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How in-race motorcycles influence cycling races, and what to do about it

Bert Blocken Tekst Bert Blocken Gepubliceerd 07 November 2019

Bert Blocken, Professor Civil Engineering

Department of the Built Environment, Eindhoven University of Technology, the Netherlands

Department of Civil Engineering, KU Leuven, Belgium

Scientific Director of the Wind Tunnel at Eindhoven University of Technology, the Netherlands

During the 2019 Tour de France, professional cyclist Bauke Mollema expressed his grievances concerning motorcycles riding closely in front of cyclists and thereby providing substantial aerodynamic assistance, a concern he had aired before. His comments were both right and fair. Right not only because the aerodynamic benefit of drafting behind motorcycles is well-known, but even more so because our recent scientific research has shown that the benefits are much larger than previously expected. Fair because he also criticises the vicinity of motorcycles in cases where he himself gains an advantage.

Cycling races indeed contain many motorcycles, including camera motorcycles, sound motorcycles, and photographers’ motorcycles. These motorcycles can assume several positions relative to the cyclists. For simplicity, we considered three positions: motorcycle in front of the cyclist, next to the cyclist, and behind the cyclist. Our scientific research shows that in all three cases undesired aerodynamic effects occur, i.e. advantages or disadvantages for the cyclist due to the presence of the motorcycle. The current rules of the International Cycling Union (UCI), including their new guidelines which were issued in 2017, are unfortunately severely insufficient to avoid each of these three effects. Moreover, these guidelines are not strictly enforced.

Figure 1: Cyclists drafting behind group of motorcycles. Taken from Twitter, Thijs Zonneveld.

In recent years, not only cyclists but also cycling teams, journalists and TV commentators have often complained about the aerodynamic benefits some cyclists gain during races by drafting closely behind motorcycles ( see Figure 1). Indeed, it is well known that a cyclist riding behind a motorcycle benefits from the slipstream behind a motorcycle. However, it had not yet been investigated how large these benefits really are, and to what distances between motorcycle and cyclist they are still present. By combining wind tunnel tests and computer simulations (see Figures 2 and 3), we obtained the following findings:  A rider who cycles at a distance of approximately 2.5 meters behind a motorcyclist experiences up to 48 % less drag. If he or she would ride at 54 km/h without the motorcycle, the presence of the motorcycle will allow him or her to ride at about 67 km/h. This provides a time gain of 14.1 seconds every minute. This advantage becomes smaller as the distance increases, but at a distance of 50 metres a 7% reduction in drag is still measurable. This represents a gain of 1.4 seconds per minute at a reference speed of 54 km/h. Because races are sometimes decided by seconds, these differences can determine whether a rider wins or loses. The often-heard complaint that motorcycles can influence the outcome of races is therefore justified. Note that these time gains are calculated and measured without head wind, tail wind, or crosswind. If there is head wind, the time gains are larger. If there is tail wind or crosswind, the time gains are smaller. The stronger the crosswind, the more difficult it is for a rider to get into a motorcycle’s slipstream.

Figure 2: Experiment with a rider cycling behind a motorcycle in the wind tunnel of Eindhoven University of Technology. Photo: Bart van Overbeeke.
Figure 3: Side view of computer simulation of a rider cycling at a distance of 10 meters from a motorcyclist. The green and yellow colors in front of the rider show that the airspeed here is substantially lower than in front of the motorcycle. Photo: Bert Blocken

The aerodynamic effects when a motorcycle is riding next to a cyclist or behind a cyclist are less well-known. When a motorcycle is next to a cyclist, the cyclist and certainly the sponsors might be happy because this usually means they are on TV. However, from the aerodynamic point of view, this is not good news, because the motorcycle will increase the drag of the cyclist. The reason, simply stated, is that the airflow approaching the motorcycle deviates sideways along the motorcycle, thereby increasing the airspeed next to the motorcycle, where the cyclist is present who therefore experiences a larger drag. This drag increase depends on the lateral distance between the cyclist and motorcycle, but can reach up to 15% more drag. If the cyclist is riding at 54 km/h, this means he or she will lose about 3.3 s per minute. A cyclist can thus lose a time trial due to the nearby presence of a motorcycle.

When a motorcycle is riding behind a cyclist, an even more unexpected effect occurs: the drag of the cyclist will decrease. The reason is directly a result of the governing equations of fluid flow, i.e. Newton’s second law applied to fluids: the Navier-Stokes equations. Below the speed of sound, every object moving in air, will not only change the flow behind it, but also in front of it. In other words, simply stated, the motorcycle is pushing the air in front of it forwards, and thereby the cyclist experiences less suction, so with the same exerted power, the cyclist can go faster. When a motorcycle is at 1 m distance behind a cyclist, the cyclist’s drag reduces by about 3.8%. At a speed of 54 km/h, the corresponding time gain is 1.1 s per minute.

It is clear that the largest benefits are obtained when drafting behind one or more motorcycles. The research results of this study have been reported and discussed on many TV channels, in newspapers, blogs, and podcasts. Most have indicated that these effects are even larger than they expected. So what are the reactions from cycling organisations? We have informed the UCI about these results, but they have chosen to provide no response. The ASO however, at the start of the 2019 Tour de France, informed all motorcycle riders about our study and asked them to give special attention to keeping large distances in front of the riders. The CPA (Cyclistes Professionnels Associés), though its spokesperson and general secretary Laura Mora, has indicated its satisfaction that finally scientific research has been able to provide numbers describing the actual advantages, and that it will use these numbers to engage in a discussion with the UCI, hoping to bring about better regulations.

One could wonder which measures could be taken in order to avoid motorcycles riding very close to riders. Some have argued that cycling is a TV sport, which is true, and that it is therefore unavoidable to have motorcycles near the riders. Others have indicated that the current state of science and technology should be able to provide proper alternatives. As stated by former cyclist Thomas Dekker in one of this year’s episodes of the Avondetappe on Dutch TV: “In an era where we can take high-resolution images of the moon, which is almost 400,000 km from the Earth, I cannot believe good TV recordings can only be obtained from a motorcycle riding directly in front of a cyclist”. Indeed. The problem is that solutions will only be implemented when the organisations involved, especially the UCI, are willing to implement them. Given past experiences, that might take quite a while.


Cover Photo: Chris Auld