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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?

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Actually, the smaller the rotor the greater the bending force placed on the fork, for a given braking force. I suspect that maximum rotor size is mainly a limitation due to where the caliper mount holes are placed. –  Daniel R Hicks Aug 5 '13 at 10:47
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@DanielRHicks: Larger rotor equals greater forces. And the holes (in post mount style tabs) are in the exact same position in all forks and larger rotor sizes are handled by adaptors. e.g cdn.media.cyclingnews.com/photos/2007/tech/features/… –  cherouvim Aug 5 '13 at 11:05
    
Lever arm. Less force is required with a long lever arm to generate the same torque (braking force). –  Daniel R Hicks Aug 5 '13 at 15:27
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Yeah, but rim brakes can just be thought of as disc brakes with really large rotors. Are we actually talking about damage to the forks, or having the brake mount physically separate from the point where it is welded to the fork? I think that something else in the system would surely break before the actual fork itself was damaged. –  Kibbee Aug 5 '13 at 17:30
    
The one point I'll concede here is that as the rotor size increases the stand-off distance between the fork arm and the caliper necessarily increases. This increases somewhat the amount of twisting motion applied to the caliper mounts and increases the chance of bending the mounting bolts or perhaps shearing the mounting lugs. But this effect would also be related to the stiffness of the rotor, and I haven't heard any argument that a non-stiff rotor will cause fork failure. –  Daniel R Hicks Aug 6 '13 at 15:28
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4 Answers

up vote 2 down vote accepted

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.....

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I'm 70kg and the brake is the feeble BR-M416A, so I think I will risk it. I am considering some cheep large disc to compensate for the weak brake. –  Vorac Aug 6 '13 at 14:44
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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.

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What did you expect them to say? - It is safe to ignore the manual..... One thing I agree with is "For most riders a 185mm rotor is more than enough." –  mattnz Aug 6 '13 at 9:39
    
Avid does 185mm rotors. I've used them once. –  cherouvim Aug 6 '13 at 11:10
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You gotta wonder how 18mm (which is really only 9mm difference in radius) can make a difference between a perfectly safe bike and one that is on the verge of falling apart. –  Daniel R Hicks Aug 6 '13 at 15:23
    
Considering how performance is such a factor in racing now, forks are machined to specific tolerances. Think how much 2mm makes a difference in shifting; why would fork tolerances be any different? Although it sounds like the person asking would not likely put that kind of strain on it. –  Aaron Aug 6 '13 at 19:21
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If they are engineering forks to tolerances within 10% of failure I am a monkeys uncle...... –  mattnz Aug 6 '13 at 22:02
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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.

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No, for a given amount or braking force a larger rotor does not generate more torque. Braking force translates directly into torque, unrelated to rotor size. And a larger rotor places the caliper farther out, meaning that less force is placed on the caliper for a given amount of torque. It's simple physics. –  Daniel R Hicks Aug 6 '13 at 1:47
    
It's absolutely true that the same torque applied to the caliper generates the same braking force. It doesn't follow that the same amount of torque on the caliper generates the same amount of torque on the fork regardless of caliper/brake mount position. –  Ryan Cavanaugh Aug 6 '13 at 4:39
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The same braking torque will, essentially by definition, produce the same torque (in the wheel rotation direction) on a fixed brake mount point on the fork. All forces must add to zero. Moving the mount point up (to allow for significantly larger disks) will reduce the force/torque at the mount point. Physics. –  Daniel R Hicks Aug 6 '13 at 11:13
    
The amount of torque generated is a function of the force multiplied by the distance that force is applied at. In the case of disk brakes, the force is being applied by the friction between the brakes and the distance is the size of the rotor. If the friction between the pads and the rotor is the same, a larger torque would be applied to the wheel/rim with the larger rotor. So with the same friction, the larger rotor would indeed produce more torque. However, I think that in most cases, the bike would actually flip over the wheel, or the front tire would skid before the fork got damaged. –  Kibbee Aug 6 '13 at 12:33
    
@ Daniel R Hicks, So larger torque on the fork for the same lever squeezing force (and more braking) or less powerful squeezing for the same torque (and stopping power). I get it. –  Vorac Aug 6 '13 at 14:50
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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.

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Why would the specific fork have impact on when do the brake mechanism calipers brake off? In my question, the maximum disc size is an individual characteristic of the fork. –  Vorac Aug 23 '13 at 13:51
    
For a given braking force the overall torque on the wheel will be the same, regardless of rotor diameter. However, as the rotor diameter increases, the force placed on the brake components (for a given braking force) decreases. –  Daniel R Hicks Aug 23 '13 at 14:28
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