MEASUREMENT SYSTEM FOR INVESTIGATION OF TYRE-ROAD FRICTION

Transcription

1 MEASUREMENT SYSTEM FOR INVESTIGATION OF TYRE-ROAD FRICTION Janusz POKORSKI 1 Andrzej REŃSKI 2 Hubert SAR 3 Abstract: Measurement system for investigation of tyre-road surface friction developed at Warsaw University of Technology will be presented in this article. The system, equipped with a dynamometrical trailer initially designed for routine and acceptance investigations of road surface friction coefficient, after improvements can determine full friction characteristics. The original construction of the measurement trailer makes it possible to determine exactly both longitudinal and vertical forces acting on tyre, and finally the friction coefficient as a function of slip. Full friction characteristic is obtained immediately after one braking process. The trailer is equipped with a moisturizing system, which makes it possible to carry out tests both on wet and on dry surfaces. Examples of friction characteristics obtained for different tyres on different road surfaces will be presented. The measurement system can be applied in: tyres tests to determine their friction properties, especially in connection with planned EU Directive, which will impose obligation of marking friction properties on tyres, roads construction for routine and acceptance investigation of road surface quality, automotive experts practice for collecting the real tyre-road friction data in the place of traffic accident. Keywords: tyre-road friction, friction characteristic, measurement system, vehicle braking 1. INTRODUCTION The problem of adhesion coefficient (friction force) evaluation in dependence on road surface and automotive tyre is very important in case of demands connected with traffic active safety. Reliable measurement of adhesion coefficient plays a major role in building new highways that have to meet European Union standards. At the same time changeability of adhesion coefficient, for instance as a function of braked wheel slip ratio influences the algorithms of anti-lock brake systems (ABS) and electronic stability program (ESP) actuators. These systems become standard equipment of automobiles and are crucial in traffic safety improvement. The scientific aim of the government project, conducted by the authors, is to design modern mathematical algorithm and acquisition-measurement system responsible for investigating anti-slip properties of road surfaces and automotive tyres. Precise description and modelling of dynamics in contact area between tyre and road [5]; [6]; [7], were needed to identify physical phenomena. Knowledge of adhesion characteristics plays significant role in design and improvement of modern tyres and modern road surface materials and technologies [8]. Institute of Vehicles of Warsaw University of Technology (IPPW) in cooperation with Road and Bridge Research Institute (IBDM) has meaningful achievements in designing and exploiting measurement systems for road surface adhesion researches [2]; [3]. Ten measurement units named SRT-3 are used in investigating road surface adhesion on Polish national roads [2]. Experimental measurement unit owned by IBDM, substantively supported by IPPW scientists, is taking part in European program of improving methods of road measurements [1]. Increasing demands of European standards force the need to search for new solutions of measurement systems. Experience gained through SRT-3 units exploitation made it possible to indicate direction of adhesion measurement units development. For appropriate implementation of these changes further modelling researches are necessary. They concern the phenomena of tyre and road adhesion, including the properties of road measurements. One of the project s aims was to design and make the new generation of dynamometer trailer named SRT-4, used to conduct measurements connected with longitudinal adhesion characteristics, both for different tyres and different road surfaces. In further part of the article the new measurement system and measurement results are presented as the introduction to evaluate traffic safety on Polish roads. 1 MSc., Ing.: Janusz.Pokorski@simr.pw.edu.pl, Assos. Prof., Ing., PhD.: ARenski@simr.pw.edu.pl, PhD., Ing.: HubSar@wp.pl,

2 2. PRESENTATION OF SRT-4 MEASUREMENT SYSTEM In Fig. 1 the new SRT-4 measurement system is shown. It is able to investigate both road surface adhesion and automotive tyre anti-slip properties. The features, which differentiate SRT-4 system from the previous SRT-3 are: Figure 1 Mobile measurement system SRT-4 towing vehicle Mercedes Sprinter, dynamometer trailer optional rim sizes from 14 to 17 inches of measurement wheel, extended acquisition-measurement system PCI-EPP, GPS system aiding the location of measurement points, acquisition-measurement software, making it possible to obtain full characteristics of adhesion (for the whole range of longitudinal slip ratio) µ(s). The kinematics of SRT-4 dynamometer trailer remained unchanged in comparison to the previous SRT-3. It was worked out in the 60s by Dr. Stanisław Kowalski and Prof. Edward Habich (Fig. 2). The specific property of this unit is double-wishbone measurement wheel suspension with doublelever coupling-balancing system. This system makes it possible to measure friction force between measurement wheel and road surface in different road conditions, both when the wheel is rolled free and when the wheel rolls with longitudinal slip ratio varying between 0 and 1 or when the wheel is completely locked. W (m 1 ) F C3 F C1 b a v=const h b a r (m 2 ) T a=h F C2 =T Figure 2 Kinematic scheme of SRT-3 and SRT-4 dynamometer trailers

3 Two measurement paths (two sensors) are connected with lever system. The first of them (F C1 ) measures braking torque W h on disc brake, the second (F C2 ) measures friction force T between tyre and road. In case when the measurement wheel is completely locked, both sensors measure the same force (in appropriate scale), which makes it possible to check under road conditions whether the whole system is working correctly. Special measurement path F C3 was added by Dr. Bogumił Szwabik and Janusz Pokorski, MSc. in the 90s to measure wheel dynamic load (normal force F z ) changes. This measurement path makes it possible to include correction of wheel normal load during measurement, which influences the measurement results on rough roads. In Fig. 3 are shown the curves of braking process of dynamometer trailer measurement wheel for steady-state velocity v s = 60 km/h. Figure 3 Curves of dynamometer Below is presented the meaning of coloured lines: green line braked wheel circumferential velocity v k, red line braking torque W h (F C1 sensor), blue line friction force T (F C2 sensor), grey line normal force F z loading measurement wheel, black (upper) range of time, in which are determined average, values of adhesion coefficients µ M and µ T, black (lower) range of time, in which water is poured under measurement wheel. Averaged values of tyre-to-road adhesion coefficients are calculated using the following equations: where: µ M adhesion coefficient calculated on the basis of braking force W, µ T coefficient calculated on the basis of friction force T. µ M = W/F z, µ T = T/F z (1) Theoretically, in case of completely locked wheel both values should be equal. In practice it is assumed that maximum difference should not be higher than µ = If the difference is higher, the measurement is not accepted. It is important that measurement systems used in SRT-3 and STR-4 units make it possible to improve compliance between results from these above mentioned different measurement paths. Fig. 4 presents full adhesion characteristics µ(s) (red curves), each obtained from single attempt of braking on one road section. It is the original property of this system that practically no other measurement system offers. It is possible through the kinematic scheme of the dynamometer trailer. Blue curve indicates average adhesion characteristic for the road section. On the basis of each adhesion characteristic presented in Fig. 4, two adhesion coefficients can be obtained: peak adhesion coefficient, lockup adhesion coefficient (slip s = 1), automatically calculated by SRT-4 system.

4 Figure 4 Differentiation of full adhesion characteristics µ(s) for one road section In further part of the article results of lockup adhesion coefficient measurements will be presented. Figure 5 Changeability of adhesion coefficient on road test section (Fig. 4); µ M = 0.36 Measurement units applied to investigate adhesion coefficients on national roads use the system of reference points that help to find locations of the measurement points. This system is available only outside the cities. If there are no reference points on the road, SRT measurement system uses global positioning system GPS. Fig. 5 illustrates the changeability of adhesion coefficient on road test section in Warsaw and Fig. 6 indicates the points where the measurement was prepared. Figure 6 Location of measurement points using GPS system

5 3. ADHESION COEFFICIENT AND TECHNOLOGY OF THE UPPER LAYER OF ROAD SURFACE To illustrate selected results of the measurements done with the use of the SRT-4 system, in further chapters the differences between adhesion coefficients on different road surfaces and on different road sections are presented. Full measurement results are available in the Institute of Vehicles of Warsaw University of Technology. In Table 1 values of adhesion coefficients for several kinds of the upper layer of road surface technology are shown. Table 1 Adhesion coefficients µ M for different road surfaces No of Adhesion Type od road surface section coefficient 1 sintered bauxites single strengthened road 0.71 surface 3 SMA new road surface double strengthened road 0.53 surface 5 cement concrete SMA SMA old road surface 0.33 It is very important to underline that all the results presented in the article refer only to the measurements on wet road surface, excluding the results in Table 2. If not indicated, the velocity during measurement was v s =60 km/h. The research was conducted in September, Table 2 Road surface on the bridge wet and dry adhesion measurement µ M (on wet) µ M (on dry) 0.37 CL 0.72 CL The highest values of adhesion coefficients were observed for the upper layer made from sintered bauxites. Wet adhesion coefficients are up to µ M = 0.86 and are higher than the values of adhesion coefficients measured on dry SMA road upper layer (compare Table 2). Technology of sintered bauxites is very rare at the moment on national roads in Poland. Relatively high values of adhesion coefficient can be observed for road surface strengthening in the range between µ M = These road surfaces are used only temporarily and practically only on local roads. Widely used on Polish national roads SMA technology is characterized by a wide range of adhesion coefficient values µ M = Cement concrete road surfaces have relatively good adhesion (µ M = 0.50) provided that the technology of grooving is implemented. It is important that road adhesion coefficient depends on many factors such as: exploitation time, traffic intensity, road lane [8], season and sliding velocity during braking. Table 3 Adhesion coefficients µ M for SMA road surface Velocity [km/h] left lane ->Warsaw right lane -> Białystok In Table 3 are presented more detailed results of the measurement on new road surface made in SMA technology. Fig. 7 presents the so-called velocity characteristic for analyzed section of the road.

6 Figure 7 Velocity adhesion characteristic (Table 3): red line left lane, blue line right lane, black line minimum values for national roads in Poland Values of adhesion coefficient vary in dependence on the direction of the motion [4]. If the sliding velocity v s increases, then the adhesion coefficient considerably decreases. 4. CONCLUSION Results of measured values of adhesion coefficient show its differentiation in dependence on the location of the measurement, traffic conditions, road surface exploitation time and many others. Because of this, estimation of road adhesion coefficient for the needs of accidents reconstruction or for expert witness opinions should be taken with maximum attention. The most appropriate solution seems to be the measurement of adhesion coefficient in support of SRT-4 system, equipped with the tyre taken from the vehicle that took part in collision or traffic accident. Simulating non-steady-state vehicle motion, it is necessary to include full adhesion characteristic µ(s), that can be obtained on the basis of the measurements in the place of the traffic accident or collision. The results presented above were realized as the government research project: KBN NR N /1417. REFERENCES 1. ANTLE Ch.E., WAMBOLD J.C., HENRY J.J., International PIARC Experiment to Compare and Harmonize Texture and Skid_Resistance Measurement, PIARC Technical Commitee on Surface Characteristics C1, SZWABIK B., MECHOWSKI T., POKORSKI J.: Effective Method of Determining Dynamic Characteristics of Road Pavement Friction, 2nd Eurasphalt & Eurobitume Congress Barcelona 2000, Book I, pp POKORSKI J., SZWABIK B., Experimental and calculation friction characteristics of vehicle tyres and surface pavements, Proceedings of the Institute of Vehicles, Warsaw University of Technology, 3(42)/2001, pp POKORSKI J., SZWABIK B.: Variability of friction coefficient in cross and longitudinal section of a road, Proceedings of the Institute of Vehicles, Warsaw University of Technology, 1(40)/2001, pp GRZESIKIEWICZ W., POKORSKI J., SZWABIK B.: Modelling and experimental research of braked wheel adhesion, Przegląd Mechaniczny, Rok Wyd. LXII, Nr 10/3, 2003, pp GRZESIKIEWICZ W., POKORSKI J.: Modeling of Tangential Interaction between Braking Wheel and Road Pavement in Aspect of Friction Coefficient Measurement, X International Conference Durable and safe road pavements, Warsaw, IBDiM, 2004, pp GRZESIKIEWICZ W., POKORSKI J.: Modelling and digital simulation of measurement equipment for evaluating adhesion characteristics of automotive tyres and road surfaces, X Międzynarodowa Konferencja Naukowa Badania Symulacyjne w Technice Samochodowej, Kazimierz Dolny, POKORSKI J., REŃSKI A., SAR H.: Investigation of Tyre-To-Road Adhesion in Dynamic Braking Conditions, Machine Dynamics Research, 2010, Vol. 34, No. 2, pp