How fast is the "supertuck" position, and why was it banned
Mon 12 April 2021
Following the initiative of the Union Cycliste Internationale (UCI) to enhance rider safety by further restricting rider positions, we have looked into the aerodynamics of the debated "supertuck" position to find out how efficient it really is.
The "supertuck" position is a road cycling descent position where the rider is sitting on the top tube while keeping the chest close to the handlebar, thereby minimizing the frontal area while coasting at high speed. This gives aerodynamic advantages but might also pose a safety risk to the rider. In order to lower the torso, the rider has to shift much of his bodyweight to the front wheel and thereby looses both stability and breaking power. The freedom of movement is also dramatically reduced, compromising the ability to react quickly to obstacles by adjusting the weight distribution on the bike. Still many riders choose to use it in critical parts of the race, and there are mixed opinions on the safety risk of such irregular positions among pro riders.
A famous example is Chris Froome's stage win at the 8th stage of the 2016 Tour de France when he attacked over the top of Col du Peyresourde and kept the lead on the following descent pedalling hard in a supertuck position. However, variations of this positions had already been practiced for a few years in the pro pelotons, noticeably by Matej Mohoric and Peter Sagan. From an aerodynamic perspective the position clearly gives a lower frontal area than a standard tuck position, but what about the drag coefficient and net power savings?
We tested the supertuck position on AeroCloud versus a standard drops position at a wind speed of 72 km/h. For simplicity, the bicycle was not included in the simulation. We used 3D-rigging to create the two poses from a single 3D-scan of a pro-rider. In the baseline position the rider is using the allowed contact points with the bike, at the seat and the drop bars. This could be considered a safe decent position. In the supertuck position the rider is sitting on the top tube, while using the same contact points at the bars. The position of the head is kept unchanged. Some riders also hold their hands on the top of the bar, which could be considered even more unstable.
The CFD simulation shows that the supertuck position results in a drag reduction of 26% for the rider isolated. The frontal area is only reduced by 16% so the more compact position also has a considerably lower drag coefficient compared to the standard position. As seen from the centreline velocity plot the compact position creates a much smaller wake compared to the standard position. In practice the rider sitting on the seat will have to put 237 watts more into the pedals at 72 km/h to keep up with the supertuck rider, and that difference will increase with the cube of the speed.
Based on these results it is not surprising that the riders choose to seek the most aerodynamic position in races. The difference here is quite clear, but there are still a range of different legal positions in between the ones we have demonstrated here. Using AeroCloud combined with 3D-scanning and rigging is an efficient methodology to evaluate the aerodynamic performance of different body postures in a range of high-speed sports, without spending a day in a wind tunnel.