Air Resistance, Biking, & Running

Posted by Paul A. Heckert Jul 29, 2008

A few weeks ago I did the running leg of a triathlon relay. My wife, who did the bicycle portion thought the headwind slowed her down, but when running I was hoping for some wind to provide a cool breeze.

If you watched the Tour de France, or any other major bicycle race, you may have noticed that sometimes riders will deliberately ride close behind another rider. This practice, called drafting, is designed to save energy. By letting the front rider take care of the air resistance, the back rider can rest a bit. However in major marathons and other running races, the runners don't draft each other to save energy by blocking air resistance.

Air or wind resistance is more of a problem when bicycling than when running. Why?

The physics of air resistance is quite complex. The air resistance depends heavily on the shape of an object. Is it aerodynamic? Air resistance also depends strongly on the speed an object is moving.

Ignore the minor difference in shape. A bicycle racer is moving much faster than a competitive runner, so air resistance plays a bigger role. A bicycle rider must exert more energy fighting air resistance than a more slowly moving runner. Therefore drafting can save a bicyclist a significant amount of energy, but it will not significantly help a runner. The one time it can help a runner is when running into a strong headwind. The headwind increases the runner's speed relative to the air and therefore the runner's air resistance. Then a runner can save energy by letting another runner block the wind.

The basic physics of air resistance makes drafting common for bicyclists but not runners.