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We all know that drafting is an important part of road cycling. As a cyclist moves forward he produces a turbulent wake of air behind him called vortices. The vortices create a low pressure area behind the rider and an area of wind that moves along with him. Think of how a motorboat creates 2 wakes of water behind it with a calm area directly behind it. Great for waterskiing.
If you draft behind another rider who is cutting the wind you obviously gain a significant advantage. Up to 40% less energy can be used in the draft. The low pressure moves you forward and the eddies push you forward.
A less known fact is that drafting not only helps the cyclist who is behind, but it also gives an advantage to the lead rider as well. This is because the trailing rider reduces the turbulence coming off the lead rider and in effect makes him more aerodynamic. If somebody pulls in behind you to draft, they fill this void of space behind you, the movement of air flows past you and then continues to flow past the second person, resulting in less turbulence and swirling air behind you. Obviously the lead cyclist still needs to expend much more energy than the cyclist who is following, but both gain an advantage.
One example of a study done on this is:
This was measured in the General Motors Wind Tunnel in 1996, and on the track using the SRM crank dynamometers. The lead rider in a 4-man pace line uses about 2 to 3 percent less energy than they would if riding solo.The next in line needs about 71 percent of the lead rider’s power, and the third and fourth riders about 65 percent. See “Racing cyclist power requirements in the 4000-m individual and team pursuits”, Medicine and Science in Sports and Exercise, v31, no.11, pp 1677-1685, 1999. J.P. Broker, C.R. Kyle and E.R. Burke.
I’ve been in many race situations where I try to bridge a gap and someone immediately follows me. When I realize that the person who has followed is not strong enough to pull through and help me, he is still valuable in helping me get across the gap. You’re obviously helping him more, however if he is not strong enough to pull through and do a turn, he’s not really a threat in the end anyway (unless he’s a gun sprinter). Help the poor guy out and perhaps he’ll repay you one day!