Possible silly question – Is the brake mount on a fork usually rated for any size disk – like you could put a 203mm on a sid if you wanted? Case at hand is actually a reba (100mm travel) and a big disk.

Leaving aside questions of why you would want to do this, I was just wondering if the mounting was strong enough by miles to take the braking forces, or if there might in fact be a risk of cracking over time.

I have cracked the rear hub flange holding a big disk after a couple of years, so was thinking along the same lines.

Yep – they are generally rated up to a max size. If they break if you go over that is another matter…

Some forks are… like the one you quote, apparently SID’s are limited to 185mm disc’s, yet Reba’s aren’t…
Anyway’s it’s only for warrenty purposes, and how would they know?

If you look on the Rock Shox site, it gives maximum rotor sizes for each fork.

it’s not the mounts that they worry about as such, more the load on the bushings and stanchions.

…and how would they know?

I imagine any mechanical engineer worth his salt would be able to tell the mount had failed through overstress, and that the stress exceeded that which could be produced by the recommended maximum size disc.

I’ve seen QR’s come undone, and the front wheel pop out, due to a too large rotor (the braking force twisted the lowers and managed to pop out the QR in a downwards motion)

Also, you’re front forks will flex quite a bit with QR + large rotor.

Rather scary when you look down and see it!

Thks all – guess I’ll dig out a 160.