How close do you need to be to benefit from drafting?

Aerodynamicists at wheel brand Swiss Side have conducted CFD modelling and wind tunnel tests to study the effects of drafting. They developed computer models simulating two identical sized riders at separations between 0.1m and 20m, calculating the effect on the lead rider as well as the follower.

Although Swiss Side was primarily interested in triathlon riders, its results are equally significant for individual time trials in cycling events. Drafting behind motorbikes and cars is a constant cause of friction in races and riders have been fined and disqualified too for drafting others in time trials.

On a tri bike, riding at 45kph, Swiss Side’s CFD modelling showed an aerodynamic drag of 250 watts. That’s not considering other causes of power loss like rolling resistance and friction.

Sit 10cm behind another rider and you’ll save 90 watts of that 250W. And the wheelsucker also benefits the lead rider, whose drag reduces by 10W. The benefit to the lead rider continues even when the follower is further behind, only dropping to zero when the separation is 5m.

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But the following rider continues to gain an advantage even beyond 5m separation. At 10m behind, there’s still a 33.5W energy saving and this only drops significantly at around 20m. Swiss Side’ simulation assumed zero wind speed. It says that in the real world the wind will negate any advantage at lower separations.

Swiss Side followed up its computations with wind tunnel tests, using a smaller rider behind a larger one on a turbo, separated by 10cm. The following rider saved 65 per cent of the effort required to overcome aerodynamic drag at 45kph.

That’s larger than the CFD calculation, due to the difference in rider sizes and the wind blocking effect of the lead rider’s turbo. The following rider experienced an effective wind speed of 26kph, in line with calculations.