Executive summary

The fact that different professional cyclists use very different hill descent positions indicates that there is no consensus in the peloton on which position is really superior, and that most cyclists did not test different positions, for example in wind tunnels, to find which position would give them the largest advantage. With two different and independently applied research methods, wind-tunnel testing and CFD simulations, that each yield the same conclusions, we prove which cyclist hill descent position is aerodynamically superior, among a sample of 6 positions used by professional riders in the past. We also discuss which positions are superior from the viewpoint of safety and power generation. The results show that the infamous “Froome” position during the Peyresourde descent of the Stage 8 of the 2016 Tour de France is not aerodynamically superior to several other positions. In fact, initial hesitation and an even less aerodynamic descent positions adopted by most the leading chasers caused Froome to win stage 8 of the Tour. The Froome position is not faster, not safer and not more powerful compared to other positions. Other positions are up to 8% faster, provide more equal distribution of body weight over both wheels, and allow more power generation.

Research team

This project was led by Bert Blocken of the Eindhoven University of Technology in the Netherlands and KU Leuven (Leuven University) in Belgium, and performed together with Thijs van Druenen and Yasin Toparlar of the Eindhoven University of Technology, the University of Liège in Belgium (Thomas Andrianne) and ANSYS International (Thierry Marchal).

What?

An independent and unfunded investigation of the aerodynamic performance of different hill descent positions in cycling. In addition, comments are given about power generation and safety associated with each position. Some misconceptions are explained.

How?

By two different and independent methods applied by researchers at three different universities and the world leader in Computational Fluid Dynamics (CFD) software. CFD simulations are performed with the ANSYS Fluent CFD code and wind-tunnel testing at the Wind Tunnel Laboratory of Université de Liège.

Past performance

In previous years, some of the members of this research team also investigated:

The aerodynamic benefit for a first cyclist followed by a second one:

link 1: Europhysics News

link 2: scientific article

The aerodynamic benefit for a cyclist by a following car:

link 1: TU Eindhoven press release

link 2: scientific article

The aerodynamic benefit for a cyclist by a following motorcycle:

link 1: TU Eindhoven press release

link 2: scientific article (Open Access)





Why this new investigation?

This work was incited by the end of stage 8 of the Tour de France (2016), in which probably the most remarkable item was the way in which Chris Froome descended the Peyresourde towards the finish line, took a 13” lead on Nairo Quintana, Adam Yates, Bauke Mollema and his other competitors, from which he broke away just before the top of the Peyresourde, and won the stage. The figure below shows Chris Froome during his descent, sitting on the top tube, with his chest on the handlebar. During the descent he was alternatively pedaling and holding the legs static. The descent covered 15.5 km, from an altitude of 1569 m to 632 m. Froome reached an average speed of 62.5 km/h and a maximum speed of 90.9 km/h. Some attributed this to the so-called “superior aerodynamic descent position” adopted by Froome. Within 5 minutes after the finish, I received three emails, from Yasin Toparlar, Thierry Marchal and Thomas Andrianne, suggesting we investigate this to see whether this particular position is really aerodynamically superior.

Figure 1. Froome during descent of the Peyresourde, stage 8, Tour de France 2016. (sporza.be)





Different hill descent positions

Different cyclists use very different hill descent positions. Figure 2 below shows (a) Chris Froome; (b) the late Marco Pantani; (c) Vincenzo Nibali in a position adopted also by many others; (d) Nibali in his particular descent position; (e) Fabian Cancellara in a position required for a technically difficult descent with many sharp bends; and (f) Peter Sagan in a position we will denote as “Top tube safe”. The latter name is chosen because as opposed to the Froome position, in this position the body weight is distributed more equally over both wheels. These different descent positions, which are still used by professional cyclists in several races, indicate clearly that there is no consensus in the peloton on which position is really superior, and that probably most cyclists did not test several positions in detail by either wind-tunnel investigation or CFD simulation to find which position would give them the largest advantage.

Figure 2. Different hill descent positions by professional riders: (a) Chris Froome; (b) the late Marco Pantani; (c) Vincenzo Nibali; (d) Vincenzo Nibali; (e) Fabian Cancellara; (f) Peter Sagan. (photos from sporza.be)





Wind-tunnel testing

The first method was wind-tunnel testing. We scanned a cyclist in four different positions, had models made by CSC cutting at scale 1:4 and the team tested those in the wind tunnel in Liège. The tests were performed at 216 km/h for Reynolds similarity with the physical reality at full scale and 54 km/h (= 15 m/s). 216 km/h is a hurricane of category four, so the models needed to be reinforced with vertical bars in the wheels. Corrections were made to remove their contribution to the aerodynamic resistance.

Figure 3. Cyclist models in the wind tunnel.





Our earlier research indicated that at and above 15 m/s, a sufficient degree of Reynolds number independence is obtained and the same drag areas result at 20 and 25 m/s. The results in terms of drag area are shown in figure 4 below. The drag area is the product of frontal area A and drag coefficient Cd that both appear in the equation of the drag force:

Figure 4. Drag area (ACd) obtained by wind-tunnel testing of four cyclist models.





Computational Fluid Dynamics (CFD) simulations

The second method was computer simulation with Computational Fluid Dynamics (CFD). The same four positions and two additional positions were analyzed with ANSYS Fluent CFD software using extremely high-resolution models of 36 million calculation cells with sizes down to 20 micrometer (= 0.020 millimeter) close to the body of the cyclist, which is needed for accurate and reliable results because the very thin laminar sublayer close to the surface of the cyclist and the bicycle needs to be resolved. These cells are so small they would be impossible to see in reality. However, after more than 6 months of testing, we found that this resolution and also the typology of the grid further away from the body has a very large impact on the accuracy of the results. Based on this experience, we developed the final grids.

Figure 5. Computational grid for "Froome" position in vertical centerplane. Total count is 36 million cells.

Figure 6. Part of computational grid for "Froome" position. Total count is 36 million cells.

Figure 7. Zoom of computational grid for "Froome" position. Total count is 36 million cells.





We combined these grids with application of the Transition SST-k-omega model whose capabilities in boundary layer transition can only be fully exploited by the type of extremely high-resolution grid as made here. We did this for twenty cyclist models but only results for 6 of them are reported here.

The wind-tunnel and CFD results are combined in Figure 8. Observations:

Wind tunnel and CFD give exactly the same order of best positions in terms of lowest aerodynamic resistance: “Pantani” first, “Back down” second, “Froome” third, “Back horizontal” fourth.

Wind tunnel and CFD agree very well (within error range for first three positions) thanks to both the careful generation of a very high-resolution grid and the careful application of the Transition SST-k-omega model.





Figure 8. Drag area (ACd) obtained by wind-tunnel testing and CFD simulation of four cyclist models.





Two additional positions

Because the comparison of the two independent methods showed that both give the same order of positions and that both methods are reliable, the study was extended with two other descent positions. The final results are shown in the figure below. Note that most of the percentages are larger than the deviations between wind-tunnel results and CFD results, as was shown above. These percentages are based on the assumption that:

Either all cyclists ride down a very steep hill without pedaling

Or all cyclists are pedaling with the same power provision

Comments about this further down.





Figure 9. Comparison of 6 positions in terms of speed, either without pedaling or when all cyclists are pedaling with the same power provision.





Why are these results so different?

One might wonder why these results are so different from each other and from what one might intuitively expect. The aerodynamic resistance or drag force is determined by the equation shown above, in which four factors are discerned: the cycling speed, the frontal area of the cyclist, the drag coefficient of the cyclist and the air density. Assuming the same cycling speed and air density, the frontal area A and drag coefficient Cd remain. The higher the values of A and Cd, the larger the drag force. The main reason for the difference in results in this study is the different drag coefficients. The drag coefficient is the result of the pressure distribution on the body of the cyclist. The pressure distribution in all positions tested is very complex and very different. Position of separation points is very important here. It cannot be obtained by intuition but should be studied by computer simulation. This difference causes the different drag forces. The pressure coefficients on the cyclists and bicycles as found by computer simulation are shown in the figure below.





Figure 10. Comparison of 4 positions in terms of speed (either without pedaling or with same power generation) and indication of pressure coefficient at cyclist and bicycle surfaces.





What are the consequences in time difference?

We cannot re-create the exact Peyresourde descent because we do not have the cycling speed and time differences between Froome and chasers at every second. We also do not have the lead chaser descent position for most of the descent (was not broadcasted as focus went to Froome). However, as a crude example, we calculate the time difference that would be obtained for the Peyresourde descent (15.5 km) based on the average speed by Chris Froome during this descent, 62.5 km/h. In reality, of course, the cycling speed varies a lot during the descent and pedaling is important, but these numbers give a rough indication of how much time can be gained, purely from the the aerodynamic point of view, by the different positions:

Position “back upwards”: (+ 1’17”)

Position “back horizontal”: (+ 8”)

Position “Froome”: 14’24” (+ 0”)

Position “back down” (- 23”)

Position “Pantani” (- 46”)

Position “Top tube safe” (- 1’07”)

Given the fact that the chasers adopted positions mainly in line with "back horizontal", this time difference is quite well in line with the advantage Chris Froome obtained in part of his descent.





Four conclusions

1. Froome did not win because his descent position was aerodynamically superior.

The position used by Chris Froome in stage 8 of the Tour de France of 2016 was not aerodynamically superior. Several other positions are faster. Chris Froome won the stage because he accelerated before the hilltop and already had a substantial lead when he reached the hilltop. When you have a lead when you start descending while others are still climbing at lower speed, your lead only increases until the others start descending. Second, the chasers, like Nairo Quintana, even when they were descending the hill, hesitated and were riding in the very non-aerodynamic position “Back upwards” for a very long while, see photos below. Later, they adopted the position "Back horizontal" and some for a brief while only position "Back down". So yes, Froome’s position was more aerodynamic than that of the chasers, but not aerodynamically superior all-round. Descending “Back down” or “Top tube safe” would have allowed the chasers maybe to catch up with Froome and prevent him from winning and taking the yellow jersey.

Figure 11. Position by Froome and by chasers during part of the descent.





2. “Top tube safe” position is faster ànd safer.

The new research shows that, if pedaling of the cyclist is not needed, the position “Top tube safe” is the best position of the 6 positions tested. It is also the safest position of the two "Top tube" positions as it allows a fairly even distribution of the weight of the cyclist over the two wheels.

3. Even considering pedaling, position “Froome” is not superior.

When pedaling is important, we should focus on positions that allow pedaling of the cyclist with strong power generation. Pedaling is hardly possible in position “Pantani” and position “Top tube safe”. But pedaling with larger power output will probably be possible in position “Back down” and certainly in position “Back horizontal”, which makes these positions more suitable for descending than position “Froome”, in which pedaling is possible but not with large power output.

4. Do not take risks for no gain.

Beyond the scientific results supported by this investigation, there are professional or recreational cyclists who might be putting their chances of victory and maybe even their life at risk by adopting a more dangerous position with the injustified hope to improve their aerodynamics. It is not unlikely that Alberto Contador lost Paris-Nice in 2017 to Henao by adopting the Froome position. With the three major tours, the Giro, the Tour and the Vuelta, just around the corner, many might be tempted to adopt this “Froome” position, taking risk for no gain . We strongly advise against that, from the viewpoints of speed, safety and power. We also encourage professional cyclists willing to consider new positions to first approach us so that we could safely investigate them on the computer and in the wind tunnel before taking any risk on the road. Send an inmail by Linked In to Bert Blocken.

QUESTIONS AND ANSWERS

Q1: How reliable are these results?

We used two independent methods (wind-tunnel testing and CFD) that both gave the same results, in terms of ranking of positions. It was not easy to get reliable results, both in wind-tunnel testing and CFD. Both require a lot of expertise. It took us 10 months to get both to the very high level of accuracy we have now, especially in the simulations. That is a big investment, but it has given us unique expertise and now we are ready to do the tests and simulations with the same accuracy but much faster.

Q2: I do not believe your results.

That is ok. It is not our intention to convince the world. We had two independent methods giving the same results. We publish our results in top scientific magazines based on anonymous peer review. We respect all beliefs, also when they are wrong. Many people (even some professionals in cycling) reason based on intuition. Intuition is often a bad advisor in aerodynamics. The debate between intuition and science can and will go on for years and decades. It is an interesting one, and keeps our work very relevant.

Q3. Team Sky is a scientific team that tests everything, so your results cannot be true.

We get this question all the time. Sure team Sky do a lot of testing. Every decent team does. But no team can test everything, not even if they would have unlimited budget. It took us – as professionals with long-standing expertise in wind-tunnel testing and CFD simulations – 10 months to get this work done as we reported it now. So personally, we believe Chris Froome when he said after stage 8: “I did not practice this, it was a hunch of the moment”. No team can test everything. Aerodynamics is too complicated for this. Mankind has been studying aerodynamics for more than 2000 years, with thousands and in recent decades millions of researchers all over the world. Still we do not understand everything. Scientists do not even agree on the definition of turbulence (!). So research in cycling aerodynamics will continue for decades, maybe centuries. Sky are excellent but also terrestrial. They cannot test everything. Nobody can.





ACKNOWLEDGMENTS

This work was sponsored by NWO Exacte en Natuurwetenschappen (Physical Sciences) for the use of supercomputer facilities, with financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Netherlands Organization for Scientific Research, NWO).