Rotational Drag Insights
- A New Definition of Drag -
Aerodynamics are probably the number one issue when it comes to road cycling developments at the moment. How can aerodynamic drag be reduced and what does it really mean to reduce drag? What is the actual enhancement for the rider out there on the road? How can wheels influence the aerodynamics of the rider and his bike? Those are the questions we are frequently asking ourselves during the ongoing aero development process. When it comes to the optimization of wheel sets there is still a lot of potential contrary to popular opinion. While everybody is speaking about the optimization of the rim shape we had a look on all the components of the wheel set and their influence on aerodynamic performance. The result is a complete new definition of drag!
is what we are traditionally talking about when referring to “drag” and what is measured in wind tunnels… until now. Translational drag is the aerodynamic force in the biking direction slowing the rider down. To minimize this translational drag, the complete bike and rider should be as streamlined as possible. As drag power increases exponentially with the speed, from 15 km/h, the aerodynamic drag is the biggest resistance the rider has to overcome. No news for the moment, as this is basic aero knowledge and almost everybody has been testing in the wind tunnel for reducing drag. But this is only half the truth. There is another very important component of drag...
Riding at any given speed the whole bike and rider are moving forward at this speed. As a matter of fact the wheels are moving at this speed too but they are also rotating whilst doing so. They are subjected to the translational drag & something more called: rotational drag. Rotational drag can be described as the additional friction that occurs between the wheel as it passes through its surrounding medium – the air. Whilst the rider and the other parts of the bike face the translational drag only, the wheels are facing both components – translational & rotational drag. In wind tunnel testing and in all wind tunnel published data, the rotational drag is not considered, because the wheels are driven by rollers. So the power required to rotate the wheels is not calculated in the total ‘real world’ drag. Therefore considering translational drag as well as rotational drag is key when it comes to the aero-optimization of wheels.
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 rotational drag makes up to 25% of the total wheel drag, and 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!
Source: DT Swiss – Aerodynamics by Swiss Side