It's no secret that disc brakes are becoming increasingly popular for road riding. The increase in braking power, as well as improved control (also referred to as "modulation") has already seen many riders make the change. It also allows for more flexibility in frame design, in particular allowing ever larger tyre sizes to be catered for.
However, for us, one of the key questions has always been around the aerodynamic impact of a disc brake setup and specifically the follow on implications for wheel design.
To no great shock, introducing a disc rotor to the airflow at the hub increases the overall drag of the wheel system. This matches what you would intuitively think would happen. However, the wheel, tyre and in this case rotor, need to be considered as an overall system, so we shifted our development work from the hub to the rim edge. Here, without the need to accommodate a braking surface, we had a freer hand to optimise the rim/tyre interface.
As part of our wind tunnel testing we wanted to quantify the effect of this optimisation.
Test 1: disc vs. rim (& rotor sizing)
The results showed that the difference between a rim brake model and a disc brake model were less than 1W, using the most common 160mm rotor diameter. As you might expect, reducing the rotor size to 140mm narrowed the gap to 0.3W.
If you remove the rotor altogether (not recommended whilst riding!) the disc brake wheel is actually 0.6W faster than its rim brake equivalent, showing the impact of the rim edge optimisation.
Now this only considers the wheel/tyre/rotor part of the system - clearly the frame design will also have a significant impact. However, if frame designers can optimise design by that 1W difference, the overall system will be faster.
Test 2: rotor brand/design
As well as rotor size, there are also a range of options for rotor design. Some manufacturers have published claims (and supporting data) for the heat dissipation of their rotors. As part of our testing we wanted to investigate whether there was any aerodynamic difference between brands and/or rotor design.
Interestingly, whilst the differences were small, there was a measurable increase in drag between the SRAM Centerline rotor and the Campagnolo AFS and Shimano Dura Ace equivalents. Despite the small differences, it does open the possibility that we may begin to see disc rotor design taking aerodynamics into consideration on top of braking performance.