Anybody who has bicycled with a group on a windy day has enjoyed the benefit of sheltering behind the person in front of them. Similarly, it is well-known in cycling that drafting riders (cyclists who ride behind another cyclist) benefit from the slipstream of the front rider. That is why in races, like the Tour de France, cyclists try to ride in a line to save strength for the final part of the race.
A collaborative research effort was conducted to study cycling aerodynamics with our colleagues at KU Leuven (Thijs Defraeye and Peter Hespel), the Flemish Cycling Union (Erwin Koninckx), ETH Zurich (Jan Carmeliet) and the Dutch-German Wind Tunnels (Eddy Willemsen). We are all now continuing these studies with the ANSYS CFD code to look into the aerodynamic effects in time trial races and in full peloton sprints.
We decided to conduct our own research by modeling a group of real cyclists, since there was a lack of consensus in previously published scientific research about the effect of the trailing rider on the leading rider. While some studies suggest there is no effect, others argue that the effect might be substantial, maybe up to 5 percent*. However, previous studies were conducted based either on field tests, which are very difficult to measure, or on CFD studies of simplified models of human bodies, such as cylinders.
Using ANSYS CFD, we performed — to the best of our knowledge — the first CFD study on aerodynamics of multiple cyclists, based on geometry of real cyclists’ bodies. Our results not only confirmed a 30 to 35 percent reduction in air resistance for the trailing (second) rider, but they revealed that the air resistance of the first (leading) rider decreases, by about 2 to 2.5 percent. We successfully validated these simulations with a few carefully selected wind tunnel measurements on two cyclists in the Dutch-German Wind Tunnels in Marknesse, The Netherlands.
So how new is this result? The effect itself is just physics — it has always existed. But to the best of our knowledge, this is the first time that the type and size of this effect has been assessed based on studies on real cyclists’ bodies (not cylinders), and that its size has also been publicly announced.
And, how significant is this 2.5 percent reduction in drag? Because time trial races are often won or lost in a matter of seconds, the 2.5 percent difference on the leading rider can be extremely valuable when competing in a cut-throat environment. Here is what that percentage means in terms of distances and time at 54 km/h (typical sprinting or time-trial speed)*:
- 1 second gain on a distance of 1 km
- 10 seconds gain on a distance of 10 km
- 50 seconds gain on a distance of 50 km (e.g. time trial distance in Tour de France)
- 15 m gain on a distance of 1 km
- 150 m gain on a distance of 10 km
- 750 m gain on a distance of 50 km (e.g. time trial distance in Tour de France)
Some might argue that 2.5 percent for the leading cyclist is only a minor difference — particularly when compared to the 30 to 35 percent benefit of the trailing cyclist. However, remember that professional cyclists, cycling teams, bicycle manufacturers and race clothing manufacturers are all spending large amounts of funding and effort these days to achieve gains on the order of 0.1 to 1 percent. Efforts like these remain very valuable and should certainly be continued and pursued, and indeed can be decisive. But this also provides a reference to put the 2.5 percent benefit you could get “for free”, as shown by this study, in the right perspective. Note that the late Belgian top-tier cyclist Frank Vandenbroucke, just before each race, cut off the tiny side edges of the small number plate, fixed to his bicycle, since he was focused on reducing weight and aerodynamic resistance (in this case, estimated to provide a gain of the order of 0.1 to 0.01 percent).
Note that the 2.5 percent applies for cyclists that we tested with identical body shape and size. When the second cyclist is larger/wider/taller than the first one, the effect (reduction in air resistance for the first one) will be larger, and vice versa: A second cyclist who is smaller/shorter than the first will have an effect that is less than 2.5 percent. This means that for time-trial races, where the cyclists are all different shapes and sizes, cyclists order plays a very important role. As far as we know, this knowledge is not currently taken into account; it could potentially change the strategy and the way teams prepare for a race.
One way for cyclists to take advantage of these findings in sprints, theoretically of course, is to position the largest team member just behind the front runner during the final sprint, as the induced gain of 2 to 3 percent could make the difference between winning and losing. But we realize that, in actual practice, many other aspects play a role. Therefore, we expect that the benefits of this study will be limited to time trials or team pursuit.
These results have opened the door to many more investigations. A system of several cyclists under varying wind conditions may give some insightful recommendations about the order of the cyclists as well as the rider’s position on the bike in order to minimize pressure/drag on the entire team and to maximize the pressure applied to competitors’ cyclists.
Editor’s Note: Guest blogger Bert Blocken is a professor in urban physics at Eindhoven University of Technology, The Netherlands, and worked in concert with Thierry Marchal, industry director at ANSYS in Belgium, to produce this post.