BY BERT BLOCKEN. Realizing a successful long breakaway, as a lone rider, against a chasing peloton or a smaller group of chasing riders, is very challenging. Not many riders in the professional cycling peloton are capable of such an undertaking. These performances appeal to our imagination and our emotions. But also from an objective, scientific point of view, these performances are exceptional.

Written by Bert Blocken, full professor Building Physics at KU Leuven and Eindhoven University of Technology (TU/e). Aerodynamic advisor to Cycling Team Jumbo-Visma. His areas of expertise are Urban Physics, Wind Engineering and Sports Aerodynamics. He tweets with handle @realBertBlocken.

Rewind to Saturday 13 July 2019. Tour de France, Stage 8. Our fellow Belgian countryman Thomas De Gendt, in the breakaway all day long, abandons his fellow breakaway rider Alessandro De Marchi at 13 kilometres from the finish line and embarks on an impressive solo ride. Thibaut Pinot and Julian Alaphilippe, the latter targeting the famous yellow jersey, give their maximum effort in chasing De Gendt, but De Gendt wins the stage with a six second lead. A prestigious and impressive stage win, after almost two hundred kilometers in the breakaway. TV commentators and journalists run out of superlatives.

Fast forward to two and a half months later. Saturday 28 September 2020. World Championships in Yorkshire. Dutch female rider Annemiek van Vleuten starts a spectacular solo breakaway at 104 kilometres from the finish. She systematically increases her lead, for the peloton never to see her again. “Masterly”, “phenomenal”, “historical” are the news headlines of the day and the newspaper headlines the day after.

From the more distant past, we remember the breakaways of key riders like the Frenchman Jacky Durand and Belgian Ludo Dierckxsens, who were also known for their nearly endless fighting spirit.

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These efforts and the subsequent victories appeal to our imagination. But how exceptional are these performances really? To answer this question, we combined our research results from wind tunnel testing, computer simulations and a mathematical cycling power model.

Breakaway rider against chasing peloton

We note that it is often impossible to predict an entire race with fysical-mathematical models. We do try this, but there are too many unknown parameters, such as the individual positions of every rider in the peloton as a function of time, the wind speed and wind direction as a function of space and time… Every research effort, whether in the wind tunnel or by computer simulation, is therefore based on some idealized situations and some assumptions. We often assume either no cross-wind or constant cross-wind, while evidently the wind speed and direction can change throughout a race. We also assume some specific peloton configurations, while a real cycling peloton is highly dynamic and variable. The translation of our model results to reality should take into account these assumptions. Nevertheless, the modeling efforts often provide predictions that are close to reality.

In a research consortium with colleagues of KU Leuven, Eindhoven University of Technology, the University of Liège and our USA partners Ansys and Cray HPC Supercomputing, we investigated the aerodynamics of a tightly packed full peloton by wind tunnel tests and computer simulations (see animation below). These computer simulations – the largest CFD simulation in sports in terms of computational demand – showed that the air resistance (so not the total resistance) in a tightly packed peloton of 121 riders can go down to less than 10 percent of a cyclist riding alone. Averaged over all riders in the peloton, the air resistance of a peloton rider is about 20 percent of that of a solo rider. If we add the rolling resistance between tires and road, the wheel-bearing friction and the friction in the drive train, then the average aerodynamic resistance in the peloton, at a speed of 50 km/h, is about 30 percent of that of a solo rider. So if a single rider escapes, the chasing peloton is tightly packed and every rider in the peloton takes an equal share of the total effort by the peloton, one can state that the solo rider has to deliver about three times more power and energy than those in the chasing peloton.

However, a peloton is not always tightly packed, certainly not when taking bends or in climbs or descents, or on very narrow roads. When the peloton stretches out, the air resistance for the riders in the peloton will increase substantially. On the other hand, not every rider in the peloton works equally hard. Riders planning a later breakaway, to chase the current breakaway, might hide in the belly of the peloton until they launch their own breakaway. So this number of 30 percent as an average for riders in the peloton remains a reasonable assumption.

Breakaway group chasing the solo rider

Let us now assume that the chase is not performed by the whole peloton, but only by a group of ten riders. From our earlier research work, we know that the average air resistance in a paceline of nine to ten riders is about 50 percent that of a solo rider (see table on the right). Including all resistances this becomes 60 percent. So in this case, one can conclude that the solo rider has to provide 1.7 more power than the chasers in order to keep the same lead time. If he or she does not achieve this higher power, the chasers will evidently narrow the gap with the breakaway rider.

If the chase is only engaged by two riders, such as in the final stage of the breakaway by Thomas De Gendt in stage 8 of the 2019 Tour de France, and both riders work equally hard in the chase, then the chasing riders have about 80 percent of the air resistance of the solo rider, and about 90 percent of the total resistance of the solo rider. In this case, Thomas De Gendt has to overcome 10 percent more resistance than the chasers to keep his time lead.

These are evidently calculations based on a number of underlying assumptions, but the results are very large percentages. The order of magnitude of these percentages do not change if these assumptions change within reasonable boundaries.

Therefore, based on scientific research, we can substantiate the statements in the media that the above-mentioned efforts and victories by breakaway kings and queens are exceptional. These are top performances of top athletes that did not only shape and color the race of the moment, but also contribute to the large public admiration for the beautiful sport that cycling is. They also contribute to the well-deserved admiration that we have for the athletes realizing these efforts. To such great athletes, we, as scientists, engineers and cycling fans, take the deepest bow.

Acknowledgements

I would like to thank the cycling part of my research team at TU Eindhoven and KU Leuven (names in sources below). I thank the partner organizations University of Liège, Ansys and HPC Cray Supercomputing. I’m grateful to the long-time partners Cycling Team Jumbo-Visma, Equipe Cycliste Groupama-FDJ, Cycling Ireland and Paralympics Ireland, for the collaboration that has co-inspired this work. I acknowledge NWO, the Dutch Organization for Scientific Research and the companies SURF Supercomputing, CustomCompany, Tenax and FlexForm.

Sources

• ^{Blocken B, van Druenen T, Toparlar Y, Malizia F, Mannion P, Andrianne T, Marchal T, Maas GJ, Diepens J. 2018a. ‘Aerodynamic drag in cycling pelotons: new insights by CFD simulation and wind tunnel testing’ Journal of Wind Engineering & Industrial Aerodynamics 179: 319-337}
• ^{Blocken B, Toparlar Y, van Druenen T, Andrianne T. 2018b. ‘Aerodynamic drag in cycling team time trials’ Journal of Wind Engineering & Industrial Aerodynamics 182: 128-145}