions in individual countries on the ambient conditions in which measurements can be made (perhaps to reduce the risk of water being applied to a surface that may freeze). However, there is no general agreement as to exactly what the influences of temperature are or how they should be taken into account. Some devices are equipped to measure air temperature, some the road temperature and these values may be used to adjust the measurements to reflect standard conditions. Others rely on the assumption that in the temperature of the tyre is largely governed by cooling effect of the water used to wet the road, that the variation from this source is generally small compared with the natural variability of the road and test tyres, and therefore makes a small contribution to the variability of the test method. Temperature, both ambient and of the road, is also an important factor for slider systems, particularly the pendulum type, because of the small contact area involved and the small mass of the slider. Specifications for using such systems often require temperature to be measured and apply a correction factor to reflect standard conditions. Date: 05/12/2008, Version: 4 24 (89)

25 4 Identifying current devices used to measure skid resistance The purpose of this report is to summarise the state of the art in relation to skid resistance measurement methods and therefore it summarises what is known about the various devices used in Europe for this purpose. It was known from earlier work (particularly that of the CEN Working Group and the FEHRL HERMES research project) that a large number of different devices were in use across Europe. Some, such as SCRIM and GripTester, which are built commercially and marketed across the world, are used in many countries and in large fleets: others devices are unique and only used in the country in which they were made. The HERMES project found that one of the factors affecting the quality of the harmonisation process being investigated was the different conditions and methods of operating the equipment that were used, even for nominally similar devices. The operation and calibration of some devices was clearly set out in published national standards, although not always consistently applied, whereas other devices appeared to rely upon the local knowledge and practice of their operating team. As a step to move the harmonisation process forward, the CEN group initiated the preparation of a series of Technical Specifications which would set out in a consistent way the fundamental features of each device and how they should be used. The idea was that devices should have a clear written technical specification before they could be operated in association with any harmonised CEN procedure for dynamic measurement of skid resistance. To date, draft Technical Specifications have been prepared for twelve devices which are described Chapter 5. Other devices used in European countries are listed in Chapter 6. Information about this second group of derives primarily from the personal knowledge of members of the TYROSAFE project team. For some of this second set of devices, CEN Technical Specifications are being prepared but were not available at the time of writing, which is why they are included in this separate chapter. Some have national or other standards describing their use, others do not. The precision of the measurements is obviously an important factor in interpreting and utilising the results of the test equipment. However, the approach taken to defining this by the various organisations that prepared the technical specifications varies widely. Some devices are operated in an environment in which formal estimates of repeatability and reproducibility following standard procedures can be made. However, others are not and there are no formal assessments, although some use of repeatability concepts are incorporated into the technical specifications. For many devices it is not always possible to assess reproducibility because there are insufficient devices of the same type available and although values for R are sometimes quoted, they may not fully represent reproducibility conditions. For these reasons it has not been possible to standardise the way in which the precision information is presented for the individual devices. Date: 05/12/2008, Version: 4 25 (89)

26 The information given in Chapters 5 and 6 has been drawn (or inferred) from the CEN draft Technical Specifications or other documents available to the project team, supplemented by personal knowledge in some instances. The information for each device is presented in a similar way: What the device looks like: a short description and photograph where available. What the device measures: a brief definition of the measurement made, including an indication of precision where this is available. How the device works: a summary of the way in which the equipment works, including (for the CEN TS devices) a table summarising the operating conditions that are standardised. Calibration information, where this is known, is included in Appendix 1. It has not been possible at the time of writing to provide some details for all machines where this information was not immediately available. Date: 05/12/2008, Version: 4 26 (89)

27 5 Skid-resistance devices identified by CEN/TC227/WG5 The twelve devices for which CEN Technical Specifications have been prepared are (in alphabetical order): 1. ADHERA; 2. BV11 and Saab Friction Tester; 3. GripTester; 4. RoadSTAR; 5. ROAR DK; 6. ROAR NL; 7. RWS NL Skid Resistance Trailer; 8. SCRIM; 9. Skiddometer BV8; 10. SKM; 11. SRM; 12. Tatra Runway Tester. The main characteristics and the test procedure for each of these devices are set out in sections 5.1 to 5.12, using information taken from the descriptions in their respective Technical Specifications [1-12]. The level of detailed description in those documents varies and so, for brevity, only the basic features of each device are summarised here. There is inevitably some variation in the output from the measurement sensors as the device travels along the road and therefore the data are typically averaged over a defined length before recording a value to represent the measured skid resistance. The detail tables, which are taken directly from the Technical Specification documents, usually include an entry for the length for the mean value. This represents the distance along the road over which individual interval sensor readings are normally averaged before recording a result. In some cases, however, while the device may report a value over a short distance, several individual results may be aggregated to represent a section of road, for example over 100m. It is not always clear from the TS information which of these situations apply so at present there may be some ambiguity when comparing devices. For ease of reference and formatting, the description of each device begins on a new page. Date: 05/12/2008, Version: 4 27 (89)

28 5.1 ADHERA What does ADHERA look like? ADHERA (Figure 1.1) is a single-wheeled trailer towed behind a vehicle that carries water and the recording equipment. It operates on the longitudinal friction principle. The trailer is supposed to represent a quarter of passenger car. Figure 5.1 ADHERA: General view (left) and trailer (right) with cover opened What does ADHERA measure? The ADHERA measures LFC using a locked wheel (i.e. a slip ratio of 100 %) in its standard configuration. For research use, ADHERA uses a variable slip ratio between 0 to 100 %. The precision of the LFC measured in standard locked-wheel conditions is given in Table 5.1. Table 5.1 Precision indicators for ADHERA measurements Repeatability r = 0.03 Reproducibility R = 0.05 The system also includes a laser-based system called RUGO to measure the macrotexure of the pavement surface: the standardised Mean Profile Depth is calculated How does ADHERA work? The measuring wheel allows the simulation and investigation of a locked braking situation. Braking sequences consist of braking and free-wheeling sections at specific test speeds (mainly 40, 60 and 90 km/hr on main roads and 60, 90 and 120 km/h on motorways). For research projects it is possible to perform measurements with a variable slip ratio in order to characterise the whole LFC-slip curve. Both horizontal and vertical forces are measured. Date: 05/12/2008, Version: 4 28 (89)

29 The water supply system enables the specification of a defined water film thickness for all measurements. The amount of water delivered is adjusted depending on the specified film thickness and measuring speed. The standard test-conditions for the ADHERA are listed in Table 5.2. Table 5.2 Standard test conditions for ADHERA air temperature > 4 C pavement temperature > 5 C (testing season: April to November) and < 50 C pavement status no pollution test wheel Smooth PIARC-tyre 165R15 inflated at 0.18 MPa method Locked wheel (slip ratio, 100 %) static wheel load 2500 N operating speed 40 to 120 km/h theoretical water film thickness 1 mm length for the mean value 20 m wheel path nearside right wheel path Date: 05/12/2008, Version: 4 29 (89)

30 5.2 BV-11 and SFT (Saab Friction Tester) What does the BV-11 or SFT look like? This device is built either as a towed trailer (BV-11) or built into a vehicle (SFT). The measuring wheel, which operates on the longitudinal friction principle, is located between two reference wheels (Figure 5.2). Figure 5.2 BV11 trailer (left) and SFT (right) What do the BV-11 and SFT measure? The BV-11 and SFT measure LFC with a fixed slip ratio of 17%. The precision of the LFC measurement indicated in the test procedure is controlled by the conditions set out in the test procedure, as indicated in Table 5.7. Table 5.3 Precision indicators for BV11/SFT measurements If the results of two repeated runs differ by more than 10 % and one of the values is less than 0.5, a complete renewed test should be made How do to the BV-11 and SFT work? The measuring wheel is engineered to give a fixed slip ratio of 17%. The wheel slips as it is towed along the wetted pavement surface at a constant speed and the slipping force is measured. Skid resistance measurements are carried out by a sensor providing continuous data which are collected, processed and stored. The water control system enables the specification of a defined water film thickness. This is normally 0.5 mm for all measurements but on airfields where 1.0 mm is used. The standard test-conditions for the BV-11 and SFT are listed in Table 5.4. Date: 05/12/2008, Version: 4 30 (89)

31 Table 5.4 Standard test conditions for BV-11 and SFT air temperature > 5 C pavement temperature > 5 C (testing season: Summer condition) pavement status no debris test wheel Trelleborg type T49 inflated at 0.14 MPa method constant slip ratio, 17 % static wheel load 1000 N operating speed 70 km/h theoretical water film thickness 0,5 mm length for the mean value 20 m wheel path Right or left wheel path Date: 05/12/2008, Version: 4 31 (89)

32 5.3 GripTester What does GripTester look like? The GripTester is a device developed by Findlay Irvine Ltd in the United Kingdom, initially for use on helipads but now widely used in many countries on airfields and roads. The device operates on the longitudinal friction principle and is a trailer with two running wheels (called the drive wheels) and a single small test wheel. The wheel dimensions are similar to those of a go-kart wheel (Figure 5.3). It can also be configured to be pushed manually for lowspeed operation in confined areas. Lifting handle Chassis Measuring wheel Transmission chain Water supply Drive wheel Towing bracket Water Connection Figure 5.3 GripTester What does GripTester measure? GripTester measures LFC using a small test wheel operating at fixed slip ratio of 15%. Precision indicators for the LFC measurement are given in Table 5.5. Table 5.5 Precision indicators for GripTester measurements Repeatability r = 0.03 R = 0.08 at 30 km/h, Reproducibility (dependent on operating speed) R = 0.07 at 50 km/h, R = 0.06 at 80 km/h. Date: 05/12/2008, Version: 4 32 (89)

33 5.3.3 How does GripTester work? The test wheel is mounted on a stub axle and is mechanically braked by a fixed gear and chain system linking it to the drive wheel axle. The gear ratio is 27:32 in relation to the drive wheels so that there is a slip ratio of just over 15%. The wheel slips as it is towed along the wetted pavement surface at a constant speed and the slipping force and vertical load are both measured. The static load on the test wheel is (250 ± 30) N when towed or (260 ± 30) N when used in push mode (in the latter case a small water container is mounted on the device itself, adding to the load). During operation, the stub axle becomes elastically deformed by the horizontal drag and vertical load forces acting on the test tyre. Two strain gauge bridges on the stub axle continuously measure the horizontal drag and vertical load forces. The two drive wheels are mounted on the main axle, which also carries a toothed wheel. A proximity sensor generates signals for distance recording. For normal wet road testing, water is deposited in front of the test tyre from a water tank fitted with a control valve. A water nozzle is mounted directly in front of the test wheel delivering a controlled amount of water to the road surface. In towing mode, water flow rate is further controlled by a pump and may be monitored with a flow meter. The standard test-conditions for the GripTester are listed in Table 5.6. Table 5.6 Standard test conditions for GripTester air temperature > 4 C pavement temperature > 5 C and < 50 C pavement status no pollution test wheel smooth ASTM-tyre 254 mm in diameter inflated at 0.14 MPa method constant slip ratio, 15 % static wheel load 250 ± 20 N operating speed 5 km/h to 100 km/h theoretical water film thickness 0.5 mm minimum recording length Optional, typically 10 m or 20 m. wheel path Normally nearside wheel path or as required Date: 05/12/2008, Version: 4 33 (89)

34 5.4 RoadSTAR What does RoadSTAR look like? RoadStar operates on the longitudinal friction principle. The measuring equipment is mounted on the rear of the chassis of a specially modified truck that also carries a water tank and the control equipment (Figure 5.4). Measuring wheel including braking torque measurement Pneumatic cylinder Wetting unit Pre-wetting system Gearbox Water tank Device storage Drivers cabin - digital data acquisition Figure 5.4 RoadSTAR general view (left) and close-up of measuring wheel (right) What does RoadSTAR measure? In normal operation, the device provides a continuous measurement of LFC using the fixedslip principle with a car-sized wheel. The standard measurement uses a slip ratio of 18 %. For other comparison measurements (such as those envisaged by the HERMES project), slip ratios of 37.5 %, 50 %, 75 % can be used. The equipment can also measure with a locked wheel or under ABS-braking conditions for research purposes. The device is also fitted with a laser sensor to measure macrotexure (as MPD) on the dry surface in front of the test wheel. Precision indicators for LFC measured in standard conditions are given in Table 5.7. Date: 05/12/2008, Version: 4 34 (89)

35 Table 5.7 Precision indicators for RoadSTAR measurements offset of the mean value between the 2nd and 3rd measurement (LFCS, 50 m-values) twice standard deviation of the offset between the values of the 2nd and 3rd measurement (LFCS, 50 m-values) Δµ σ How does RoadSTAR work? RoadSTAR is based on the Stuttgarter Reibungsmesser (see 5.11) but was developed so that it could provide measurements under a wider range of conditions, including selected slip ratios to reflect proposals for a possible reference device that was one of the outputs from the FEHRL HERMES project. The measuring wheel at the rear of the vehicle is mounted on the right of the machine (in the nearside wheel path for driving on the right-hand side of the road) and is applied to the road surface under a known vertical force using a pneumatic controlled loading unit. The current wheel load is recorded and used in computing the skid resistance values. Different slip ratios are achieved using a specific gear box. Continuous measurements can be made along the entire measuring section in fixed-slip mode. Individual braking sequences can be selected for locked-wheel or ABS measurements. These braking sequences consist of braking sections and free-wheeling sections that can be selected from a defined range. A controlled flow of water pre-wets the road surface immediately in front of the test wheel to provide a defined theoretical water film thickness. The amount of water required is automatically adjusted to reflect the film thickness required and the measuring speed. Water film thicknesses between 0.5 mm and 2 mm and measuring speeds up to 120 km/h can be pre-selected. Due to the construction of the skid resistance unit and the forces caused by the vehicle roll (chassis movements) at higher speeds, there are limitations on the radius of curves through which the machine can be operated that depend on the test speed. The standard test speed of 60 km/h allows measurements of LFC in curves with a radius > 85 m. If the curve radius is less than 85 m, the operating speed has to be reduced. The standard test conditions for the RoadSTAR are listed in Table 5.6. Date: 05/12/2008, Version: 4 35 (89)

36 Table 5.8 Standard test conditions for RoadSTAR air temperature > 3 C pavement temperature > 5 C (testing season: April till November) and < 50 C pavement status no pollution test wheel ribbed PIARC-tyre method const. slip ratio, 18 % static wheel force 3500 N operating speed 60 km/h minimum operating speed 30 km/h theoretical water film thickness 0.5 mm length for the mean value 50 m Wheel path nearside wheel path Date: 05/12/2008, Version: 4 36 (89)

37 5.5 ROAR DK What does ROAR DK look like? This is the version of the ROAR device that is operated in Denmark. The device operates on the longitudinal friction principle. The test wheel mechanism is mounted within a trailer with drive wheels and a single loaded test wheel (Figure 5.5). Laser sensors Towing vehicle Roar units and watertank Measuring wheel Water system Figure 5.5 A general view of ROAR DK (left) and close-up of the test wheel mechanism (right) What does ROAR DK measure? ROAR DK measures LFC using the fixed-slip method at a slip ratio of 20 %. The system is also capable of measuring skid resistance at a pre-set slip ratio, which can be fixed from 1% to 99%. The device is also fitted with a laser sensor to measure macrotexure (as MPD). This is mounted on the front of the towing vehicle in order to measure on the dry pavements on the same path as the skid resistance measurement. Precision indicators for the precision of the LFC measured in standard conditions are given in Table 5.9. Table 5.9 Precision indicators for ROAR DK measurements For 90% of the runs the difference of mean between the two runs(10 m- values) Δµ 0,04 For more than 90% of the runs the standard deviation of the offset between the two runs (10 m-values) Δσ 0,01 Date: 05/12/2008, Version: 4 37 (89)

38 5.5.3 How does ROAR DK work? A hydraulic braking system controls the slip ratio. The slipping force is measured as the test wheel passes along the wetted pavement surface at a constant speed. The measurement is continuous. The pre-wetting function enables the specification of a defined water film thickness of 0.5 mm for all measurements. Optional theoretical water depths of 0.0 mm (dry road) and 1.0 mm can also be used. The standard test conditions for the ROAR DK are listed in Table Table 5.10 Standard test conditions for ROAR DK air temperature > 5 C pavement temperature > 5 C (testing season: April till November) and < 50 C pavement status no pollution test wheel ASTM 1551 tyre inflated at MPa method const slip ratio, 20 % static wheel load 1200 N operating speed 60 km/h for routine measurements and 60 km/h and 80 km/h for control of new pavements theoretical water film thickness 0.5 mm length for the mean value minimum 5 m wheel path Both wheel paths Date: 05/12/2008, Version: 4 38 (89)

39 5.6 ROAR NL What does ROAD NL look like? This is the version of the ROAR device used in the Netherlands, operating on the longitudinal friction principle. The test vehicle is a three axle tanker truck with two measuring systems mounted at the rear of the chassis. The tank capacity is about 12,000 litres. The measuring units are mounted to align with the right wheel track, the left wheel track and/or in the centre line of the truck (Figure 5.6). Figure 5.6 A general view of ROAR NL (left) and close-up of the measuring system mounted at the rear in the right wheel track (right) What does ROAR NL measure? The ROAR NL measures LFC using the fixed-slip method. The normal slip ratio for the Netherlands is 86% but the equipment can be set to maintain any slip ratio between 5 and 95%. A laser system is fitted at the front of the truck measure macrotexure as MPD. Precision indicators for the LFC measurements under the standard conditions derived from monthly correlation trials are given in Table The reproducibility figure may be an underestimate since there are only two ROAR units and both are attached to the same vehicle. In the monthly trials the machine is included with the Netherlands Skid Resistance Trailers (Section 5.7) with which it has been found to agree quite closely. Table 5.11 Precision indicators for ROAR NL measurements Repeatability (100 m average LFC) r = 0.04 Reproducibility (100 m average LFC) R = 0.05 Date: 05/12/2008, Version: 4 39 (89)

40 5.6.3 How does ROAR NL operate? A hydraulic braking system controls the slip ratio. The slipping force is measured as the test wheel along the wetted pavement surface at a constant speed. The measurement is continuous. The pre-wetting function enables the specification of a defined water film thickness of 0.5 mm for all measurements. Optional theoretical water depths of 0.0 mm (dry road) and 1.0 mm can also be used. The standard test-conditions for the ROAR NL are listed in Table Table 5.12 Standard test conditions for ROAR NL air temperature > 2 C and < 30 C pavement temperature > 2 C and < 45 C pavement status no pollution test wheel ASTM 1551 tyre tyre pressure equivalent to 0.2 MPa at 20 C method Constant slip ratio, 86 % static wheel load 1200 N operating speed 70 km/h and 50 km/h. Deviation from target test speed maximum 5%. theoretical water film thickness 0.5 mm, deviation maximum ± 10% length for the mean value minimum 5 m and 100 m wheel path wheel path of near side lane 5.7 RWS NL Skid Resistance Trailer What does the RWS Netherlands Skid Resistance Trailer look like? The RWS skid resistance trailer is a single- axle trailer that carries a measuring wheel mounted centrally between the road wheels (Figure 5.7). Water and control equipment are carried in the towing vehicle. A number of these devices are operated in the Netherlands. Figure 5.7 RWS NL skid resistance trailer Date: 05/12/2008, Version: 4 40 (89)

41 5.7.2 What does it measure? The RWS trailer measures LFC using the fixed slip method at a slip ratio of 86%. Precision indicators for the LFC measurements derived from monthly correlation trials are given in Table Table 5.13 Precision indicators for RWS Skid Resistance Trailer measurements Repeatability (100 m average LFC) r = 0.04 Reproducibility (100 m average LFC) R = How does it operate? The measuring wheel is connected via a mechanical transmission to one of the bearing wheels to achieve the slip ratio of 86%. This means that the circumferential speed of the standard test tyre is 14% of that of the bearing wheels. A water film thickness of 0,5 mm immediately in front of the test wheel is used for all measurements. The standard test conditions for the RWS Trailer are listed in Table Table 5.14 Standard test conditions for the RWS Skid ResistanceTrailer air temperature > 2 C and < 30 C pavement temperature > 2 C and < 45 C pavement status no pollution test wheel PIARC smooth 165 R15 tyre pressure equivalent to 0.2 MPa at 20 C method Constant slip ratio 86 % static wheel load 1962 N, deviation maximum ± 10 N operating speed 70 km/h and 50 km/h. Deviation from target test speed maximum 5%. theoretical water film thickness 0,5 mm, deviation maximum ± 10% length for the mean value minimum 5 m, mostly 100 m wheel path wheel path of nearside lane; towing vehicle drives on an offset line to achieve this Date: 05/12/2008, Version: 4 41 (89)

42 5.8 SCRIM What does SCRIM look like? The SCRIM was originally designed in the UK by the then Road Research Laboratory and has been manufactured under licence by WDM Limited since the 1970s. The device operates on the transverse friction principle and uses special narrow test wheel which set at an angle to the direction of travel. The wheel is lowered on to the road surface under the action of a static load. The test wheel is mounted to the side of a tanker lorry between the front and rear axles of the truck so that it runs in the vehicle wheel path. SCRIM is used widely across Europe with many countries operating more than one machine. There is a wide variety of truck chassis and bodywork in use, ranging from small units for use on local roads to very large three-axle trucks for long-distance main highway work. Figure 5.8 shows the measuring wheel assembly on a SCRIM built for UK main road use, with its test wheel on the left side of the truck. European mainland machines normally carry the test wheel on the right side and some machines (both in the UK and Europe) are fitted with two test wheels. Figure 5.8 SCRIM wheel assembly for a left-side test wheel (wheel in its raised position) What does SCRIM measure? SCRIM measures SFC using an angled wheel. Some machines are also fitted with laser sensors to measure macrotexure. Indicators for the precision of SCRIM measurements are given in Table These have been estimated from data from the 2008 annual comparison trial in the UK involving fourteen machines operating on seven different test surfaces. Reproducibility values may vary in other countries depending on whether the machines have been maintained and compared with the UK fleet. Date: 05/12/2008, Version: 4 42 (89)

43 Table 5.15 Precision indicators for SCRIM measurements Repeatability (100 m average SFC) r < 0.03 Reproducibility (100 m average SFC) R < How does SCRIM work? A freely rotating wheel fitted with a special pneumatic, smooth, rubber tyre, is mounted midmachine in line with the nearside wheel path and set at an angle to the direction of travel of the vehicle. The wheel is lowered on to the road surface under the action of a static vertical load defined by the mass of the wheel assembly, which is able to move freely up and down on vertical linear guides. The force acting along the axle of the test wheel is measured and used to calculate the SFC. On some machines, particularly those operating in the UK, the dynamic vertical load is also simultaneously measured and used in the computation of SFC. The standard test conditions for the SCRIM are listed in Table Table 5.16 Standard test conditions for SCRIM air temperature > 4 C pavement temperature > 5 C (testing season: April till November) and < 50 C pavement status no pollution test wheel smooth tyre 76/508 mm inflated at 0.35 MPa method constant slip ratio from slip angle slip angle 20 static wheel load operating speed theoretical water film thickness length for the mean value wheel path 1960 N Varies from country to country. Typically 50 km/h is used as a reference speed but other speeds are sometimes used in operation for safety reasons with measurements corrected to the reference speed. 0.5 mm Minimum typically 10 m but other options available Normally nearside wheel path or as required Date: 05/12/2008, Version: 4 43 (89)

44 5.9 Skiddometer BV What does the BV-8 look like? The Skiddometer BV-8 was developed by the Statens Väginstitut, National Swedish road research institute of Stockholm to perform routine measurements of friction for long roadsections or point measurements at different speeds to characterise a particular section. The device, which operates on the longitudinal friction principle, is a 2-wheel trailer, with a measuring wheel mounted in the centre of the trailer between the running wheels and is applied to the road surface under a known and controlled vertical load (Figure 5.9). Figure 5.9 Skiddometer BB-8 (example used in Switzerland) What does the Skiddometer BV-8 measure? The Skiddometer BV-8 measures LFC using either a locked wheel or a fixed slip ratio of 14%. Precision indicators for the Skiddometer BV-8 are given in Table 5.17 Table 5.17 Precision indicators for Skiddometer BV-8 measurements Offset of the mean friction value between two test runs (in a time lap shorter than 2 hours) Offset of the standard deviation between the first and the second run on a test section after the calibration Δμ ± 0.03 σ 0.02 Reproducibility of single sample ± How does the Skiddometer BV-8 operate? The test wheel assembly on the Skiddometer BV-8 assembly comprises a spring controlled load system that provides a vertical load of 3500 N which is measured in both static and dynamic conditions. A torque axle, which has a strain gauges system to measure the Date: 05/12/2008, Version: 4 44 (89)

45 frictional force on the wheel, a brake system to lock the wheel are provided, together with an E-clutch that is used to provide the constant slip ratio when this is required. The equipment includes a self-wetting system to wet the road in front of the test wheel. Different water film thicknesses (0-1 mm, usual is 0.5 mm) and different measuring speeds can be selected. Due to the construction of the system, with its test wheel in the middle of the trailer and the measuring speeds used, for safety reasons limits are applied to curve radius and lane width where the machine could encroach into the lane required for oncoming traffic. The standard test conditions for the Skiddometer BV-8 are listed in Table Table 5.18 Standard test conditions for Skiddometer BV-8 air temperature > 10 C pavement temperature > 10 C and < 30 C pavement status test wheel method static wheel load operating speed theoretical water film thickness length for the mean value position of measurement no pollution AIPCR ribbed tyre Dimension 165 R15 with four longitudinal grooves Locked wheel or 14% ±1% slip ratio 3500 N 40, 60, 80 km/h 0,5 mm m usually in one of the wheel path Date: 05/12/2008, Version: 4 45 (89)

46 5.10 SKM What does SKM look like? The SKM was developed in Germany; it was originally based on SCRIM (see Section 5.8) but has some specific modifications related to its use in Germany. The device operates on the transverse friction principle using a special narrow test wheel, similar to a motorcycle wheel, set an angle to the direction of travel, mounted on the side of a tanker lorry. Figure 5.10 illustrates the SKM test-wheel assembly, mounted on the right-hand side of the test vehicle. Figure 5.10 SKM test wheel assembly (this example is on the machine operated by BASt) What does SKM measure? SKM measures SFC using a wheel angle of 20. Indicators for the precision of SKM measurements are given in Table Table 5.19 Precision indicators for SKM measurements Repeatability (100 m average SFC) r = 0.03 Reproducibility (100 m average SFC) R How does SKM work? A freely rotating wheel fitted with a special pneumatic, smooth, rubber tyre, is mounted midmachine in line with the nearside wheel path and set at an angle to the direction of travel of the vehicle. The wheel is lowered on to the road surface under the action of a static vertical load defined by the mass of the wheel assembly, which is able to move freely up and down on vertical linear guides. The force acting along the axle of the test wheel is measured and used to calculate the SFC. Date: 05/12/2008, Version: 4 46 (89)

47 A special device is used to control the water flow appropriate to the operating speed to provide the required water film thickness. The SKM is fitted with systems to measure road, air and water temperature; measurements used in Germany to adjust the reported SFC value. The standard test conditions for the SKM are listed in Table Table 5.20 Standard test conditions for SKM air temperature > 5 C pavement temperature > 5 C and < 50 C (For Road Monitoring and assessment (ZEB) - testing season: May till October) water temperature > 8 C and < 25 C pavement status no pollution test wheel smooth tyre large diameter method constant slip ratio from slip angle 20 º Slip angle 20 ± 1.0 Camber 0 ± 1.0 Static wheel load 1960 N ± 10 N operating speed theoretical water film thickness length for the mean value wheel path 40, 60 or 80 km/h 0.5 mm optional, usually 100 m normally nearside wheel path or as required Date: 05/12/2008, Version: 4 47 (89)

48 5.11 SRM What does the SRM look like? The SRM was developed by Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS). It uses the longitudinal friction principle with the test wheels mounted on the rear of a tanker vehicle to run in each vehicle wheel path (Figure 5.11). Figure 5.11 The SRM showing the test wheels mounted on the rear of the vehicle (this example is from Switzerland) What does SRM measure? The SRM measures LFC and can be operated with a locked wheel, a fixed slip ratio of 15% or in simulated ABS conditions. Precision indicators provide for the SRM are given in Table Table 5.21 Precision indicators for SRM measurements Offset of the mean friction value between two test runs (in a time lap shorter as 2 hours) Offset of the standard deviation between the first and the second run on a test section after the calibration Δμ ± 0.03 σ How does SRM work? The SRM uses the longitudinal friction method to measure skid resistance. A pneumatic load unit controls the load on the measuring wheel. The applied water film thicknesses can be varied between 0 and 3 mm if required, although 0.5 mm is usual. Measuring speeds of 40 km/h, 60 km/h, 80 km/h and 100 km/h can be selected. Date: 05/12/2008, Version: 4 48 (89)

49 Because the measurement vehicle is a 20-tonne truck, limitations are applied to the measuring speed and curve radius, and to road lane width on single-carriageway roads with oncoming traffic. The standard test-conditions for the SRM are listed in Table Table 5.22 Standard test conditions for SRM air-temperature > 10 C pavement-temperature > 10 C and < 30 C pavement status no pollution test wheel AIPCR ribbed tyre Dimension 165 R15 with four longitudinal grooves method Blocked wheel or 15% ±1% slip ratio or ABS wheel load 3500 N operating speed 40, 60, 80 km/h theoretical water film thickness 0.5 mm length for the mean value 20 m position of measurement nearside wheel path right and left Date: 05/12/2008, Version: 4 49 (89)

50 5.12 TRT What does the TRT look like? The TRT was developed by Tatra Kopřivnice in the Czech Republic to perform routine, continuous measurements of friction for runways and long road sections or point measurements at different speeds to characterise a particular section. It is not manufactured under license. The measuring equipment is contained within a specially modified vehicle (Figure 5.12) and operates on the longitudinal friction principle. Figure 5.12 The TRT What does TRT measure? The TRT measures LFC with a fixed slip ratio of 25%. Variable slip between 0 % and 100 % can be used for research purposes. Indicators provided for the precision of TRT measurements are given in Table Table 5.23 Precision indicators for TRT measurements Repeatability r = 0.03 Reproducibility R How does TRT work? TRT uses a hydraulic cylinder in combination with a pressure sensor to apply and control a constant vertical load on the test wheel during a measurement run. The vertical load can be adjusted within the range 500 to 1400 N but 1400 N is normally used. The braking system can be set to provide a fixed slip ratio between 1% and 100% percent, although 25% is normally used for routine measurements. The system can also automatically change the slip ratio in the range 0 % 100 % to measure the friction/slip curve. Date: 05/12/2008, Version: 4 50 (89)

51 A defined theoretical water film thickness can be specified for all measurements, normally 0.5mm and the flow rate required is adjusted to match the specified measuring speed. The standard test-conditions for the TRT are listed in Table Table 5.24 Standard test conditions for TRT air temperature > 4 C pavement temperature > 5 C (testing season: April till November) and < 50 C pavement status no pollution test wheel smooth ASTM-tyre method constant slip ratio, 25 % static vertical force N operating speed 40 km/h to 140 km/h theoretical water film thickness 0.5 mm length for the subsection optional, usually 20 m wheel path left side wheel path Date: 05/12/2008, Version: 4 51 (89)

52 6 Other devices As well as those for which draft CEN Technical Specifications have been prepared (described in Chapter 5), a number of other devices have also been identified that are used in Europe to measure skid resistance on roads or, sometimes, runways. There is less standardised information available about these devices but they are presented here in a similar format to that used for the CEN devices where possible. Devices covered in this Chapter are listed below (in alphabetical order). Most are vehiclebased but those marked with an asterisk are either static on the road or pushed by a pedestrian when making measurements. 1. DFT* 2. IMAG 3. Mu-Meter Mk5 and Mk6 4. Odoliograph 5. OSCAR 6. PFT 7. SALTAR friction meter 8. Skiddometer BV Skiddometer BV SRT (pendulum tester)* 11. T2GO* 12. VTI portable friction tester* 13. VTT friction lorry Date: 05/12/2008, Version: 4 52 (89)

53 6.1 DFT Dynamic Friction Tester The DFT (Figure 6.1) is a static device operating on the rotating-head rubber-slider principle. The device is produced commercially in Japan and sold in Europe. An early version of the device was included in the original PIARC experiment of 1991 and the device is marketed as a means of comparing other friction measuring devices with the International Friction Index. It has not been assessed as part of more-recent harmonisation exercises in Europe. Figure 6.1 Dynamic Friction Tester Date: 05/12/2008, Version: 4 53 (89)

54 6.2 IMAG What does the IMAG look like? IMAG was developed by the French Civil Aviation Technical Division, primarily for use in assessing the friction condition of runways. It consists of a towing vehicle and a two-wheel trailer carrying a centrally-mounted test wheel (Figure 6.2) using the longitudinal friction principle. Figure 6.2 The IMAG What does the IMAG measure? IMAG measures LFC using a fixed slip ratio of 15% How does IMAG operate? IMAG uses the standard PIARC smooth test tyre (as also used by ADHERA). The wheel load is 1500 N.Theoretical water film thickness is 1 mm. The normal operating speed is 65 km/h, but in principle the device can operate at any speed up to 140 km/h. Date: 05/12/2008, Version: 4 54 (89)

55 6.3 Mu-Meter Mk-5 and Mk What does Mu-Meter look like? Mu-Meter is a three-wheel-trailer, shown diagrammatically in Figure 6.3. The device was designed for use on runways and operates on the transverse force principle. Unlike the other side-force devices covered in this report, Mu-meter uses two measuring wheels that are pushed apart by the frictional forces rather than one which is mounted on a vehicle and forced towards it. Figure 6.3 Diagram of the Mu-meter What does Mu-Meter measure? Mu-meter measures SFC based on the aggregated force developed between two linked test wheels, each angled at 7.5 to the direction of travel. Slip ratio is fixed at about 13 %, How does Mu-Meter operate? The Mu-meter is towed along the runway at a constant speed and the transverse force on the two angled wheels tends to force them apart. A sensor mounted between the two test wheel arms measures the tension force developed. The third wheel provides distance measurement and helps to keep the trailer operating in a straight line. A separate water tank is used, either mounted on a separate trailer or fitted within the towing vehicle. Typical measurement speeds range from 20 to 80 km/h. The device was designed for use on airfields which do not generally have the well-defined wheel paths that traffic generates on roads. Because the device effectively measures the average skid resistance along two test lines which are rather far apart compared with a road wheel path, this device is not well-suited for use on trafficked roads. Date: 05/12/2008, Version: 4 55 (89)

56 6.4 Odoliograph What does Odoliograph look like? The Odoliograph was developed in Belgium and is used in Wallonia and Flanders in Belgium. It operates on the transverse force principle but uses a car-size tyre. The test wheel is mounted within a front-wheel drive car, which follows a separate water spray tanker to measure on wet roads (Figure 6.4). Figure 6.4 Odoliographs following water tankers devices from MET (left) and CRRB (right) What does Odoliograph measure? Odoliograph measures SFC with a wheel angle of How does Odoliograph work? Odoliograph has its test wheel mounted within the test vehicle. The test wheel uses the smooth PIARC tyre. The wheel is set to the normal straight line for general travel but is set to the required angle in order to carry out a measurement. The static vertical load is 2700 N and the normal operating speed is 80 km/h. The target water film thickness for the spray bar on the tanker is 0.5 mm. It is understood that a newer version of the Odoliograph has been developed that integrates the test vehicle and tanker and has additional measuring capability but information on this device is not available at the time of writing. Date: 05/12/2008, Version: 4 56 (89)

57 6.5 OSCAR What does OSCAR look like? OSCAR (Figure 6.5) is used in Norway and was developed by the Norsemeter company [17]. The device is used in Norway. The test equipment operates on the longitudinal friction principle using a test axle carrying the measuring wheel, mounted on the rear of a small truck. Figure 6.5 OSCAR (the test wheel and water feed nozzle can be seen on the left) What does OSCAR measure? OSCAR measures LFC at a controlled slip ratio, normally 18%. Other slip ratios between 3% and 75% can also be chosen How does OSCAR work? The test axle, which can be raised when not in use for measurements, carries a test wheel on the left fitted with a standard ASTM E-524 smooth tyre. The right wheel provides speed and distance measurement. A servo braking system is used that automatically adjusts the brake force to maintain the required slip ratio as the speed and friction vary. Water film thickness is 0.5 mm; vertical load is 4826 N. Date: 05/12/2008, Version: 4 57 (89)

58 6.6 PFT (TRL) What does PFT look like? The PFT, which is owned by the Highways Agency and operated by TRL, is a special version of the Dynatest T1290 ASTM E-524 friction trailer. It is used exclusively for research work in the UK. The ASTM trailer is used in a number of States in the USA but this is thought to be the only version currently operating in Europe. The device (Figure 6.6) is a 2-wheel trailer towed by a pick-up truck that carries the water tank and control equipment. Either of the two trailer wheels can be used as the test wheel (but not both simultaneously). The PFT operates on the longitudinal friction principle with a locked wheel. Figure 6.6 The PFT What does the PFT measure? The PFT measures LFC in locked-wheel conditions. It is also possible to measure the peak friction either by inference from the friction-time curve during a standard locked-wheel test or using a mode in which the device makes repeated measurements close to the peak without fully locking the wheel How does PFT work? PFT uses an automatic air-over-hydraulic braking system to lock and release the test wheel. The test brake cycle can be initiated manually, automatically at a fixed distance from a defined reference point or automatically repeated at defined distance intervals. The vertical load is nominally 5000 N, determined by the mass of the trailer acting on the test wheel. A normal lock and release test cycle takes five seconds. Strain gauge bridges on a sensor fitted to the test wheel axle are used to measure the vertical load and drag force and Date: 05/12/2008, Version: 4 58 (89)

59 these are recorded, together with the speed of the test wheel and the vehicle, every 0.01 s throughout the test cycle. The LFC is determined from the average frictional force over a 1-second period after the wheel has locked and been allowed 0.5 seconds to settle. The peak friction is interpolated from the friction/time curve using a five-point moving average. All the friction-time data are recorded to allow separate detailed analysis of each skid if required Test speed depends on the use for the measurements: ranges of speeds from 20 to 130 km/h are used to characterise the friction speed curve for different road surfaces. The length of road sampled depends on the test speed since all tests occur within a fixed time interval. An ASTM E-524 smooth tyre is normally used. Both trailer wheels are equipped as test wheels although in the UK the left wheel is normally used. Water film thickness is controlled by pumps linked to the drive shaft that automatically increase the flow as speed increases. Water depth of 1 mm is normally used in the UK although the ASTM standard of 0.5mm is also an option. Date: 05/12/2008, Version: 4 59 (89)

60 6.7 SALTAR friction meter How does SALTAR work? The SALTAR device [16] was developed primarily for making skid resistance measurements on ice and snow without the addition of water using a patterned tyre. The device uses the longitudinal friction principle and can be fitted to a wide range of vehicles (Figure 6.7). Its symmetrical design allows it to be fitted to measure in either wheel path. Figure 6.7 Griffigkeitsmesseinrichtung SALTAR What does SALTAR measure? SALTAR measures LFC with variable slip ratio from free-rolling to locked wheel How does SALTAR work? A compressed air system applies a vertical load of 700 N (155 lbs) to the test wheel which is fitted with a Bridgestone 8F R X 12 tyre, inflated to 207 kpa (30 psi). An electronic brake system controls the variable-slip braking cycle. Date: 05/12/2008, Version: 4 60 (89)

61 6.8 VTI Skiddometer BV What does BV-12 look like? The VTI Skiddometer BV 12 [13][14] BV12 was developed by VTI in Sweden as part of the EC VERT research project about eight years ago and is used purely for research purposes. The measuring wheel is mounted on the rear of an old (1973) Scania LB80 tanker truck (Figure 6.8) and has a special water flow system designed to deliver the water in front of the test wheel close to it and near-horizontally over a range of water depths. The device can use the longitudinal friction measurement principle, or the transverse friction principle, or use both in a range of combinations. Figure 6.8 The VTI Skiddometer BV What does the BV-12 measure and how does it work? The BV-12 measures the LFC based on the frictional force on the test wheel which can be progressively braked through a sequence of slip ratios from 0 to 100%. The wheel can also be rotated to change the slip angle up to 20 in either direction. A wide range of different combinations of slip ratio can therefore be achieved. An additional feature of the machine is that it can accelerate the wheel to 100% slip ratio. A measurement cycle consists of 10 measurements from 0 up to 100% slip ratio and back to 0% slip ratio. Measuring tyres are of the same dimensions as normal passenger car tyres. Date: 05/12/2008, Version: 4 61 (89)

62 6.9 VTI Skiddometer BV-14 Skiddometer BV-14 (Figure 6.9) is a skid-resistance measurement unit that can be mounted on different cars. It measures skid resistance using the longitudinal principle in both wheel tracks and is designed specifically for measurements on snow and ice surfaces. Measurements are done without wetting at about 17 % slip ratio. Vertical load is 1000 N, consisting of 400 N of kerb weight and 600 N of additional load. A Trelleborg T 49 tyre with a size of is used. Theoretical water film thickness is 0.55 mm for wet tests. Figure 6.9 VTI Skiddometer BV-14 Date: 05/12/2008, Version: 4 62 (89)

63 6.10 SRT Pendulum The Skid Resistance Tester (SRT) pendulum was originally designed by TRL (then the Road Research Laboratory) as an easily-transported static device to make spot-checks on road surfaces (Figure 6.10). It is widely used throughout the world. Figure 6.10 The Pendulum Tester It contains, at the end of its articulated arm, a rubber pad that slides on the surface to be measured. The pendulum arm is locked in a horizontal position and the road surface is thoroughly wetted. The arm is then released and allowed to swing freely, being caught by the operator on the backswing after it has reached its maximum height. A spring mechanism applies the sliding pad onto the surface with a known force. The swept length is kept within predetermined limits. The maximum height of the pendulum rise is identified by a needle positioned in front of a scale directly graduated to show readings of friction coefficient measured by the pendulum or Pendulum Test Value. In fact, PTV results correspond to the work done by the slider rubbing on the road surface. The device was originally developed to provide values similar to those of the LFC from a patterned tyre (of 1950s properties) skidding at 50 km/h. PTV measurements are widely referred to as a measure of LFC and even as a measure of road surface microtexture. In practice the device is very sensitive to factors such as macrotexure and must be used with care. Date: 05/12/2008, Version: 4 63 (89)

64 6.11 T2GO The T2GO portable friction tester (Figure 6.11) is a commercial development available from ASFT Swiss AG (in Switzerland) and ASFT Industries AB (in Sweden). The device is designed to be operated by a pedestrian to make measurements in confined areas, being mainly used for road markings. It is also used in other areas where vehicles could not be used, such as on footways and in shopping malls. It operates on the longitudinal principle with two very small test wheels. The measuring wheel and guidance wheel are of 3 (75 mm) size, and a toothed belt system generates a fixed slip ratio of 20 %. The product information sheet claims a good correlation to the OSCAR device [19]. Figure 6.11 T2GO Date: 05/12/2008, Version: 4 64 (89)

65 6.12 VTI Portable Friction Tester (PFT) The VTI Portable Friction Tester (Figure 6.12) was developed for skid resistance measurements especially for road. It is a slow-speed pedestrian-operated device using longitudianl friction fixed-slip principle. A good correlation to the SRT pendulum test has been shown [18]. Only few devices exist. Figure 6.12 The VTI Portable Friction Tester 6.13 VTT Friction Lorry There is only limited information available for this device. VTT friction lorry is used in Finland and is capable of measuring sideway friction coefficient as well as skid number with a locked wheel. Sideway friction coefficient is measured at a speed of 60 km/h at a inclination angle of 8 degrees and a wheel load of 390 kg. Directly in front of the tyre, 1 l of water per m² is applied to the surface. Measurement sections between 400 and 1000 m are common. Date: 05/12/2008, Version: 4 65 (89)