Drafting 101

he most obvious benefit of running behind a pacer is that you don’t have to think about how fast to go. Want to run a 3:30 marathon? Tuck in behind the 3:30 pace leader, turn your mind off and run. But ever since Eliud Kipchoge’s elaborately detailed assaults on the two-hour marathon barrier began in 2017, runners have started to pay more attention to another benefit: reducing the effects of wind and air resistance. While these effects are more noticeable at 2:00 marathon pace than 3:00 or 4:00 pace, tucking in behind someone else on a breezy day can help at any pace.

But where, exactly, should you tuck? In a recent issue of the Journal of Biomechanics, French researchers led by Fabien Beaumont of the University of Reims Champagne-Ardenne analyzed the drafting positions used by Ethiopian star Kenenisa Bekele in the 2019 Berlin Marathon. In the first part of the race, he ran behind three pacers, either directly behind the one in the middle, directly behind one of the side pacers, or behind and between two of the pacers.

Using computational f luid dynamics, the researchers showed that running directly behind someone was the best option, reducing the drag force they experienced from 7.8 to 3.3 newtons. (A force of one newton is roughly equivalent to the weight of an apple.) Running

Tbetween and behind two runners, in contrast, only reduced the drag force to 4.8 newtons. It was better than nothing, but not as useful as tucking directly behind.

In practice, there’s a reason runners tend to gravitate to the between-and-behind position: it’s more comfortable to be able to see where you’re going, and feels like less of a trip hazard. In the new study, the researchers assumed Bekele was about 1.3 metres behind his pacers, but wind tunnel studies have estimated that you should be within a metre of the runners in front of you to maximize the benefits. That would feel uncomfortably close for most people, so you have to accept a trade-off among comfort, politeness and aerodynamic efficiency.

There’s one other nuance in Beaumont’s results, which is wellknown to cyclists but perhaps more surprising to runners. Thanks to the f low of air around a group of runners, the lead runner also experiences a slight decrease in drag force when someone tucks in behind. That means that any group of runners can benefit by taking turns leading and drafting. And it means that when you’re running in a pack for a prolonged period, you have no excuse not to take a turn at the front.