This is a summary of a research study done by Dr. Bonita Sawatzky and fellow researchers at ICORD. Dr. Sawatzky’s research focuses on mobility for individuals with SCI, with the specific goal of improving the function of manual and powered wheelchairs.
Click here for access to the original paper, published in Journal of Rehabilitation Research and Development.
Does size matter? As you are probably aware, propelling a wheelchair is not a very efficient use of your energy. You have probably heard that the type of tires you use and the importance of inflating them regularly makes a big difference to how much energy you use, but have you thought about the size of your front wheels, called casters?
What was the purpose of this study?
This study examined the size of wheelchair castors and their effect on rolling resistance (or drag) on a smooth surface. The researchers thought that:
- Casters with larger diameters would reduce the rolling resistance of a wheelchair.
- Rolling resistance would increase if the weight was shifted more over the casters rather than the rear wheels
How was this study conducted?
A treadmill and a rigid frame, ultra-light wheelchair were used to perform a series of drag tests. The drag tests included two variables: the size or diameter of the caster (3”, 4” or 5”) and the the amount of weight to be supported by the casters (weight distribution). Other factors, such as the rear wheels and caster forks, were kept constant.
Before each trial, the rear wheels’ tire pressure was inflated to 700 kPa (approx. 100 psi), a set of casters were installed on the wheelchair and a standardized weight of 60 kg (typical female adult weight) was placed on the seat. The weight distribution was adjusted starting at 10% of the total weight and increased by 10% for each subsequent set of tests up to 60%.
After the wheelchair was adjusted, it was placed on the motor-driven treadmill. The forces were measured using “drag” protocol. Three trials were performed at a linear velocity of 1.11 m/s, registering the drag force at different inclines of the treadmill, from 5.5% to 0.5%, reducing the incline by 0.5% for each measurement. The drag force at 0% incline was calculated by linear regression using the data collected during the 3 trials per condition.
What was measured?
The effects of the caster diameter and weight distribution on the drag force were analyzed.
What were the results?
Using the 3” diameter casters increased overall drag force when propelling a wheelchair. However, this was not significant until the weight was more than 30% on the front casters. The largest caster tested did not show a significant difference in the rolling resistance force measured compared to the 4” than 5’’ thus, not necessarily better.
More important for reducing rolling resistance was the weight supported by the front casters. Weight distribution affects the drag force regardless of casters used. Knowing this, you and your clinician can choose the set of castors that best suits your needs as long as most of your weight is over your wheelchair’s rear tires. An optimal combination of caster size and weight distribution is best to conserve energy when propelling on smooth surfaces. The next question is how do these results change on other indoor and outdoor surface. When are the bigger casters more useful or are they?