What limits the rotor size in flat-mount brakes?

Question

Let's consider Shimano BR-RS305 caliper. The only numbers which are floating around are 140 mm and 160 mm for the rotor.

My question is -- what stops this caliper from using 180 mm or bigger rotor? Mount position or construction? Or let me put this -- let's say you are building custom bike, and you can set mount position even for 220 mm rotors. Would such caliper as above be suitable then or not?

Update: I've just read the bolt distance for flat-mount is 34mm (in post-mount it is 74mm), so another issue would be whether flat-mount is really prepared for bigger rotors in sense of torque applied.

Answer 1

From a purely numerical standpoint, the mounting interfaces should be able to handle any rotor size. From this website, the highest torque generated seems to be around 100Nm, which for a 34mm spacing flat mount interface would translate to around 3kN force on the bolts. From this other website, I see that a reasonably high-grade M5 bolt is in theory capable of handling this load. However, this doesn't mean it's necessary a good idea in practice, as the mounting interface is not an isolated system--it's attached to the frame/fork, which may not have the same strength.

For what it's worth, a friend of mine once experimented with putting TRP Hy/Rds (powerful hybrid hydro-mechanical brakes) on his gravel bike (Trek Checkpoint) with 180mm rotors. Although he really liked the resulting braking power, he noticed that under heavy braking the fork was oscillating somewhat, which is not very safe. The setup itself was easy enough though (140 flat to 160 flat combined with a 140 flat to 160 post mount adaptor), and you could certainly stack more adaptors to get you to an even more ludicrous rotor size.

Note that some new MTBs are coming with flat mount rear brakes. As 180mm is possible to install on such bikes (as one would expect from a MTB in this day and age), it really does look like a limitation of the bike, rather than one of the brake mounting system.

Answer 2

Disk brakes create a lot of torque on the frame or fork, where the calipers are mounted. Neither the caliper nor the disk are the limiting factors, but the frame or fork, that have to be designed to withstand the forces. Typically, for forks the manufacturer specifies the maximum disk size allowed. I assume it's the same for frames, but i haven't seen that yet.

About the actual disk size: Typically, the mount points and calipers are designed for the smallest feasible brake disk size, e.g. 160mm for mointain bikes. If you want to mount larger disks, you use an adapter, as found at your LBS or online shop of your choice.
As a side note, there are also adapters that allow mounting an iso-type caliper to post-mount forks or frames, and vice versa. The restrictions of frame or fork as far as disk size is concerned still apply, though.

Answer 3

As you increase the rotor's diameter, the shearing forces on the bolts attaching the calipers to the fork or frame are only one of the limiting forces.

Beyond the usual diameters, you will need to pay special attention to the rotor's overall rigidity, starting by its thickness.

We can take a hint from cars, where the rotors consist of two solid pieces, themselves connected to provide a larger lateral rigidity.

The problem is that once you apply Euler's critical load formula on the rotor, you will find you need to increase its rigidity to the point where it has an unacceptable weight for a bicycle. Even though, as juhist and Burki argue, the frictional force remains the same for the same surface materials and caliper opposing forces, the length of the arc distance along which the rotor needs to transmit that force to the hub (opposing the force at the calipers) increases (by π).

This is the reason why increasing the rotor's diameter will result in oscillations under heavy braking, as MaplePanda reports. To see this, think of a rotor made of rubber. The rubber will stretch behind the calipers, which is not a problem for metal rotors, but it will also be pushed in front of the calipers, which, whether the rotor is made of rubber or metal given enough force, will cause the rotor surface to bend unless it's sufficiently rigid. It is this bending that the rider will feel as an oscillation.