Aerodynamic benefits for a cyclist by drafting behind a motorcycle

Abstract

Motorcycles are present in cycling races for reasons including television broadcasting. During parts of the race, these motorcycles ride in front of individual or groups of cyclists. Concerns have been expressed in the professional cycling community that these motorcycles can provide aerodynamic benefits in terms of drag reduction for the cyclists drafting behind them. However, to the best of our knowledge, no information about the extent of these benefits is present in the scientific literature. Therefore, this paper analyses the potential drag reduction for a cyclist by drafting behind a motorcycle. Wind tunnel measurements and numerical simulations with computational fluid dynamics were performed. It was shown that drafting at separation distances d = 2.64, 10, 30 and 50 m can reduce the drag of the cyclist down to 52, 77, 88 and 93% of that of an isolated cyclist, respectively. A cyclist power model is used to convert these drag reductions into potential time gains. For a non-drafting cyclist at a speed of 54 km/h on level road in calm weather, the time gains by drafting at d = 2.64, 10, 30 and 50 m are 12.7, 5.4, 2.7 and 1.6 s per km, respectively. These time differences can influence the outcome of cycling races. The current rules of the International Cycling Union do not prevent these aerodynamic benefits from occurring in races.

Impact of a motorcycle on cyclist aerodynamic drag in parallel and staggered arrangements

“…The time step of 0.002 s was selected such that the Courant-Friedrichs-Lewy (CFL) number was equal to unity or below unity in the area between the motorcycle and the cyclist. A time step convergence analysis confirmed the suitability of this time step [8]. The number of time steps required to obtain a constant moving average of the sampled drag values was 8000.…”

“…Combined with our previous work on the aerodynamic impact of a nearby motorcycle on cyclist drag [7,8], we now have a strong overall picture of the effect of a motorcycle, which can be in front, behind or to the side, on the aerodynamic drag of a cyclist. Each of these motorcycle positions has a clear effect that can change the outcome of a race.…”

“…Details of the models and the wind tunnel can be found in [8,13]. The WT set-up was described in an earlier publication [8] and the description is summarized here for completeness. The models were placed on an elevated sharp-edged smooth horizontal plate to limit boundary layer development.…”

“…The computational settings are identical to those in [8] and are summarized here for completeness. At the inlet, a uniform velocity of 15 m/s was imposed that represented the riding velocity.…”

“…Blocken et al [7] demonstrated that a motorcycle following a cyclist can provide a drag reduction for the cyclist of 3.8, 0.3 and 0.1% for separation distances of 1, 5 and 10 m, respectively. More recently, Blocken et al [8] focused on quantifying the aerodynamic benefits for a cyclist by drafting behind a motorcycle. While it was well known in professional cycling that drafting behind a motorcycle can yield large drag reductions, the actual values had not yet been determined.…”