Cheating the Wind: The Physics of Draft-Legal Racing

Unless an event’s rules specifically allow it, drafting is illegal in triathlon.

Draft-legal races are much more popular in Europe where the triathlon culture is more entwined with road cycling. It’s my general observation that Americans are less comfortable with the idea because we don’t often ride in large packs. We also tend to ride “triathlon bikes,” which are set up to conform to the rules of long-course racing rather than Olympic (i.e., draft-legal) events. We find drafting equally despicable for its lack of fairness and safety. But whether you’re concerned about just how much advantage someone is obtaining by cheating in an Ironman® or thinking about jumping into draft-legal racing, it helps to have a perspective on just how much drafting influences outcomes.

cyclists, pace line, cycling, drafting FASTER

The basic principle of drafting is this: the closer you get to another cyclist, the less air resistance you encounter. That “sweet spot” of reduced drag caused by the lead rider’s disturbance of the air is called the slipstream. But where is the slipstream and how much advantage does it offer?

Back in the 1970s, the U.S. Olympic Cycling Team consulted Doctor Chester Kyle from the Massachusetts Institute of Technology to answer these questions. Kyle is something of a pioneer in the history of American cycling aerodynamics. He was an innovator of recumbent frame geometry and his revolutionary bike designs helped the 1984 Olympic team haul away more medals than the next four U.S. Olympic cycling teams combined. Much has changed since then—air has not—so the findings from his research in 1979 are still relevant.

In tests run both on the road and in wind tunnels, Kyle discovered that a rider can reduce wind resistance by 38% if they ride within twelve inches of a leader’s rear wheel. At a distance of two meters, the follower still maintains a 27% drag savings. He was able to establish the following equation describing the relationship between distance from the lead rider and the savings in drag.



From this equation, Kyle concluded that at a distance of three meters the effects of trailing in someone’s slipstream is negligible. Plug 3 in for in the equation and the result is 1, meaning you get 100% of the wind resistance. That conclusion is still the commonly accepted model by the people trying to make cyclists better at drafting.

Here are a few items of interest for draft-legal racing:

  1. It doesn’t matter whether you’re the second, third, or even fourth competitor back in the pace line. Kyle found that everyone gets the same relative advantage regardless of position.
  2. The only thing you might want to consider while jockeying for a spot is the size of the person you get behind. Bigger people offer more blockage.

In a research paper published in the European Journal of Applied Physiology in 1998, Tim Olds established the relationship between the proximity of two cyclists and the VO2 savings gained by the trailing rider (see graph).

Faster triathlon science: drafting

This is why drafting is such a big part of competitive strategy in ITU racing. There is a direct correlation between the physics of your surroundings and your internal physiology. Not only is there an influence on your speed on the bike itself, but the reduced effort you experience by drafting can save your energy for the run as well.

This cuts both ways, though. If you exit the swim too late to catch on to the “lead draft pack,” then you will either have to settle for riding with the second group or risking the muscle burn to catch up. Of course, it can be of relative detriment to you if someone drafts on your wheel as you “bridge the gap” to the leading riders; you do all the work, only to have them possibly run away from you after the transition. How professional ITU athletes balance their swim-bike-run effort depends both on individual training and overall team strategy, but at the end of the day it all comes back to the same scientific principles that affect us all.

In the next post, I’ll discuss how a poor understanding of science actually penalizes athletes for obeying the rules in races that don’t allow drafting.

If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make simple changes that add up to free speed and faster racing.

The Dynamic of Bicycle Finals: A Theoretical and Empirical Analysis of Slipstreaming. Alexander Dilger and Hannah Geyer, 2009.
The Science of Drafting: Easy Riding in the Slipstream. Jeffrey P. Broker, Ph.D. USA Cycling Performance Conditioning Cycling, Volume 11, #2.
The Mathematics of Breaking Away and Chasing in Cycling. Tim Olds. European Journal of Applied Physiology, 1998.
Maybe It Looks Funny, but Chester Kyle’s New Olympic Bike Could Turn Out a Winner. Monty Brower. People Magazine, August 6, 1984.
Analytic Cycling.

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