Aerodynamics of Soapbox Racers

Aerodynamic drag is by far the largest factor affecting a carties top speed, but how big an effect is it? I've been doing some maths and have come up with some interesting typical figures. The three sources of drag on a cartie are;

• Aerodynamic drag
• Rolling resistance
• Wheel inertia

Rolling resistance is almost constant at all speeds, and wheel inertia actually decreases as speed increases (or - more correctly - as acceleration decreases), but aerodynamic drag increases as the square of the speed. It starts off very low, but soon becomes the dominant factor.

As the graphs on the right show, at 10mph the aero' drag is just 19% of the overall drag, but by the time the cartie has reached 20mph, aero' drag is nearly half of the total. (Click on the graphs for the full size versions).

 

Speed (mph)	Aero' drag as a % of total drag
10	19.0%
20	46.5%
30	64.7%
40	75.5%

So what does this mean in real terms? If it's a relatively slow race with much of the time spent at speeds below about 20mph, you're probably better off concentrating on keeping the rolling resistance as low as possible. If the race is faster than that, make sure your cartie is as aerodynamic as possible. And if you want to learn how to do that, get hold of a copy of The Leading Edge: Aerodynamic Design of Ultra Streamlined Land Vehicles by Tamai Goro.