Air Resistance and Drafting in Bicycling and Running
In the Tour de France, or any other major bicycle race, riders will often 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 the Boston Marathon, other major marathons, or other running races, the runners don’t draft each other to save energy by blocking air resistance. Air resistance is more of a problem for bicycle riders than runners. Why?
Physics of Air Resistance Simplified
The physics of air resistance is quite complex. For the mathematically unintimidated, details can be found in Analytical Mechanics by Fowles, G.R. and Cassiday, G.L. (2005, Thomson Brooks/Cole). I provide only a conceptual overview here.
In the equations describing a moving object, the terms accounting for air resistance (also called drag) contain a constant coefficient to quantify the amount of drag. This constant coefficient depends heavily on the shape of an object. Is it aerodynamic? Aerodynamic objects have a low numerical values for this coefficient. Objects, such as parachutes — designed to have as much air resistance as possible — have a very high value for this coefficient. There is no easy way to calculate these coefficients for different objects, but they can be measured experimentally in wind tunnels.
The constant drag coefficient is them multiplied by either the velocity or at higher speeds the velocity squared to get the complete air resistance term. Physically that means that the faster an object is moving the greater the air resistance will be. If the speed is high enough, the velocity in the air resistance term is squared. Hence the air resistance increases much more rapidly as the speed increases.
Summarizing, the air resistance that a bicycle rider, runner, or any other object will have depends on both the object’s shape and its speed.
Air Resistance of Bicyclists and Runners
The minor difference in shape between a person on a bicycle and a runner makes a small difference in the constant air resistance coefficients. The real difference, however, is the speed.
A bicycle racer is moving much more rapidly than a competitive runner, so air resistance plays a bigger role for a bicycle racer. A bicycle rider must therefore exert more energy fighting air resistance than a slower runner. Drafting can save a bicyclist a significant amount of energy, but it will not help a runner as much.
The one time drafting 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 explains why drafting is common for bicyclists, but not for runners.