Rolling, Rolling, Rolling

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August/September 2002

By Bonita J. Sawatzky,PhD; Ian Denison, PT; and Won O. Kim, BSc Won O. Kim, BSc

Won O. Kim, BSc, readies the roll down apparatus for the loaded wheelchair in preparation for the tires’ rolling resistance test.

For many children and adults with mobility impairments, wheelchairs are essential mobility aids. Manual wheelchairs are more economical and have more fitness benefits for users when compared to motorized units.¹

Unfortunately, the upper extremity was not designed to bear the repetitive strain that is generated during manual wheelchair propulsion. Research shows that up to 75% of wheelchair users suffer from repetitive strain injuries (RSIs) of the shoulder and wrist.² These injuries are insidious in nature and the joint damage starts long before the patient is aware of any trauma. Therefore, further research is necessary to investigate the factors that likely contribute to such injuries.

Our researchers and clinicians have noted that a significant number of wheelchair users do not maintain their tire pressures up to the manufacturer’s recommended level. It is not uncommon to find pressures even lower than 25% of the recommended level.

We decided to perform a set of standardized roll down tests for five of the more popular wheelchair tires in the market to determine their rolling resistance characteristics. The rationale for this comparison study is threefold. First, it would render an objective and quantitative measure of the rolling resistance characteristics of pneumatic and solid tires. Second, the study results would help formulate recommendations for patients, taking into consideration their individual needs, tire maintenance habits, and financial constraints.

Finally, statistically significant results among different tires in rolling resistance may be extended and correlated to physiologically significant findings in current and future studies. In time, such correlation may help establish a link between RSIs in wheelchair users and an issue as simple as tire pressure maintenance.

Figure 2

METHODS

Five pairs of brand-new 24-inch wheelchair tires were selected for the study. We used three types of pneumatic tires: Tires A, Tires B, and Tires C; and two types of solid tires: Tires D and Tires E. Each pair was weighed and its retail costs were investigated. A lightweight manual wheelchair (weight: 13 kg; width: 17 inches; wheel base: 14 inches; 3° camber; casters: 5-inch, 125/37.5-50 soft roll) was mounted with a 56-kg load and a 76/24 (rear/front) weight distribution to simulate a person in the wheelchair. A pair of standard 24-inch wheels with radial spokes were used for all the tires tested.

The roll down tests were performed on the smooth surface of a gymnasium floor. A wooden ramp with an 8° incline was used to propel the wheelchair down its top surface. Specific markers on the ramp and the floor of the gymnasium were used to ensure consistency of the starting position for each roll down. Prior to each daily sampling, the rolling path and the tires being tested were cleaned with a dry cloth to remove any debris that would influence the rolling resistance of tires (the photo above illustrates the roll down apparatus).

The total weight of 69 kg (wheelchair plus load) was used in the roll down test. Four tire pressures (100%, 75%, 50%, 25% of the manufacturers’ recommended level) were selected for each of the three pneumatic tires for the roll down tests. The tire pressures were measured using a digital gauge. Five runs of each sampling set were measured and recorded. A straight line on the floor was used to ensure runs did not deviate more than 10 cm laterally.

Roll down tests for all five tires were performed and data were collected on 4 separate days within a 1-month period. A repeated measures analysis of variance was used to compare the mean rolling distances between tire types at varying tire pressures in the four sample groups. A Bonferroni/Dunn correlation was used to examine the relationship between the differences in mean rolling distance and independent variables.

Figure 3

MEAN ROLLING DISTANCE AND TIRE PRESSURES

There were no statistically significant differences in mean rolling distances at pressures between 100% and 75% of the recommended tire pressures for all pneumatic tires (p<.0001). However, there were significant differences in mean rolling distances at pressures between 100% and 25% for all tires and additional significant differences at pressures between 100% and 50% for Tires A and Tires B.

MEAN ROLLING DISTANCE AND TIRE TYPES

Tires B consistently recorded the furthest rolling distances among the five tires tested at all pressure levels. At 100% of the recommended tire pressure, Tires C traveled less than Tires A; however, Tires C rolled further than Tires A at and below 50% of their recommended tire pressures. As for the two solid tires (Tires D and Tires E), their rolling distances were markedly inferior to those of the three pneumatic tires. Even Tires D—the better of the two solids in terms of the rolling distance—at best were comparable to the rolling distance at the lowest pressure tested (16 psi for Tires A) for the pneumatic tires. Tires E, the heaviest of all five tires, consistently traveled the shortest distance in all 4 days of sampling. The Bonferroni/Dunn correlation demonstrated that the differences in mean rolling distances between each of the five tire types were statistically significant (p < 0.0001).

The aim of a well-designed wheelchair is to allow its user to maneuver through space with minimal energy cost and physical strain.³ In achieving that goal, tire pressure is a key determinant of the performance of a pneumatic tire.⁴ Our study confirmed that—depending on the type of tire—decreased tire pressure becomes a significant factor in wheeling resistance at and below 50% of the manufacturer’s recommended pressure. Since wheelchair tires lose 10%-25% of their tire pressure in the first 2 weeks and 25%-40% after 1 month,⁴ it is rational to assume that many wheelchair users who do not regularly monitor tire pressure experience a significant increase in rolling resistance as their tires lose air. According to our study, this implies that wheelchair users using Tires A and B experience a statistically significant increase in wheeling resistance when going without tire reinflation for a little more than 1 month. Even more rolling resistance is noted in all three pneumatic tires when the tire pressure drops to 25% of the recommended level. How much of that actually translates into added physical strain remains to be seen through ongoing and future studies.

Interestingly, of the three pneumatic tires tested—Tires C—showed the least amount of percentage drop in mean rolling distance with decreasing tire pressure. This may be attributed to its sidewall construct, which is more rigid (hence resisting the flattening out of the tires at lower pressure) than those of the other pneumatics tested.

Our study showed that Tires B outperformed the other four tires tested in rolling distance. They not only rolled consistently the farthest at all pressure levels, but are also the lightest of the five tires. The weight differences among the five tires tested are minor when compared to the combined weight of the wheelchair and its user. The small weight advantage in Tires B may or may not be a factor physiologically distinguishable; however, it certainly is an incentive for its users to consider.

It has previously been contended that the rolling resistance of solid tires is up to 35% higher than conventional pneumatics.^5,6Our results showed that the mean rolling distances of Tires D and Tires E were 42%-58% shorter than those of the three pneumatics at 100% recommended pressures. Tires E were not only the tires with the shortest rolling distance but also the heaviest tires tested in this study. Tires D were only marginally better than Tires E in rolling distance, although they are approximately 30% lighter. Despite their advantages over pneumatics as being more durable, puncture resistant, maintenance-free, and nonmarking, our roll down tests confirmed the solid tires’ relatively high rolling resistance compared to the pneumatics.

When addressing the issue of reducing the energy expenditure of the wheelchair user, it appears that pneumatic tires offer significantly less rolling resistance than the solids. This claim can be made, however, only if the tire pressure is maintained relatively close to the recommended tire pressure. Other advantages of choosing the pneumatics over the solids are that they provide a more cushioned ride and better traction for braking and propulsion.^5,6 For those looking to save money, the initial cost of purchase for pneumatic tires is also lower than that of the solids, although maintenance costs and risk of puncture must be taken into account. Another advantage of pneumatics is that, depending on the characteristics of the terrain, tire pressure level can be adjusted to ease the user’s effort in propelling a manual wheelchair.⁴ Tires A—the least expensive and the heaviest of the three pneumatic tires tested—may be an adequate investment for a wheelchair user who regularly monitors the tire pressure, because their rolling resistance also increases the most with decreasing tire pressure. Solid tires, despite their poor performance in our roll down tests, are more expensive to purchase. Due to their inherent structural flaws (out of round), they also mean a bumpier ride and inconsistent brake effectiveness for their users.⁵ There are also some anecdotal reports of the solid tires soaking up water when driven on wet grounds, adding to their weight and affecting their structural integrity. However, as mentioned earlier, they are fairly maintenance free, puncture resistant, and durable. Thus, for a wheelchair user who seeks hassle-free, durable tires without much concern for the aforementioned shortfalls of solid tires, they may be acceptable mobility aids.

LIMITATIONS

One of the limitations of this study is that we tested only five of the more common models in the market; numerous other tire models not tested still lack the data gathered through this study. The five tires tested were also brand-new, which suggests that the effects of tire wear over time on rolling resistance of each tire type could not be accounted for in this study. Moreover, only one type of floor surface was used to study the effects of tire pressure on rolling distance and, therefore, the results of this study cannot be directly applied to any other types of ground surface commonly encountered by wheelchair users. Lastly, day-to-day fluctuations in atmospheric pressure, air density, temperature, and humidity may influence tire pressure, air drag, and surface texture of the tires and the floor.

CONCLUSIONS

Although a statistical difference is not necessarily a physiologically or clinically detectable difference, we hope our study results represent a useful tool for wheelchair users, clinicians, and medical equipment specialists in making better-informed decisions regarding mobility aids to optimize their performance. Continued research in this area is needed to further promote the health of wheelchair users and improvement of mobility aids by manufacturers.

ACKNOWLEDGEMENTS

We would like to thank British Columbia’s Children’s Hospital for providing the facility for this study; Advanced Mobility and Motion Specialties for generously supplying us with the various equipment needed for this study; and the Wheelchair Services Department of GF Strong Rehab Centre for their tireless effort in tire changes and wheelchair alignment adjustment.

REFERENCES

Cowell LL, Squires WG, Raven PB. Benefits of aerobic exercise for the paraplegic: a brief review. Med Sci Sports Excerc. 1986;18:501-508.
Curtis KA, Tyner TM, Zachary L. Effect of a standard exercise protocol on shoulder pain in long-term wheelchair users. Spinal Cord. 1999;37:421-9.
Waters RL, Mulroy S. The energy expenditure of normal and pathologic gait. Gait and Posture. 1999;9:207-231.
Denison I, Shaw J, Zuyderhoff R. The effect of components on manual wheelchair performance. Wheelchair Selection Manual. Vancouver, BC: BC Rehab; 1994:55-58.
Fields CD. Groundwork: tires. TeamRehab Report. November/December 1991:22-24.
Gordon J, Kauzlarich JJ, Thacker JG. Tests of two new polyurethane foam wheelchair tires. J Rehabil Res Dev. 1989;26(1):33-46.

Bonita J. Sawatzky, PhD, is assistant professor in the Department of Orthopedics, Faculty of Medicine, at the University of British Columbia, Vancouver, Canada. She can be reached via email: bsawatzky@cw.bc.ca. Ian Denison, PT, is an equipment specialist at GF Strong Rehab Centre, Vancouver, and Won O. Kim, BSc, is a third year medical student at Queen’s University, Kingston, Ontario, Canada.

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August/September 2002