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Hello all!
In a previous article, I spoke about rolling resistance in manual wheelchairs. Lets take a step back and look more closely at what is meant by the term "rolling resistance" and what factors contribute to the rolling resistance in a manual wheelchair. Before we look at rolling resistance, lets have a quick physics lesson.
Our high school physics classes taught us about Newton's first law of motion, which states that a body at rest tends to stay at rest, while a body in motion tends to stay in motion, unless acted upon by an external force. When we apply this law of motion to manual wheelchairs, we can think about the forces required to initiate propulsion and the forces that cease movement.
Inertia, the tendency of an object to resist change in its motion,1 must be overcome in order to initiate propulsion in a manual wheelchair. Greater force must be used in order to begin propulsion from a stopped position compared to maintaining a constant self-selected pace.2 In order to maintain a constant speed over a level surface, a person using a wheelchair must overcome rolling resistance.3
Rolling Resistance
Rolling resistance is defined as the force that resists the motion when an object rolls on a surface.4 Rolling resistance will limit the distance a manual wheelchair coasts after a force, such as from a push stroke, is applied. Minimizing the rolling resistance in a manual wheelchair enables more efficient propulsion, reducing the force and frequency of strokes required to propel the wheelchair, which helps to reduce the risk of upper extremity pain and injury. For those people who foot propel, minimizing rolling resistance in a manual wheelchair allows for greater ease of movement of the wheelchair and may help to prevent sliding from the wheelchair caused when the effort required for the heel strike and pull through pulls the pelvis forward in the wheelchair.
Several factors contribute to rolling resistance in a manual wheelchair. These factors include the mass of the user and the mass of the system, the weight distribution between the front casters and the rear wheels, and the size and type of casters and tires selected, and the surface on which the wheelchair is used3.
Weight Distribution Between the Front Casters and Rear Wheels
The load distribution between the front casters and the rear wheels will have a significant impact on rolling resistance in a manual wheelchair. I have previously written on this topic, so I will not repeat myself here. If you would like to read more on this, please see two of my blog posts at Centre of Gravity and Manual wheelchairs and More on rear wheel position and standard manual wheelchairs.
Size and Type of Casters and Tires
Just as the forward/rearward position of the rear wheels affects rolling resistance, so too does the choice of rear tire. It is the inelastic deformation of the materials of the tire and/or the surface on which it rolls that affects the rolling resistance.5 Often solid tires are chosen due to lack of maintenance required. Studies have shown, however, that solid tires have greater rolling resistance than pneumatic tires.5,6 As load on the tires increases, solid tires demonstrate even greater increases in rolling resistance than pneumatic tires.5 From a clinical perspective, this means that the weight of the user will impact upon rolling resistance of the tires. Solid tires demonstrate greater deformation as the load increases than pneumatic tires. This means that a heavier individual will experience even greater rolling resistance with solid tires than will someone who weighs less. Tire tread and tire size also effect rolling resistance. Low profile tires have less rolling resistance than full profile tires.5 A larger tire radius will have less rolling resistance than a similar tire of smaller radius.7
Tire pressure also has an effect on rolling resistance. A study assessing the effect of tire inflation on rolling resistance found that pneumatic tires "showed no statistically significant difference in rolling resistance until pressures had decreased to 50% of the recommended value"6 (p. 1480). The study further found that pneumatic tires inflated to at least 50% of the recommended value had less rolling resistance than solid tires. The authors indicated that wheelchair tires need to be pumped once per month to maintain adequate pressure, based on the finding that wheelchair tires lose 10 to 25% of their pressure in the first two weeks following inflation and 25 to 40% of their pressure after one month following inflation.
Caster selection also influences rolling resistance in a manual wheelchair. In a study in which casters were classified into soft roll casters, standard casters and roller casters, standard casters were shown to have the highest rolling resistance, followed by the soft roll casters, and finally by the roller casters, when comparing similar radius.7 As the radius of a caster increases, the rolling resistance factor decreases.
Thus, rolling resistance is effected by a number of different factors. The weight distribution between the rear wheel and front casters, the size and type of rear wheels and front casters, and the surface on which a wheelchair is propelled (e.g., hard, smooth surface versus a soft surface such as a carpet) will all effect rolling resistance. Because rolling resistance increases the effort required to propel a manual wheelchair, it is important to look at unique requirements and maintenance capabilities when configuring a manual wheelchair for an individual.
References
- Caspall, J.J., Seligsohn, E., Dao, P.V., & Sprigle, S. (2013). Changes in inertia and effect on turning effort across different wheelchair configurations. Journal of Rehabilitation Research & Development. 50(10), 1353-1362.
- Koontz, A.M., Cooper, R.A., Boninger, M.L., Yang, Y., Impink, B.G., & van der Woude, L.H.V. (2005). A kinetic analysis of manual wheelchair propulsion during start-up on select indoor and outdoor surfaces. Journal of Rehabilitation Research & Development. 42(4), 447-458.
- Tomlinson, J.D. (2000). Managing maneuverability and rear stability of adjustable manual wheelchairs: An update. Physical Therapy, 80(9), 904-911.
- Wikipedia. (2014). Rolling resistance. Retrieved from http://en.wikipedia.org/wiki/Rolling_resistance.
- Kwarciak, A.M., Yarossi, M., Ramanujam, A., Dyson-Hudson, T.A., & Sisto, S.A. (2009). Evaluation of wheelchair tire rolling resistance using dynamometer-based coast-down tests. Journal of Rehabilitation Research & Development, 46(7), 931-938.
- Sawatzky, B.J., Kim, W.O., & Denison, I. (2004). The ergonomics of different tyres and tyre pressure during wheelchair propulsion. Ergonomics, 47(14), 1475-1483.
- Sauret, C., Bascou, J., de Saint Remy, N., Pillet, H., Yaslin, P., & Lavaste, F. (2012). Assessment of field rolling resistance of manual wheelchairs. Journal of Rehabilitation Research & Development, 49(1), 63-74.