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Qualification
The biggest objection from the triathlon community to disc brakes is that they are aerodynamically inferior to rim brakes. However, there is not a significant amount of strong data on the subject. Herein are some wind tunnel results of a bike tested with both disc and rim brakes. This data is a sample from our ongoing aerodynamic research at Cannondale.
There is a limit to this testing. Modern triathlon and TT bikes are integrated systems. To date this has included the development of integrated rim brakes, and a bike designed around rim brakes is unlikely to perform as well with disc brakes if they are added as an afterthought. The reverse also applies. While this will always be a difficult question to answer perfectly, it is possible to provide some insight.
The biggest objection from the triathlon community to disc brakes is that they are aerodynamically inferior to rim brakes. However, there is not a significant amount of strong data on the subject. Herein are some wind tunnel results of a bike tested with both disc and rim brakes. This data is a sample from our ongoing aerodynamic research at Cannondale.
There is a limit to this testing. Modern triathlon and TT bikes are integrated systems. To date this has included the development of integrated rim brakes, and a bike designed around rim brakes is unlikely to perform as well with disc brakes if they are added as an afterthought. The reverse also applies. While this will always be a difficult question to answer perfectly, it is possible to provide some insight.
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Wind Tunnel Tests
We took a prototype TT frame and tested it in the San Diego wind tunnel with both disc brakes and rim brakes. This frame was designed for disc brakes and so had flat mount fixtures in the fork and chain stay. The disc brake setup used standard Shimano flat mount hydraulic calipers and centerlock rotors (160mm front, 140mm rear). The rim brake setup used a TriRig Omega front brake attached at the fork crown and a Shimano direct mount brake mounted under the chainstays.
As noted, this is not a fully integrated solution for rim brakes as may be seen on some models. However, the two braking setups are similar in their level of integration. Furthermore, this style of rim brake configuration remains common on many TT and triathlon bikes and does not restrict the results to proprietary components. This configuration is also the same as that used by Cannondale athletes at the Ironman World Championships in Kona this year.
We took a prototype TT frame and tested it in the San Diego wind tunnel with both disc brakes and rim brakes. This frame was designed for disc brakes and so had flat mount fixtures in the fork and chain stay. The disc brake setup used standard Shimano flat mount hydraulic calipers and centerlock rotors (160mm front, 140mm rear). The rim brake setup used a TriRig Omega front brake attached at the fork crown and a Shimano direct mount brake mounted under the chainstays.
As noted, this is not a fully integrated solution for rim brakes as may be seen on some models. However, the two braking setups are similar in their level of integration. Furthermore, this style of rim brake configuration remains common on many TT and triathlon bikes and does not restrict the results to proprietary components. This configuration is also the same as that used by Cannondale athletes at the Ironman World Championships in Kona this year.
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All runs were conducted with a bicycle only. The same wheelset was used in both configurations; however, rotors were removed for the rim brake test.
The chart above and table below show that there is very little separating the two braking systems. It is unsurprising to see the curves follow a similar shape as the rest of the bike is unchanged and therefore dominates the overall profile. However, some differences can be seen. There is a crossover point in the curves. In the negative side of the yaw curve the rim brakes have lower drag (beyond -5 deg). However, towards the positive side (at yaw angles greater than -5 deg) the disc brakes have lower drag. In these tests negative yaw angles are defined as non-drive side windward. Positive yaw is then drive side into the wind.
The chart above and table below show that there is very little separating the two braking systems. It is unsurprising to see the curves follow a similar shape as the rest of the bike is unchanged and therefore dominates the overall profile. However, some differences can be seen. There is a crossover point in the curves. In the negative side of the yaw curve the rim brakes have lower drag (beyond -5 deg). However, towards the positive side (at yaw angles greater than -5 deg) the disc brakes have lower drag. In these tests negative yaw angles are defined as non-drive side windward. Positive yaw is then drive side into the wind.
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Given what we know about the primary importance of low yaw angle performance, this graph shows disc brakes to have a very slight advantage over the rim brake configuration. However, this is minimal given the uncertainty of the wind tunnel test.
These wind tunnel results include data for the bike with no braking system installed. It should be noted that this should not be taken to represent a fully integrated braking system. Even for a fully enclosed rim brake system the brake pads and shoes must remain close to the rim and so exposed to the air. Furthermore, the volume and frontal area of the frame must increase in order to accommodate braking hardware.
Table 1 below shows the numerical average of CDA over different yaw angle ranges; between ±5, ±10 and ±15 deg.
These wind tunnel results include data for the bike with no braking system installed. It should be noted that this should not be taken to represent a fully integrated braking system. Even for a fully enclosed rim brake system the brake pads and shoes must remain close to the rim and so exposed to the air. Furthermore, the volume and frontal area of the frame must increase in order to accommodate braking hardware.
Table 1 below shows the numerical average of CDA over different yaw angle ranges; between ±5, ±10 and ±15 deg.
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Limitations
Like any wind tunnel investigation, these results are limited to the scope of this test. It has already been stated that the level of integration of either braking system will affect the results. In addition, the same wheels and hubs were used in all tests. In some cases, a wheel with rim brakes can potentially have a smaller hub and lower spoke count which may provide an additional small reduction in drag.
Like any wind tunnel investigation, these results are limited to the scope of this test. It has already been stated that the level of integration of either braking system will affect the results. In addition, the same wheels and hubs were used in all tests. In some cases, a wheel with rim brakes can potentially have a smaller hub and lower spoke count which may provide an additional small reduction in drag.
