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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.