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Test 2: disc brakes vs. rim brakes

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 aerodynamic penalty of a disc brake setup. 

Introducing a brake rotor to the airflow at the hub has been shown to increase the overall drag of the wheel.  So, when working to develop our disc brake wheel range, we shifted our focus to the tyre/rim interface, aiming to take advantage of no longer needing a brake track at the rim.  As part of our wind tunnel testing of the disc brake range we wanted to quantify the effect of this optimisation, as well as provide a direct comparison between equivalent disc and rim brake wheels.

Drag chart

Learn more about how to interpret a drag chart here

Drag comparison

The results show the difference between a Passista rim brake wheel and a Passista Disc brake wheel with a standard 160mm rotor to be less than 1W.  Reducing the size of the brake rotor to 140mm further narrowed the gap to 0.3W.  Taking things even further, removing the brake rotor completely (not recommended whilst riding!) showed the Passista Disc to be marginally (0.6W) faster than the equivalent rim brake wheel.

A final point to note – all of our testing was conducted on a wheel-only basis.  One of our initial goals was to quantify the size of the gap that a frame designer would need to “fill” to overcome the disc brake aerodynamic penalty.  Even with a 160mm brake rotor, this gap is only 0.9W.  Worth considering when the difference between an aero brake caliper and a standard caliper has been tested at ~2W.

Following on from the original test, looking at size of rotor, we also considered the impact of different designs and patterns of the rotor itself.  To that end, we also tested the aerodynamic drag from the three main groupset providers:

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.

    All tests were conducted using the Passista and Passista Disc 56mm wheels fitted with a Schwalbe Pro One tyre (23mm)