I have been told that every fork has a maximum rotor size characteristic. For example, for my $75 Suntour this is listed as 185mm. The explaination that I have been given for this limitation is that any larger rotor will have significant chance of breaking the forks! That is, the braking force will be too high and the brake mount system will shutter.
Is this correct and precise information or is it just some legal insurance?
My guess is its not so much about the forks themselves breaking - I cannot imagine that sort of force can be generated by brakes when the forks themselves have to handle the forces generated in prangs etc that would easily exceed any braking forces..... Yep - forks break in a bad prang, but no one would buy forks that did not survive a moderate fall without damage
The failure point is most likely the mounting posts, or the wheel being loosened or even ejected by the brakes on a QR system (well known issue and one argument for through axles).
However, the idea that bigger disks give more braking and bigger forces is overly simplistic. A cheap setup with 200mm discs ridden by a 100pound wimp will not produce the same force as a top quality 160mm disc ridden by a guy with the weight and strength of a gorilla.....
I believe its most likely the manufactures covering themselves and weasel words to get out of warranty claims from guys who buy cheap shocks, put on the biggest brakes they can find and ride them on the biggest drops they can find - and then claim "It broke.......". The kind of guys that do this would not be seen dead on a bike with small discs.....
Ok, I like the above two answers a lot ... since they say what I want to hear. However, here is what SR SunTour responded to my official support request (after less than two hours of the request!):
Dear Mr. Vitkov,
the leverage is much higher if you go for a bigger rotor than 185mm.
You risk that the fork breaks if you go for a 203mm rotor!! So we strongly recommend not to do this if you don´t want to get seriously injured!
For most riders a 185mm rotor is more than enough.
Best regards Benjamin Rees
I still do not know what to believe. For example, I do not know of 185mm rotors, the closest I have seen is 180mm.
Another controversial point is that their more expensive forks - EPICON and AXON also are limited to 185mm, I managed to locate only 1 fork limited to 210mm - a dirt jump intended one.
One possibility that I can think of is that the stanchion tubes could brake off the lower tubes at very high deceleration. However, deceleration is as much function of the rotor size as of the brake type e.g. high-quality hydraulic or the cheapest mechanical brake.
cherouvim nailed it in the comment.
First, imagine the case where somehow (maybe a system of gears) the rearmost edge of the disc rotor were inside the fork itself. When the pads grip the rotor, the rotor tries to push the brake assembly up further into the fork. Most materials would fare very well under this arrangement. Let's call this the 0° case.
Now, think about the current case. The force is still almost along the axis of the fork, but slightly further out, so there's some "sideways" torque being applied to the fork. This is maybe 5-10°, but the larger the rotor, the more torque.
Finally, the claim that rim brakes are just disc brakes with large rotors. This isn't right, because rim brakes are not anchored at the end of the fork. If you imagine ideal calipers anchored at the bottom of the fork that could grab the rim at the back (where a huge disc brake rotor would) with force equivalent to a disc brake system, they'd probably snap the end of the fork off (or the brake mount itself) very easily. That's the 90° case.
My fork for fatbike bent over in mounting place, i was put 203mm rotors and two cylinder hydraulic brakes, goodly fork was made from steel if it wasnt steel it would be shattered in pieces, braking from 40-50kmh and pressed too high pressure and fork bent over in upper mount, then brake disc together with support bent inside and support got in disc, that was so fast that i even didnt slow just snapped brakes and goodly cars braked.. So if you put 203mm brakes you cant brake on full or need good fork with mount. E-bike weight probably 50kg and my 110kg :D
Why greater rotor puts more stress on fork (no matter how and where the caliper is mounted) from physics point of view:
Friction depends on the materials touching and the force they are pushed together.
The materials depend on the quality of rotor/pads.
The force depends on how hard you can squeeze the brake lever and also on quality of the brake (caliper design, brake hose strength against stretch, brake fluid, etc.).
So given a brake and given how strong your fingers are the brake's maximum friction force it is able to generate is always the same.
Now - the further the braking point is from the center of rotation the more leverage there is and the greater braking torque can be achieved. Higher braking torque puts more stress on the fork.
The other thing and non-relevant is if you can use all that braking torque. That depends on the weight of the rider, the tire (contact patch and tire tread), and the ground (asphalt, gravel, mud, ice,..).
And the simple answer is because its a radial (turning around a fixed point) force then the calculation (centripetal) force requires the size of the circle to be squared (thats it multiplied by itself), so if you increase the virtual rotor size by 45mm (160 to 205) then you square the difference (45x45 = 2025) then the forces involved are going to be MUCH bigger at the centre point and at the periphery of the circle, in short if you dont follow the manufacturers guidelines then you may or may not cause the system (fork, hub, spokes, calipers, mounting, axle, pads, etc) to fail at its weakest point. Having had this happen to me at Fort William downhill course (it was the pads, that i ripped the pad material clean off the backing plate) that failed and I went over a rather large double jump a little (a lot) too fast and the results are one year on a case of tendenitis and a lot of physio.
