Since the introduction of disc braking on road and time-trial/triathlon bikes, a debate has existed as to the benefits from enhanced braking performance versus the suggested aerodynamic penalty of a disc brake setup. Prior testing of our first generation of disc brake wheels has shown that by optimising the tyre/rim interface, taking advantage of no longer needing a brake track at the rim, we can reduce any aerodynamic penalty from the brake rotor. With the most common rotor size of 160mm, the aerodynamic penalty was less than 1W, whilst using a smaller 140mm rotor would reduce the penalty to only 0.3W. Removing the rotor altogether (not recommended for ride use!), the disc brake wheelset was 0.6W faster than its rim brake equivalent.
Full details of the initial testing can be found here: https://www.parcours.cc/blogs/news/the-great-disc-brake-debate
With the introduction of our #thinkwider rim profiles on the Strade and Ronde wheelsets, we have moved beyond the direct comparisons as these wheelsets are now optimised around a 28mm tyre. As a result, we can no longer compare like-for-like with the rim brake line-up in a fair test.
However, as disc brakes have become more prevalent, there are an increasing number of rotor designs available to riders. We therefore wanted to test (and quantify) the difference in aerodynamic performance between these designs.
Wheel | Brake rotor | Aerodynamic drag delta at 30mph |
---|---|---|
Passista Disc | SRAM Centerline 2-piece (160mm) | - |
Passista Disc | Campagnolo AFS (160mm) | + 0.3W |
Passista Disc | Shimano Dura Ace | + 0.3W |
Notes:
Wheel | Brake rotor | Aerodynamic drag delta at 30mph |
---|---|---|
Ronde | Shimano XTR RT-MT900 (160mm) | - |
Ronde | SRAM Centerline 2-piece (160mm) | + 0.6W |
Ronde | Shimano Dura Ace Ice Tech (160mm) | + 1.1W |
Notes:
Based on this testing, it is clear that there is a material (and measurable) difference in aerodynamics between brake rotor designs.
It is also noteworthy that the difference between the SRAM Centerline rotor and Shimano Dura Ace rotor has grown from 0.3W in the January 2019 test to November 2020 test. Whilst it is possible some of this can be attributed to margin of error in the wind tunnel, the difference is still sufficiently consistent to suggest other factors at play. The most significant influence is likely to be the difference in hub design between the first generation Passista Disc and the current generation Ronde.