Rotational drag – the hidden resistance!

posted by Fabienne Dirksen
May 09, 2017

Swiss Side in collaboration with DT Swiss and the launch of the new DT Swiss ARC 1100 DICUT aero wheels releases new research into aerodynamic drag. It’s time for a new definition of aerodynamic drag and exploiting this untapped potential. Did you know that 25% of the “real world” aerodynamic drag of wheels is not measured in the wind tunnel? This hidden 25% is called rotational drag.

Today DT Swiss launched the new ARC 1100 DICUT aero wheels, developed in collaboration with the aerodynamic experts from Swiss Side. This is the second wheel launch as part of the #roadrevolution18 campaign and the technical collaboration between DT Swiss and ‘Aerodynamics by Swiss Side’. Jean-Paul Ballard CEO and Technical Director of Swiss Side says: “What is especially exciting about this launch today is the first publication of our new combined research and development into aerodynamic drag in particular rotational drag. It is one of the most unexplored performance factors in the aerodynamic optimization of cycling wheel sets. Pushing for new understanding in aerodynamics is the core mission of Swiss Side!”

At Swiss Side we first identified the importance of rotational drag during the original development of the HADRON aero wheel profiles back in 2013. The advantage of CFD (computational fluid dynamics) is that we can quantify all aerodynamic forces on each component individually. From our very first CFD simulations we saw immediately that up to 30% of the total drag on a wheel comes from rotational drag. This was of course considered as part of the development of the Swiss Side HADRON aero wheels. The challenge however, was that rotational drag is not measured in the wind tunnel by traditional means because the wheels are driven by rollers. Therefore the power required to rotate the wheels is not calculated into the total measured drag. Of course out on the road the riders power also overcomes the rotational drag. To measure it accurately is not an easy task. As part of our technical partnership with DT Swiss we embarked on a collaboration project to quantify, understand and reduce rotational drag like no other has done before us.

Our major findings from the research into rotational drag are that rims, spokes and nipples play important roles and also plenty of scope for optimization. For example the difference between round spokes and aero spokes is 1.5W, which is 12% of the total wheel drag. Similarly hidden nipples make a difference of 0.5W, another 4% saving. In a market where most top aero wheels are within 2W of each other these are significant findings. Naturally the Swiss Side HADRONs as well as the new ARC 1100 DICUT aero wheels are designed with aero spoke & hidden nipple concepts. These findings are just the tip of the iceberg. In collaboration with DT Swiss we are continuing to research and optimize rotational drag, the results of which will flow into new product development moving forward, as part of our continued mission to revolutionize aerodynamics and real world performance in the cycling industry.

 … Watch this space!



Daniela Ryf: DT Swiss - Aerodynamics by Swiss Side athlete