Impact of Toe in/out due to Rolling Resistance Losses in Manual Wheelchair Propulsion

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Impact of Toe in/out due to Rolling Resistance Losses in Manual Wheelchair Propulsion

Joseph Ott, M.S.1,2, Holly Wilson-Jene, M.S.1,2, Travis Henderson, B.S.1, M. Mendel Marcus, B.S.1, Jonathan Pearlman, Ph.D.1,2

1University of Pittsburgh, Pittsburgh, PA, United States, 2International Society of Wheelchair Professionals, Pittsburgh, PA, United States

I, Joseph Ott, do not have affiliation (financial or otherwise) with equipment, medical device, or communications organization. Speakers who have no involvement with industry should inform the audience that they cannot identify any conflict of interest.



Rolling resistance (RR) is the primary force acting against propulsion forces for manual wheelchair users (MWU) and is linked with pain and other health issues in the wrist and shoulders. Air resistance & bearing resistance are considered to be negligible in most cases [1-3]. The RR results from an energy loss of the tire deforming and reforming while contacting the ground. The material of the tire is critical in the loss of energy due to hysteresis (inelastic deformation); it accounts for almost all of the loss of kinetic energy in rubber-based tires [4]. A simplified free body diagram of the forces is shown in Figure 1. Rolling resistance can be referenced as a force or as a coefficient proportional to the weight: µRR = FRR/W.

Figure 1: Rolling Resistance Free  Body Diagram Ft is the tangential  force, V is angular velocity, W is the  load on the axle, FRR is the rolling  resistance force.

For clinicians, it is imperative to understand the biomechanical impact of rolling resistance and related consequences for MWU. Increased RR increases the input force required for a MWU to propel their wheelchair, which is linked to an increase in the risk of upper extremity injury and pain, such as rotator cuff injuries [5, 6]. Pain and injuries can lead to reduced activity and participation [7]. Wheelchair setup, such as adjustments in rear axle position changes the load on the rear wheels and consequently changes RR [8]. Other parameters such as the toe and camber of the rear wheels, tire type, and tire pressure can also affect rolling resistance [9] [10]. 

A scoping review of 40 articles on rolling resistance test methods was performed and the articles were categorized into seven testing method groups: deceleration, motor draw, treadmill, physiological expenditures, drag tests, ergometer/dynamometer, and robotic test rig. Each article was used to evaluate test methods against three criteria: direct or indirect testing method, the ability to test on a component level, and the ability to test multiple setup parameters (camber, toe, tire type, tire pressure, load distribution, and surfaces). The direct test methods include treadmill testing and drag testing. The five indirect testing methods calculate rolling resistance by means of other measurements and consequently are less accurate than direct testing. None of the test methods measured RR at a component level, but rather tested the whole wheelchair as a system. Some test methods can have the ability to measure the influence of a limited number of setup parameters, but no one study tested every possible setup parameter. Additionally, testing parameters (such as force applied and tires tested) varied across different manuscripts, making it difficult to compare RR results. To better understand the influence of different parameters on RR so that users, designers, and providers of wheelchairs can be more informed, it is important to develop a robust, repeatable and component-level test method. The goals of this study were to design this test method and use it to measure the influence of different factors on RR. Additionally, we investigated the prevalence of toein/out in the community, after we found it to be one of the most influential factors on RR.



With the drawbacks of the previous testing methods, a new testing machine was designed and built utilizing a drum-based testing method at the University of Pittsburgh, shown in Figure 2. The drum (1) is four feet in diameter and it rotates at a constant speed. The frame (2) is split and has a top section that holds the arm assembly. The arm (3) consists of two parallel one-and-a-half-inch precision ground rods that sit tangent to the surface of the drum at top dead center. A truck (4) utilizes four air bushings, which slide on two parallel rods of the arm assembly. Compressed air is pumped into these bushings, causing them to float on the rods and eliminate friction, which improves the accuracy of the RR measurements. A load cell (5) is mounted at the front of the arm, with an eye bolt to connect it to the truck. The machine has the capability to test tire type, tire pressure, toe, camber, load, surfaces, and casters type at a component level. For testing, the drum is rotated at a constant speed and the air bearings are released so the truck floats on the rods. The load cell measures the pullback force (FRR) on the truck which is a direct measurement of RR.

Figure 2: Drum based rolling  resistance testing machine. (1) Drum,  (2) Upper Frame, (3) Arm, (4) Truck,  (5) Load Cell

Preliminary testing showed that toe (when the rear wheels are not parallel to each other) can be a major influencer with RR, doubling at two degrees of toe. While this correlates with other studies from the literature review, it seemed to be an understudied area of wheelchair setup [9]. Database searches revealed that a study had yet to be conducted on the prevalence of toe in/out in manual wheelchairs. A laser measurement system, as seen in Figure 3, was developed to measure the severity of toe and slop (excessive play in the axles) in the rear wheels. The lasers are accurate down to 1-millimeter precision and are set to axle height to negate the effects of camber. Integrated into the design are 1-pound constant force springs that apply pressure to the rear wheels to induce toe if slop is present. A field study was conducted on 200 manual wheelchairs users to collect toe in/out of their rear wheels, slop of the rear wheels when loaded nominally inward/outward, use habits and other device characteristics. Simpler devices such as a tape measure and digital level were used for other measurements of the wheelchair. The study was deemed IRB exempt and followed inclusion criteria of a wheelchair between 14 and 32 inches wide, a MWU weight under 300 pounds, and a MWU over 18 years old. 

Figure 3: Laser measurement system for toe and slop in  manual wheelchairs


The results from the drum testing reveal the degree of influence on RR for common wheelchair selection, setup and environmental parameters. Table 1 shows the average change in RR across the six tires (4 for tire pressure since two were airless) by parameter. They are compared to the worst-case scenario for each parameter. Camber and speed, which are typically self-selected, were not found to be highly influential on RR. Level-loop carpet was tested through three varieties of pile and medium pile ended up being the biggest increase in RR compared to the drum. The results also show that there are high levels of variations between tires, emphasizing the importance of informed selection when choosing tire type. The four pneumatic tires had the lowest RR compared to the airless insert or solid polyurethane tires. The airless insert performed the worst but was less susceptible to be influenced by parameters such as toe.

Table 1: Comparing Test Parameters

Adaptive sporting events were the primary recruitment location for participants in the field study, because of the high number of wheelchair users attending. Table 2 shows the average offset between the front and rear of the rear wheels, which translates to 0.92 degrees of toe out (the front of the wheels is farther apart than the rear of the wheels). Furthermore, there is an average 0.54 degree of slop in addition to the toe out. It is important to note that while slop and toe are independent, they could combine to have about 1.5 degrees of toe. Toe and RR have a parabolic relationship; therefore, larger toe values have a much larger increase in RR. The weight equivalent of 1.5 degrees of toe would be adding 47.24 pounds to the MWU’s lap. Camber and tire pressure were also collected and revealed severe under inflation and reported as percentage of the max inflation possible.

Table 2: Results from the Field Study

The results from the field study were used to identify the parameter levels to be tested in combinations of two factors. One tested interaction was camber of 3 degrees was tested with 0.5 and 1 degree of toe out. Every parameter, except for speed, was included in the combined factors testing with the parameter levels being chosen to replicate the field observations. A common observation was underinflated tires in combination with toe or slop in the system. The results were analyzed briefly to determine if there are any interaction effects in the combined factors as compared to the results of an individual factor. Only one combination shows the possibility of an interaction with low load and low tire pressure, which shows an average of 14 percent lower RR than the cumulative result of both the individual factors.



The importance of this research is to inform clinicians, end users, and manufacturers on the biomechanical impact to upper extremities of manual wheelchair users from design, product selection, and setup. To better understand the effects of each parameter on RR, each parameter was ranked by their potential increase in RR across tire types. Two degrees of toe has a significant increase in RR, and a few devices measured in the field study had that level of misalignment. The field study revealed a widespread issue of tire under inflation, and at 40% of max inflation pressure, RR increases by one-third. To better understand the impact of each parameter, the change in RR was converted to weight to provide insight into the perceived increase in weight of the MWU that occurs due to a parameter. To simplify the calculations, some parameters were held constant, such as the tire type, where the most common tire found in the field study was used. Since toe is a measure of both wheels, it is calculated as only affecting one side, whereas the other parameters are assumed to be influencing both rear wheels. Large increases in weight can be seen across the parameters, and manufacturers are competing to produce lightweight manual wheelchairs. However, that influence is going to be relatively small compared to impact of a MWU’s weight. 

The results from the field study show disparities in maintenance across the field that have a lasting impact on the MWU. Toe in/out found in the field study, results in a perceived increase of 24 extra pounds to the MWU during propulsion. Similarly, the reduction in tire pressure is equivalent to the MWU adding a 20-pound weight on their lap and propelling. Due to lack of proper maintenance, the equivalent addition of weight negates the benefits of using an ultralight wheelchair. One concerning point is that toe and low tire pressure were observed together, and these would be combined to have a cumulative influence on RR.

Factors like carpet and user weight (load) are uncontrollable but do have an impact on RR, whereas speed and camber are self-selected but not as strong of an influence on RR. The controllable factors of tire selection and rear axle position (load) greatly change how the MWU propels and the biomechanical impact. Maintenance factors arise over time in toe and tire pressure loss. The lack of maintenance is obvious Measurement Average Standard Deviation Equivalent Weight Added Toe (mm) 9 12 24 lbs. Toe (Deg) 0.9 1.4 Slop (mm) 5 8 5 lbs. Slop (Deg) 0.5 0.8 Camber (deg) 3.0 1.5 7 lbs. Tire Pressure 35% 24% 20 lbs. Table 2: Results from the Field Studydue the severe underinflation and the severity of toe in the field study. This study was focused on predetermined parameter and can be expanded in the future. Tighter intervals of parameters may be selected, but the drum-based testing provides accurate component level data. The combined factors testing can also be expanded, but overall shows cumulative effects of parameters. The field study was limited by the fact that measurements were taken by researchers where recording errors could be made. The field study revealed that more education should be conducted on the proper maintenance of manual wheelchairs using already published resources such as the Wheelchair Maintenance Training Program [11]. Currently, no maintenance or testing to identify toe is recommended. Further work needs to be explored with the drum-based machine since only a small subset of wheels were tested and more information can be deduced from the field study. Through increased testing, a standard for RR or toe variance could be implemented into product testing. The results of this research have the ability to be compiled such that clinicians can input variables to understand the effect of RR on the MWU. 



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Contact Information

Joseph Ott, M.S. 6425 Penn Ave., Suite 401, Pittsburgh, PA 15206