From testing sample and prototype brakes to see if they can meet CPSC standards, I can tell you that rotor diameter does make a difference. For these tests, I'm only doing single stops from 15mph on flat ground, not enough to get the rotors past warm. Going to a larger rotor often made a difference whether a brake would be able to stop in the required distance, especially with many of the cheaper mechanical calipers.

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"I'm thinking of a number between 1 and 10, and I don't know why!"

OK, I agree that, in terms of clamping force needed at the rotor, assuming the coefficient of friction is the same, you need more clamping force with a smaller rotor because of the reduced leverage. I'm thinking of this from the rider's standpoint, where the additional rotor clamping force needed at the rotor can be compensated for with different hydraulic leverage ratios. There's plenty headroom within a hydraulic bicycle braking system to create more clamping force with the same lever force. Probably enough to stop a bike easily (once, anyway) with a 100mm rotor.

The trade-off of the greater hydraulic leverage would be a loss of modulation. So, all else being equal, a 160mm rotor will provide better modulation as well as better fade resistance than a 140mm rotor. In terms of single-stop stopping power, however, the 160 can't stop the bike any shorter, as long as the resulting heat from that one stop doesn't saturate the 140mm rotor.

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"They're made of latex, not nitroglycerin"

For a given application of lever force and a given leverage ratio within the hydraulic system a larger rotor will apply more brake force at the wheel (also assuming the same coefficient of friction).

The other disadvantage to having a greater leverage ratio in the hydraulic system is that pad spacing must be closer which increases the likelihood of brake rub.

*ting ting ting ting ting ting...*

...just like a creaky bottom bracket it gets old quick.

I categorically disagree comparing Omega X + Jet Black rims to XT 8000 brakes 160mm front and rear. Clermont doesn’t have much in the way of long hills but it has some very steep hills.

This. At least Roval had a good idea with their 21 spoke front wheels laced 2:1 with hubs shaped to minimize the drag hit from the rotor.

In the Air Force we called this Drinking the Coolaid! Agreed

I came across this today from a May 2019 web article at https://road.cc/...-and-how-wind-tunnel

A final note on disc brakes
Disc brakes, in their current incarnations, aren't aero. I'm sure we all guessed that. The magnitude of the difference is fairly significant.
"We've measured a 16% increase in wheel drag between a disc-braked wheelset and a standard wheelset", Jean-Paul told us. "We performed a direct back to back test of the Zipp 303FC in standard version and disc brake version, for our own competitor comparison purposes. That 16% is a constant offset in the performance curve across the entire cross wind angle range."
So there's work to be done there, but how much can actually be done? The extra drag essentially comes from three sources. The rotor itself adds drag, and because disc wheels need more spokes to cope with braking forces there's more drag there too. On top of that, a disc hub contains more material and needs to be built to withstand torque across the hub body, as the braking forces are on one side only. Because of that, the hub body is generally bigger and that increases drag as well.
What can be done about those three sources of extra drag? In reality, probably not a huge amount, and disc systems will likely continue to be at a disadvantage in terms of aerodynamics. So if you want to be as slippery as possible, it's rim brakes for the time being. And hide them, if you can.

Until frame and fork design is optimised for discs it's not a reasonable comparison. Without the need for proximity between the tyre and fork crown, I reckon there are decent gains to be made to compensate for the drag at the rotor and hub.

Manufacturers are already producing disc-only optimized models and consumers are buying them, so I think it's fair to start comparing at this point. The fact that after optimization the P5d is similar to the old P5 should be enough to tell us that disc aren't helping the aero cause.

This, and to be fair, it is really hard to make a faster bike than the P5. That they were able to get close with disc brakes is impressive. I guess my next bike will be disc, but it is really just because by then I won't have a choice.

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Heath Dotson
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