Keywords: wheelchair base frames, frontal-impact crashworthiness, crash testing, wheelchair transportation safety, surrogate seating system

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Garey Boone
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1 Patterns of Occupied Wheelchair Frame Response in Forward-Facing Frontal-Impact Sled Tests Julia E. Samorezov, Miriam A. Manary, Monika M. Skowronska, Gina E. Bertocci*, and Lawrence W. Schneider University of Michigan Transportation Research Institute *University of Louisville ABSTRACT The frame response of occupied forward-facing wheelchairs secured by a surrogate four-point strap-type tiedown was characterized using data from 575 WC19 frontal-impact sled tests. Powerbase, manual, and stroller-type wheelchairs with fabric or planar seats and seatbacks were tested. Loading mechanisms that caused damage to the base frame were identified and the effect of seating system type was considered. The most frequent types of wheelchair frame damage were failure or deformation of the rear securement brackets or frame members to which these are attached, main frame members, front caster assembly, footrests, seat tilt and seatback recline mechanisms, and seatback canes. Wheelchairs with fabric seats were more likely to experience frame damage, but there was no relationship between seatback type and frame damage. There was no evidence of a relationship between base frame damage and wheelchair rotation. The patterns of frame damage and the relationships between damage and seating system type will be used to guide the design of a reusable surrogate wheelchair seating system (SWCSS) for use in frontal-impact tests of wheelchair base frames independent of commercial seating systems. Keywords: wheelchair base frames, frontal-impact crashworthiness, crash testing, wheelchair transportation safety, surrogate seating system BACKGROUND Section 19 of ANSI/RESNA WC/Vol.1, Wheelchairs Used as Seats in Motor Vehicles (WC19), describes test methods and performance criteria for evaluating frontal-impact performance of an occupied forward-facing wheelchair secured by four-point, strap-type tiedowns. However, only an entire wheelchair, consisting of a base frame and seating system, can be tested to WC19 (1). WC20, Seating Devices for Use in Motor Vehicles, is a new standard expected to be published in 2009 that will allow for frontal-impact testing of wheelchair seating systems independent of base frames by installing the seating system on a surrogate wheelchair base (SWCB) (2). Independent crashworthiness testing of seating systems and base frames is important because wheelchair users needs may require a unique hardware combination from different manufacturers and it is not practical to crash test every possible seat/frame pairing. Independent testing of a wheelchair base or frame will require the use of a surrogate wheelchair seating system (SWCSS) that will result in nominally worst-case loading of the wheelchair base on which it is installed. Development of the SWCSS requires an understanding of how wheelchair base frames respond to WC19-type crash testing, and the role that the wheelchair seating system plays in causing or preventing base frame damage. OBJECTIVE This study documents wheelchair frame impact responses and post-test conditions of wheelchair base frames subjected to WC19 frontal-impact sled tests. These results can be used to determine relationships between wheelchair and seating system characteristics and base frame damage.

2 METHODS Data from WC19 frontal-impact tests of forward-facing adult (N=412) and pediatric (N=163) wheelchairs conducted on the impact sled facility at the University of Michigan Transportation Research Institute (UMTRI) between January 1998 and May 2007 were reviewed and compiled in a database. These tests were conducted using the 48-kph, 20-g crash pulse specified in WC19 and constitute a wide sample of wheelchairs including models from almost every U.S. manufacturer. Included in this analysis were 165 manual wheelchairs (28.7%), 182 power wheelchairs (31.7%), and 228 strollers (39.7%) with both fabric or planar seats and seatbacks. Wheelchair and seating system type were determined from test photographs. For each test, the wheelchair size, type and mass, the anthropomorphic test device (ATD, a.k.a. crash test dummy) size, the type of belt restraint system, the ATD and wheelchair peak excursions, peak loads in the rear tiedowns and occupant restraint belts, and test outcomes in accordance with WC19 performance criteria were documented. Pre- and post-test photographs, along with descriptions from the UMTRI test reports, were used to identify the location and nature of frame damage. When available, side-view high-speed videos were reviewed to determine loading mechanisms based on wheelchair and ATD kinematics. The wheelchair base frame was defined as all wheelchair components excluding the seat, seatback, and the seat/seatback attachment hardware. Damage to the wheelchair base frame was grouped into nine categories described in Table 1. Categories A-F were considered to be seating-system-dependent outcomes because in these cases seating type could affect how loads are distributed to base frame components. Categories G-I were considered to be seating-system-independent outcomes because it was thought that seating system type was unlikely to have influenced loading and damage of the base frame. Right sideview wheelchair rotation was categorized as clockwise rotation (forward pitching), counterclockwise rotation (rearward pitching), no rotation, or not applicable (wheelchair release because the rear securement brackets failed or other catastrophic failure). RESULTS Table 2 and Figure 1 show the frequency of different categories of frame damage. Components of the base frame fractured or deformed in 183 tests (31.8%). Of these, 160 (87.4%) had damage or deformation significant enough to constitute failure by WC19 performance criteria. In many tests, multiple damaged frame components were noted and counted in more than one category. The most common outcome was deformation/fracture of the rear securement brackets or the adjacent frame member to which securement brackets attach (29%), which occurred primarily on power wheelchairs. Not coincidentally these are the heaviest wheelchairs with an average mass of kg (+/- 27.3SD), compared with 23.4 kg (+/- 6.2 SD) for manual wheelchairs and 20.7 kg (+/- 6.0 SD) for strollers. Other failures that occurred almost exclusively on power wheelchairs included deformation of the seat support member or of the rear wheel assembly. Front caster assembly deformation (16.4%) and footrest failures (16.9%) were also common types of frame damage. In 43 the tests, moderate front caster deformation was noted but did not constitute failure as defined by WC19. Footrest failures ranged from detached footplates to tubing fractures to complete detachment of one or both footrests. Contact with the sled platform or bolster that occurred in 14.8 tests with footrest failure may have contributed to failure, but loading of the footrests by the ATD feet was considered to be the primary cause.

$Main Frame Member Fracture fracture of the seat rails, any other lateral and longitudinal$ $Front caster assembly deformation bending and/or fracture of the caster forks/shafts or$ wheels, and/or the vertical frame members to which the front casters are attached C.

wheels, and/or the vertical frame members to which the front casters are attached C.

3 Table 1. Categories used to classify wheelchair base frame damage A-F seating-system-dependent, G-I seating-system-independent Category Description of Outcome Example Photograph A. Main Frame Member Fracture fracture of the seat rails, any other lateral and longitudinal frame members, or the crossbars of folding manual wheelchairs. B. Front caster assembly deformation bending and/or fracture of the caster forks/shafts or wheels, and/or the vertical frame members to which the front casters are attached C. Tilt/recline locking mechanism failure slipping or failure of manual seat and/or seatback tilt or recline locking mechanism D. Seatback cane fracture complete or partial fracture of one or both seatback canes

E. Deformation of seat support member severe deformation and/or failure of the

complete separation of the seat frame from the base F.

resulting in longitudinal spreading of the front and rear wheels without frame

$Securement bracket or bracket attachment failure fracture or detachment of one$ $Footrest failure fracture or detachment of one or both footrests or footplates$

4 E. Deformation of seat support member severe deformation and/or failure of the components connecting the seat to the wheelchair base, sometimes leading to complete separation of the seat frame from the base F. Severe downward deformation of frame downward displacement of the base resulting in longitudinal spreading of the front and rear wheels without frame failure G. Securement bracket or bracket attachment failure fracture or detachment of one or both rear securement-point brackets, or of the frame member to which the brackets are attached H. Footrest failure fracture or detachment of one or both footrests or footplates I. Damage to the rear wheel assembly fracture or significant deformation of the rear wheels or wheel hubs, or of the frame members to which the rear wheels are attached

5 The most common of the seating-dependent outcomes was a fracture of one or more components in the main base frame (21.3%) and most of these fractures occurred in seat rails or other horizontal members around the seat. Failure of a tilt or tilt-and-recline locking mechanism was also common, particularly for stroller-type wheelchairs, comprising 12.6 all frame failures. Fracture of the seatback canes (7.7 all frame damage) occurred at similar frequencies in all wheelchair types and occurred either during forward rotation of the seatback while the base of the wheelchair was held stationary, or, more commonly, by ATD rebound loading. Severe downward deformation (splay) of the main wheelchair frame (5.5 all frame damage) occurred mostly in strollers. Seating-dependent Seating-independent Failure Mechanism A. Main Frame Member Fracture B. Front Caster Deformation C. Tilt/Recline Locking Mechanism Failure D. Seatback Cane Fracture E. Deformation of Seat Support F. Severe Downward Deformation Seating-System- Dependent Total G. Securement Bracket or Attachment Failure N Table 2. Frequency of Various Frame Outcomes Manual Power Stroller Total all all all frame frame N frame N frame N tests tests tests failure failures failures failures s all tests % 12.1% 6 8.7% 3.3% % 5.7% % 6.8% % 6.7% 3 4.3% 1.6% % 7.0% % 5.2% 3 7.3% 1.8% 4 5.8% 2.2% % 7.0% % 4.0% 3 7.3% 1.8% 5 7.2% 2.7% 6 8.3% 2.6% % 2.4% 0 0.0% 0.0% % 6.0% 2 2.8% 0.9% % 2.3% 0 0.0% 0.0% 1 1.4% 0.5% % 3.9% % 1.7% % 19.4% % 15.9% % 23.2% % 19.8% % 4.2% % 18.7% % 5.3% % 9.2% H. Footrest Failure % 4.2% % 4.9% % 6.6% % 5.4% I. Deformation of Rear Wheel Assembly Seating-System- Independent Total 1 2.4% 0.6% % 6.0% 0 0.0% 0.0% % 2.1% % 10.9% % 28.6% % 11.4% % 16.2% Note: Since a wheelchair can have multiple failures, numbers do not add to 100%

6 Figure 1. Frequency of frame failure at various locations. A-F seating-system-dependant, G-I seating-system-independent C. Tilt/Recline Locking Mechanism Failure N=23 (12.6%) D. Seatback Cane Fracture N=14 (7.7%) F. Severe Downward Deformation of Frame N=10 (5.5%) A. Main Frame Member Fracture N=39 (21.3%) E. Deformation of Seat Support Member N=13 (7.1%) G. Securement Bracket or Bracket Attachment Failure N=53 (29%) H. Footrest Failure N=31 (16.9%) I. Damage to Rear Wheel Assembly N=12 (6.6%) B. Front Caster Assembly Deformation N=30 (16.4%) Seating-system-dependent loading patterns that caused fracture or deformation of wheelchair base frame components are shown in Table 3. Some of these wheelchair frames exhibited deformation caused by more than one loading type, and thus were counted in more than one category. The type of loading is unknown for some wheelchairs because videos were unavailable, but pre- and post-test photos indicate that the seating-dependent damage of the wheelchair frame did occur. The majority (55.8%) of seating-dependent damage occurred because of the initial forward and downward loading of the ATD through the seat. However, rebound loading of the ATD accounted for 38.7 such damage. Frame outcomes associated with forward/downward loading were often in the categories of main frame fracture, failure of the tilt/recline mechanism, and severe downward deformation. Frame outcomes associated with ATD rebound loading included mostly the categories of main frame fracture, seatback cane fracture, and failure of the tilt/recline mechanism. On rare occasions, the shear force from seating system moving forward while the base frame was effectively secured by the rear tiedowns also caused frame damage.

7 Table 3. Loading Patterns for Seating-dependent Frame Outcomes Loading type Forward/downward loading through seat Loading from ATD rebound Shear force between seat and base # of wheelchairs Other / Unknown 12 8 Wheelchairs with fabric seats exhibited base frame damage more often than wheelchairs with planar seats, especially under ATD rebound loading. This suggests that a fabric seat could provide worst-case loading of base frame components. Table 4 compares the occurrence of seating-dependent frame outcomes in wheelchairs with planar and fabric seats. Fabric Seat (n = 111) Planar Seat (n = 462) Table 4. Occurrence of Seating-System-Dependent Frame Outcomes by Seat Type Seating-dependent Frame Damage (categories A-F) 27.0% (n=30) 18.2% (n=84) Seating-dependent Frame Damage due to Forward/ Downward Loading 14.4% (n=16) 10.4% (n=48) Seating-dependent Frame Damage due to Rebound Loading 10.8% (n=12) 5.2% (n=24) As shown in Table 5, power wheelchairs rotated most frequently of all WC types (50.3 %), followed by stroller-type (24.2%) and manual wheelchairs (14%). Power wheelchairs most often rotated counterclockwise, while manual and stroller-type wheelchairs were more likely to rotate clockwise. Of manual wheelchairs, those with fabric seats and seatbacks were more likely to rotate than those with planar seats and seatbacks; this rotation was usually clockwise. There was no conclusive evidence to suggest wheelchair rotation as a cause of base frame damage. In fact, review of high-speed test video showed that in many instances (for example front caster assembly deformation) failure of wheelchair components occurred before wheelchair rotation, suggesting that failure can actually cause rotation. Table 5. Relationship Between Wheelchair Type and Rotation During Impact Loading Manual Power Stroller Total count count count count manual power stroller all WC Rotated CW Rotated CCW No Rotation All Rotated

8 Table 6. Occurrence of Wheelchair Rotation Based on Wheelchair and Wheelchair Seat Type Manual Power Stroller Total Seat Seat back Seat Seat back Seat Seat back Seat Seat back Clockwise Counterclockwise All Rotation Rotation Rotation N Percent (%) N Percent (%) N Percent (%) planar (N=140) % 2 1.4% % fabric (N=24) % 1 4.2% % planar (N=93) 3 3.2% 1 1.1% 4 4.3% fabric (N=61) % 1 1.6% % planar (N=164) % % % fabric (N=17) 0 0.0% % % planar (N=93) % % % fabric (N=78) % % % planar (N=158) % 8 5.1% % fabric (N=70) % % planar (N=149) % 8 5.4% % fabric (N=78) % 0 0.0% % planar (N=462) % % % fabric (N=111) % 6 5.4% % planar (N=335) % % % fabric (N=217) % % % DISCUSSION The results of this study have several implications for surrogate seating system design. The SWCSS cannot strengthen any components and must allow full testing of all base frame components. This includes the seat tilt or seatback recline mechanisms; the design cannot lock the angle of the seat or seatback in order to fully test these locking mechanisms. To test all other components, the SWCSS must both maximize forward/downward loading through the seat, and maximize rebound loading on the seatback. Because fabric seating may introduce worst case loading of the wheelchair frame, but most specialized seating systems will be planar or customized, two tests may be necessary to test a wheelchair base. The first of these would be with a fabric seat to maximize rebound and inward frame loading, the second with a planar seat to better simulate the seating systems most often prescribed. Wheelchair rotation is not related to base frame damage, and shows no clear relationship with any one specific failure mode. Although data on seating-system-independent damage are less useful to the SWCSS development process, studying these frame response patterns yields some insight into designing WC19 complaint wheelchairs. The most common of the seating-independent outcomes, securement bracket failure, can easily be addressed by strengthening securement brackets and their attachment hardware. Others, such as footrest failure and front caster deformation also highlight issues of interest to wheelchair manufacturers. LIMITATIONS This study used a retrospective analysis of data gathered from test reports, before and after photographs, and high-speed test videos; the wheelchairs tested were not available for further

9 inspection. Also, seating system type was determined from photographs, which could introduce classification errors. CONCLUSIONS Compilation and analysis of data from 575 frontal-impact sled tests of commercial wheelchairs revealed clear patterns in wheelchair frame response. Wheelchair frame failures were divided into seating-dependent and seating-independent outcomes. The most common type of frame damage was failure of a rear securement bracket, occurring in 29 all tests with frame damage. The most common seating-dependent outcome was the fracture of a main frame member, which occurred in 21.3 all tests with frame damage. Forward and downward loading through the wheelchair seat and rebound loading of the ATD against the seatback were found to be the primary base frame loading mechanisms. Wheelchairs with fabric seats experienced frame damage more often than wheelchairs with planar seats, especially damage caused by rebound loading (10.6 wheelchairs with a fabric seat failed in this way, compared to only 5.2 wheelchairs with a planar seat) and may provide worst case loading of the wheelchair frame. A SWCSS is necessary to test wheelchair bases or frames independent of seating systems, and should approximate worst-case loading by a commercial seating system. The SWCSS must not strengthen the frame or divert loading from any structural components of the wheelchair, and must maximize both rebound and forward/downward loading. ACKNOWLEDGMENTS The RERC on Wheelchair Transportation Safety is funded by the National Institute on Disability and Rehabilitation Research (NIDRR), Department of Education, Washington, D.C., Grant # H133E The opinions expressed herein are those of the authors and do not necessarily reflect NIDRR opinions. REFERENCES 1. Section 19, ANSI/RESNA Wheelchairs/Volume 1(2000). ANSI/RESNA WC/Vol.1, Section 19:Wheelchair Used as Seat in Motor Vehicles. 2. ANSI/RESNA Committee on Wheelchairs and Transportation (2008). RESNA WCT Section 20: Seating Systems Used in Motor Vehicles. COWHAT preballot draft standard. Author contact information: Miriam Manary, UMTRI, mmanary@umich.edu.

INTERNATIONAL STANDARD

INTERNATIONAL STANDARD ISO 10542-1 Second edition 2012-10-01 Technical systems and aids for disabled or handicapped persons Wheelchair tiedown and occupant-restraint systems Part 1: Requirements and test