What actually breaks if you over spec a rotor?

I think it’s partly legal/warranty based

Partly correct. It’s entirely legal/warranty based.

Your bike will not explode in flames if you fit a bigger rotor. I know this because I’ve been running oversized rotors for 20 years. It makes no difference, unless you’re a lawyer.

Will nobody think about the poor hub ?
It has a hell of a time with all these forces pulling/pushing/twisting it this way and that.

That’s the only damage I’ve actually seen that may have been down to rotor size – cracked hub flange around the disk mount on the rear with a big disk.

For a given braking force on the ground, the overall torque on the fork leg generated by the braking system will be the same whatever the rotor size. But that torque will be generated by a different balance of forces on the two brake mounts and the axle/dropout interface. Smaller rotors place more force at the axle/dropout, larger rotors will exert more of the leverage via the two caliper mounting points, with less force at the dropout.

In the real world, I can definitely brake harder, more often, with larger rotors. So that is going to place more stress on the fork bushings per ride (if I am in a gung-ho mood). The concern of the manufacturers may be about acceptable wear life not safety.

I assume that someone has decided that larger rotors look rubbish on gravel/road bikes. So have decided to stop people ruining the look of the bikes.

I think there is some engineering sense in a max rotor size. I’m sure some where lawyers are involved but I think it makes some sort of engineering sense. For most of us the peak braking force is limited by pitching over the bars. But the forces for a tall 130kg man with dropper down and his bum over the rear axle may well be higher for a 200mm rotor than a 160mm. in this case the the 160mm rotor might not be able apply enough force to lift the rear wheel

I think it’s a bit easy to blame engineering decisions on lawyers. Sadly I’m a member a member Canyon Neuron facebook group. One of the most depressing places on the internet. The Neuron is rated for drops of 60cm. It’s a bike for useless people like me. However they seem to be bought by people who suddenly decided they are an Enduro rider and they need longer forks for their new found a taste for bike parks. Some one posted a Canyon video which they said prooved canyon really did intend the Neuron as a bike park bike, it’s just the lawyers wouldn’t let them admit it. The bike showed a man on a trail centre blue lightly hoping off the odd root or really modest undulation (so very much a video aimed at people like me). Canyon do sell bikes rated for bike park use. Presumably it’s an engineer that decides which rating a bike gets not a lawyer

Presumably it’s an engineer that decides which rating a bike gets not a lawyer

Lawyers write the warranties. The most common question along these lines is about putting longer forks on bikes. The difference in stresses from putting a 50mm longer fork on an XC bike are not enough to cause problems in themselves. The problem is that it’s still a lightweight XC bike and it’s not robust enough to bash down rough DH courses on. Same with brakes. Putting bigger brake rotors on won’t cause anything to snap, but if you thrash the snot out of a lightweight bike, it’ll break. That’s why lawyers basically just say that the warranty is voided if you change anything.

^ same as putting a whale tail on your Escort, the insurance goes up. May not change the car performance in reality but it’s a sign that you’re probably a boy-racer liability.

Such a good point, it’s not neccesarily about instantaneous failure. I would be concerned for the fork bushing wear (on suspesion forks) by overspeccing a rotor.

I have zero data to back this up but the logical optimum stress point to my mind is the fork crown/steerer interface. Bigger disc = an ability to stop more quickly, which results in a higher impulse. The front wheel is trying it’s best to remain stationary while the mass of the rider is pushing the frame into the steerer. The B.B/rider side of the bottom of the head tube acts as the knee over which one would break a stick.

I would be concerned for the fork bushing wear (on suspesion forks) by overspeccing a rotor.

It won’t make any difference to bushing wear unless you are constantly doing panic stops on every ride. If you are just riding like a normal person, the rotor size won’t break anything or cause anything to wear out faster.

Surely the issue here isn’t the warranty. That is an optional extra provided by the manufacturer. They can opt out of not covering anything they aren’t happy with. The big factor must be liability for death and injuries due to failure. That liability must be why they didn’t say use any rotor you like and why they need an engineering basis for the specification they offer

Oh here is a thought. Are any suspension forks rated for tandem use. That would be an application were rotor size makes a difference. Looks like Alpkit will have one

Ok, but what about if you’re braking while on a treadmill?

That’s only for the cautious.