Handrim Diameter
The handrim acts like the gearing for the wheel chair.
If a small diameter handrim is used, the athlete has selected a "high" gear that produces poor acceleration but a high top speed.
Conversely, if a large diameter handrim is used, the benefit is greater acceleration at the cost of a lower top speed.
In general, stronger athletes are able to effectively push smaller diameter handrims, and thus the optimum handrim diameter is a function of the size of the athlete, the relative importance of acceleration and top speed in the race being run, and the strength of the athlete.
Most handrims are between 14 and 16 inches (0.35 and 0.41 meter) in diameter and athletes usually experiment to determine what works best for them.