A following car influences cyclist drag: CFD simulations and wind tunnel measurements

Abstract

It is well-known in elite cycling that a cyclist riding behind a car experiences a substantial reduction in aerodynamic resistance or drag. However, the upstream effect by a following car on the cyclist in front of it is not well-known and has, to the best of our knowledge, not yet been reported in the scientific literature. It is also not taken into account in elite cycling, as for individual time trials, the rules of the International Cycling Union (UCI) only specify a minimum distance between rider and car of 10m because of safety reasons. Furthermore, during actual races, this limit is often not kept and not strictly enforced. Nevertheless, during individual time trials, there is always at least one, but often more, following cars, potentially influencing the cyclist drag. This study presents the results of CFD simulations and wind tunnel measurements to ascertain and quantify the upstream effect by a following car on the drag of the cyclist in front of it. CFD simulations are performed based on the steady-state Reynolds-Averaged Navier–Stokes equations with the standard k–ε model for closure. The simulations are validated based on a series of wind tunnel measurements. The drag reduction for the cyclist ranges from 3.7% over 1.4% to 0.2% for realistic separation distances of 3, 5 and 10m, respectively. For a typical 50km individual time trial, the potential time reduction by exploiting this effect (e.g. by a car following the rider at this short distance versus no car behind) is 62.4s, 24.1s and 3.9s, respectively. As elite cyclist time trials are often won based on seconds or sometimes even less, these differences can be decisive for who wins the stage. Therefore, it is recommended that the UCI not only raises its current minimum separation distance of 10m to at least 30m, but also strictly enforces it, to avoid this unwanted aerodynamic effect that can influence the outcome of the race.