Racing Wheelchair Aerodynamic Drag Reduction
Background
There are two sources of resistance that an athlete encounters when pushing a chair on level ground: rolling resistance (from the tires on the ground) and air resistance (aerodynamic drag). Rolling resistance is more important at low speeds and is reduced by using high pressure tires. Air resistance is more important at high speeds. The chart below shows the relative importance of rolling resistance and air resistance for varying wheelchair speeds. At typical race speeds of 16-19 mph, air resistance makes up about 75% of the total resistive force on the chair, so the largest gains can be made by reducing drag.
Fig. 1 Relative importance of rolling restance and air resistance at various wheelchair speeds, expressed as a percentage of the total resistive force on the chair. Air resistance data are from wind tunnel results, rolling resistance data are from coastdown results.
Reductions in aerodynamic drag can be accomplished by reducing the athlete's frontal profile (e.g., by tucking) or by streamlining the chair. Current wheelchair designs position the athlete to have a small profile, but do not incorporate a significant amount of streamlining. A measure of the amount of streamlining is given by drag coefficient. The lower the drag coefficient, the more streamlined the shape. A parameter that incorporates both the frontal profile of an object (i.e., its size) and the amount of streamlining of an object (its shape) is called the drag area, or CdA. It is the product of the drag coefficient and a reference area, usually the frontal area for ground vehicles. For reference, a typical car has a CdA of about 8 ft^2 and a person standing upright has a CdA of 9 ft^2 (Ref 1).
In similar applications, streamlining has been shown to potentially generate very large reductions in drag. In the most extreme cases, bicycles constructed to break the human powered vehicle land speed record incorporate full fairings, which are essentially shells enclosing the bike and rider. Well-designed full fairings, such as that shown in the figure below, have been shown to reduce drag coefficient by more than 10-fold compared to a bike without a fairing (in some cases, reducing drag coefficient from 0.68 to 0.045, according to Ref 2). Of course, full fairings are not practical for racing chairs -- they tend to be heavy and would restrict the athlete's movement and visibility.
