THE AFFECTS OF SECUREMENT POINT LOCATION ON WHEELCHAIR CRASH RESPONSE

Gina E. Bertocci, MSME, PE, Douglas A. Hobson, PhD Rehabilitation Technology Program University of Pittsburgh

ABSTRACT

ADA has led to an increase in disabled travelers, many of whom are required to use their wheelchairs as vehicle seats. Proper securement of the wheelchair is crucial to the safety of these wheelchair users in a crash. To promote proper wheelchair securement, the ANSI/RESNA Transportable Wheelchair Standard currently under development will require that all transportable wheelchairs be equipped with four securement points, compatible with belt-type tiedowns. Through computer simulations, the location of these securement points has been found to influence the response and loadings of a wheelchair in a frontal crash. Accordingly, placement of securement point locations by wheelchair manufacturers can serve as a strategy to control crash response, and may eliminate the failure of critical wheelchair components in a crash.

BACKGROUND

With the advent of ADA many wheelchair users are seeking both public and private transportation to access employment, particpate in leisure activities and carry out the needs of daily living. In doing so a large number of these travelers are unable to transfer from their wheelchairs to a vehicle seat and are therefore required to remain in their wheelchairs during transport. By using a wheelchair as a vehicle seat, the wheelchair is required to meet new performance capabilities for which it may not have been originally designed. In an effort to establish design and performance criteria for wheelchairs used in transportation, ANSI/RESNA is currently developing a Transportable Wheelchair Standard. As an initial requirement, the ANSI/RESNA standards committee intends to require that transportable wheelchairs must be equipped with four securement points compatible with belt type tiedown securement systems. Currently vehicle operators must select the points on the wheelchair which they believe will provide adequate and safe securement during normal driving manuevers as well as in a crash. Compliance with the ANSI/RESNA Standard will eliminate the "guessÐwork" associated with this process by providing four easily accessible securement points for repeated use. Since these securement points will become a permanent part of the wheelchair, it is prudent that insight to selecting the placement of these points be provided. To assist wheelchair manufacturers in establishing securement point placement, this study investigates the affects of securement point location on wheelchair response and loadings during a crash. Consequential tiedown loadings as a result of securement point location are also examined.

RESEARCH QUESTION

This study seeks to determine the influence of wheelchair securement point location on wheelchair crash response, as well as wheelchair and tiedown loadings during a crash event.

METHODS

To evaluate the effects of rear securement point location on the crash response of a wheelchair, a lumped mass model of the SAE surrogate wheelchair (1) with a 50th percentile male Hybrid III anthropomorphic test dummy was employed. The SAE surrogate wheelchair, a structurally enhanced wheelchair, was constructed for the purposes of repeated sled testing to evaluate the performance of wheelchair tiedowns and occupant restraints (WTORS). Wheelchair securement in the model is accomplished using a four point tiedown system, while the occupant is restrained with an integrated lap belt and vehicle mounted shoulder belt. This model (2), developed for research associated with the SAE J2249 Wheelchair Tiedowns and Occupant Restraint Standard (3), uses the Articulated Total Body/Crash Victim Simulator code (Wright-Patterson Air Force Base). Validation of the SAE surrogate wheelchair model has been conducted through interlaboratory sled impact testing (4). The model subjects the vehicle transporting a forward oriented wheelchair and occupant, to a 20g, 30 mph frontal crash. Using the wheelchair/occupant model described above, simulations were run with the rear securement point height at 7.5" above the wheelchair center of gravity, level with the center of gravity (CG), and 7.5" below the center of gravity. Although these conditions may represent the extreme of securement point locations for actual installations, they provide insight as to the trends which occur in a crash. For each case, response of the wheelchair and it's occupant are recorded, while front and rear wheel forces, front wheel vertical displacement, and tiedown forces are evaluated.

RESULTS

A pictorial representation of each of the three simulations with varying rear securement point height is shown in Figure 1 at 90 msec into the crash event. Case A which secures the wheelchair above the wheelchair CG tends to rotate the wheelchair and occupant counterclockwise, or rearward. Conversely, Case C which places the securement below the CG tends to rotate the wheelchair forward. Locating the securement point at the same height as the vertical wheelchair CG as depicted by Case B, provides the most controlled wheelchair response to a crash. Peak wheelchair excursions and loads associated with each of the simulations are provided in Table 1 through a time of 120 msec. Plots of each evaluated

Figure 1. Wheelchair-Occupant Response for Varying Rear Securement Point Heights

parameter are shown in Figures 2 through 4. Figure 2 shows that maximum rear wheel forces decrease as the securement point is moved from a point 7.5"above the wheelchair CG, to a point 7.5" below the CG. This type of relationship is as expected since as shown by Figure 1, the wheelchair and occupant rotate about the rear wheel when the securement point is above the CG (Case A), leading to an excessive peak rear wheel loading of 7399 lbs. Front wheel loading is negligible in Case A since the wheel is lifted off of the floorboard during the crash. Moving the securement point to below the CG (Case C) causes the wheelchair to rotate forward on the front wheel, reducing the maximum rear wheel loading to only 118 lbs. However, this relief of rear wheel loading upon forward rotation of the wheelchair is accompanied by an increased front wheel peak loading of 2022 lbs. The motion of the wheelchair during the crash event can be captured by evaluating the wheelchair front wheel excursion. Figure 3 shows that the front wheel hub rises 4.8" above it's initial position when the securement point is above the wheelchair CG as in Case A. This result occurs since rearward, or counterclockwise rotation of the wheelchair tends to lift the front wheel upward. Conversly, the forward rotation associated with Case C which locates the securement point below the CG, causes the front wheel to be depressed into the floorboard. This deformation produces a downward deflection of the front wheel hub equal to 1.7". The controlled

Table 1. Affects of Varying Wheelchair (WC) Tiedown Securement Points (t=0 to 120 msec)

Figure 2. Front and Rear Wheel Forces vs. Difference Between Rear Securement Point and Wheelchair Vertical CG

response of the wheelchair under conditions where the securement point is at the same level as the CG, can be confirmed through the minimal front wheel downward excursion of just 1.2". Tiedown forces are also affected by location of the securement point as shown by Table 1 and Figure 4. Placement of the securement points above the wheelchair CG produces the largest tiedown forces of the three scenarios. In Case A, maximum front tiedown forces are 6170 lbs., while rear tiedowns experience a peak loading of 7000 lbs. (Since an integrated lap belt adds the load of the occupant to the wheelchair tiedowns, resulting tiedown loads are greater than in a scenario where an independent lap belt is used.) This relatively high loading placed upon the front tiedowns is due to the rearward rotation of the wheelchair which must be countered by the front tiedowns. Relocating the securement point to below the CG decreases the maximum force imposed upon both the front and rear tiedowns. In this scenario (Case C), front tiedown loads are negligible and rear tiedown loads are reduced to 4513 lbs. This decrease

Figure 3. Front Wheel Vertical Displacement vs. Difference Between Rear Securement Point and Wheelchair Vertical CG

Figure 4. Front and Rear Tiedown Forces vs. Difference Between Rear Securement Point and Wheelchair Vertical CG

in maximum rear tiedown loading is close to a 36% reduction when compared to Case A maximum loads.

DISCUSSION

Results of this study indicate that rear securement point location relative to wheelchair CG can influence loads experienced by both the wheelchair and securement system in a crash. Selection of securement point location on the wheelchair is critical since differences in wheelchair component loadings may equate to failure in a crash. Wheelchair crash component forces, such as those experienced by the front and rear wheels, vary greatly depending upon rear securement point location. For example, maximum front wheel forces are shown to increase by more than 1600% with moving rear securement points from 7.5" above the CG, to 7.5" below the CG. Failure of components critical to wheelchair stability in a crash, such as casters, can lead to excessive occupant excursion. It is these excessive excursions which often result in "secondary impact", or occupant impact with vehicle surfaces, that cause serious injury. Rear securement point placement has also been found to influence the dynamic crash response of the wheelchair. With rear securement at the level of the vertical CG (Case B), wheelchair response to the crash is relatively controlled as compared to Cases A and C which secure the wheelchair above or below the CG. The controlled response in Case B can be accounted for since the rear tiedown more directly opposes the forward crash forces of the wheelchair which are concentrated at the CG. Eccentric location of the securement point relative to the wheelchair CG will induce a moment causing the wheelchair to rotate as seen in Cases A and C. Increasing the distance between the CG and the securement point serves to increase the moment arm, thereby increasing the rotating moment. Wheelchair securement point locations also affect the loads which securement systems must withstand in a crash. In designing securement systems, manufacturers must be aware of the consequences associated with poor placement of securement points. Placement of securement points above the CG to the extend of that in Case A, for example, will tend to lead to front tiedown loads which are higher than typically expected since front tiedowns must resist the severe backward rotation of the wheelchair. To comply with the ANSI/RESNA Transportable Wheelchair Standard, wheelchair manufacturers will soon be required to provide securement points on wheelchairs intended for transport. As shown by this study, locating the rear securement points at or close to the height of the wheelchair vertical CG can improve the response of the wheelchair and occupant to a crash. Additionally, when securement points are placed at the vertical CG level, wheelchair and tiedown loadings can be better managed.

REFERENCES

1. Society of Automotive Engineers. SAE J2252 Draft Surrogate Wheelchair Drawing Package and Maintenance Manual, 1994.

2. Digges K, Bertocci G. Application of the ATB Model to Wheeled Mobility, Tech Rpt #4. University of Pittsburgh RERC on Wheeled Mobility, 1994.

3. Society of Automotive Engineers. SAE J2249 Draft Wheelchair Tiedowns and Occupant Restraints (WTORS) for Use in Motor Vehicles, 1995.

4. Shaw G, Lapidot A, Scavinsky M, Schneider L, Roy P. Interlaboratory Study of Proposed Compliance Test Protocol for WTORS. SAE Paper No. 942229, 1994.

ACKNOWLEDGEMENTS

Research conducted as a part of this study was supported by NIDRR Project #H133E30005, RERC on Technology to Improve Wheelchair Mobility, and the ANSI/RESNA Transportable Wheelchair Standard's development effort. The authors personally acknowledge Dr. Kennerly Digges and Dr. Lawrence Schneider for their input to this research.

Gina E. Bertocci, MSME,PE Rehabilation Technology Program University of Pittsburgh UPARC-915 William Pitt Way Pittsburgh, PA. 15238 412-826-3138 ginaber@pitt.edu