How many watts you save in the peloton: quantifying the draft after Blocken (2018)

aerodinámicaJuly 23, 20267 min readBorja Alfaraz

Drafting

7-10 %

Of an isolated rider’s drag is what a cyclist experiences deep in a large peloton (Blocken et al., 2018).

In power terms: holding 45 km/h on the flat drops from ≈ 424 W solo to ≈ 70 W inside the bunch.

Abstract. This article quantifies the effect of drafting on power demand as a function of position within the group. Using the drag fractions documented by Blocken et al. (2018) through CFD simulation and wind tunnel testing, a physical model estimates the power required to hold a racing speed, from the isolated rider to the interior of a large peloton.

In mass-start racing, drafting is not a tactical detail: it is the dominant energetic factor. Aerodynamic resistance accounts for most of the power at racing speed, and riding in the wake of others reduces it drastically. Quantifying that reduction explains why position in the group decides more races than individual maximal power.

Physical mechanism

A moving cyclist leaves behind a wake of air with reduced dynamic pressure, partly dragged along in the direction of travel. A rider placed in that wake meets a lower relative air speed, so the drag force to overcome falls. The effect compounds: the more riders surround one, the more shielded from the free stream, and the lower the effective drag coefficient.

Blocken et al. (2018) simulated a 121-rider peloton with CFD and validated it in the wind tunnel. Their key finding: inside the group, a rider's drag can fall to 5-10% of that of an isolated rider at the same speed, far lower than earlier small-group studies suggested.

Quantification by position

Applying those drag fractions to the AthletePro physics engine, the power needed to hold 45 km/h on the flat (road-position CdA 0.32, rider-bike mass 78 kg) by position is:

Isolated (solo)

424 W

On another rider’s wheel

289 W

In a paceline / echelon

193 W

Deep in a large peloton

70 W

Figure 1. Power required to hold 45 km/h on the flat by position in the group, computed with the AthletePro physics engine and the drag fractions of Blocken et al. (2018): on a wheel ≈ 65%, paceline ≈ 40%, deep in the peloton ≈ 8% of isolated drag.

The reading is stark: on a single rider's wheel, some 135 W are already saved (32%); deep in the peloton, the saving reaches ≈ 350 W (over 80%). A well-placed rider can hold 45 km/h on the power another would barely need to freewheel alone.

Operative rule. Position in the group yields more watts than any aerodynamic component. No deep wheel or TT helmet saves the 300+ W saved by sitting well sheltered. In group events, placement tactics are the first “marginal gain”.

Caveats

The front of the group does not benefit. The leading riders face drag close to that of an isolated rider; the saving concentrates from the third or fourth row inward.
Wind changes everything. In a crosswind, the peloton splits into echelons and the useful wake shifts; being outside the echelon is equivalent to riding alone.
The saving is not free in nervous energy. Holding position demands constant attention and micro-efforts; the cost is real, even if it does not appear in the power equation.

In AthletePro, drafting is modelled as a reduction of the effective CdA per segment, so that the power plan of a group event reflects the expected position rather than that of an isolated rider.

References

Blocken, B., van Druenen, T., Toparlar, Y., et al. (2018). Aerodynamic drag in cycling pelotons: new insights by CFD simulation and wind tunnel testing. Journal of Wind Engineering and Industrial Aerodynamics, 179, 319-337.
Kyle, C. R. (1979). Reduction of wind resistance and power output of racing cyclists and runners travelling in groups. Ergonomics, 22(4), 387-397.
Broker, J. P., Kyle, C. R., & Burke, E. R. (1999). Racing cyclist power requirements in the 4000-m individual and team pursuits. Medicine and Science in Sports and Exercise, 31(11), 1677-1685.