Velosplit

Cycling drafting calculator

How many watts does riding in a pace line save? Enter a speed and the group, and this works out the power to hold that speed alone and in the group, with the same physics as the Velosplit race planner.

The group size includes you. Gradient is negative downhill.

Power saved

118 W

35% less than riding alone

Alone
338 W
In the group
220 W

Your drag is 60% of riding alone, averaged over equal turns on the front.

How the saving is calculated

A rider in a slipstream meets slower, disturbed air, so their drag is a fraction of what it would be alone. The calculator multiplies your drag area (CdA) by that fraction and solves the same equation of motion as the planner, once alone and once in the group. Only drag changes: rolling resistance and gravity cost the same wherever you sit.

The fractions for single file come from wind-tunnel measurements of a four-rider team pursuit line, which found savings of 5, 45, 55 and 57 percent in positions 1 to 4 [1]. The leader saves a little too, because the rider behind fills the low-pressure wake. Those were elite riders in a tight line; earlier studies of small in-line groups put drafting riders at 70 to 50 percent of solo drag [2], so a looser line may save less. Riders past fourth wheel are held at the fourth rider's value.

Taking equal turns, you spend the same time in every place in the line, so your drag is the average over all of them. Sitting in, you stay at the back and get the lowest drag the whole time.

RidersSingle-file pace line, turns / sitting inTwo-abreast pace line, turns / sitting in
275% / 55%–
460% / 43%78% / 55%
654% / 43%67% / 45%
851% / 43%61% / 43%
1248% / 43%55% / 43%
2046% / 43%50% / 43%

Drag as a share of riding alone. The two-abreast values are Velosplit's own, unvalidated assumption. Two riders side by side pay over 6 percent more drag than one alone [3], so the front pair is put at solo drag, and each row behind is assumed to shelter like the same place in single file. That makes a small two-abreast line cost more than single file when taking turns.

Flat road against a climb

A 75 kg, 178 cm rider on the hoods needs 338 W to hold 40 km/h alone on the flat, and 241 W to hold 15 km/h up a 6% climb. Here is what the group saves in each case:

Group40 km/h, flat15 km/h, 6%
Second wheel behind one rider207 W (−131 W)234 W (−7 W)
4-rider single file, equal turns220 W (−118 W)235 W (−6 W)
4-rider single file, sitting in172 W (−166 W)232 W (−9 W)
8 riders two abreast, equal turns224 W (−115 W)235 W (−6 W)

On the flat most of your power goes into pushing air, so the group saves a lot. On a climb gravity takes most of it, and the same group saves only a few watts. That is why groups hold together on flat roads and split on climbs.

Limitations

The fractions assume a tight line, with wheels close together and the wind from straight ahead. A gap of a metre or two, a crosswind or a rider who sits up changes the saving a lot. Taking turns, the power shown is the average: you ride well above it on the front and below it behind. The calculator also leaves out the extra effort of pulling through and closing gaps when turns change.

You can plan a whole course in a group in the race power planner, which takes the same formation for the whole route. The methodology page has the rest of the model.

References

  1. Barry N, Burton D, Sheridan J, Thompson M, Brown NAT. Aerodynamic drag interactions between cyclists in a team pursuit. Sports Engineering. 2015;18(2):93–103. doi:10.1007/s12283-015-0172-8
  2. Blocken B, van Druenen T, Toparlar Y, Malizia F, Mannion P, Andrianne T, Marchal T, Maas GJ, Diepens J. Aerodynamic drag in cycling pelotons: new insights by CFD simulation and wind tunnel testing. Journal of Wind Engineering and Industrial Aerodynamics. 2018;179:319–337. doi:10.1016/j.jweia.2018.06.011
  3. Barry N, Sheridan J, Burton D, Brown NAT. The effect of spatial position on the aerodynamic interactions between cyclists. Procedia Engineering. 2014;72:774–779. doi:10.1016/j.proeng.2014.06.131