I usually put 203mm disc rotors and the appropriate Shimano calliper adaptors on my XC bikes and have never had a problem (although I use 160mm rotors in races to save weight). I notice that most forks, apart from expensive ones, specify a maximum disc rotor size limit usually 180mm. Has anyone actually broken a fork leg using a big disc rotor? I was just wondering if the rotor size limit is an indication of the strength of the fork or is there some other reason?
2 replies to this topic
#2
The Wrench
-
- Members
-
- 20 posts
Bike GURU Member
Posted 01 May 2016 - 09:09 AM
Hello CMO,
Great question. Rarely comes up but the answer is quite important, especially with older forks.
Here is the skinny on rotor size recommendations on suspension forks -
The adapter mounts that are needed to make the brake work with a larger rotor are placed under more stress (especially the top adapter bolt) when used with large diameter rotors. The top adapter bolt is actually compressed when used with a small or properly sized rotor, but when a large diameter rotor is used the top adapter bolt is actually under more of a shearing force that can cause it to snap (Usually the bolt head snaps clean off).
A lot of modern forks (2010 and up) can handle up to 203mm rotor but IF you are using an older fork (pre-2010) you should not attempt to go above the recommended rotor size of 160mm or 180mm (as stamped on the fork or included in owner's manual).
Also, some older forks have bolt holes that do not go as deep as the bolt holes in modern forks.
So, basically the fork recommendation has less to do with fork leg strength and more to do with the stresses placed on the fixing bolts that adapt the brake caliper to said fork leg. Additionally, IF your particular brake caliper / rotor size requires use of extra adapter spacers then the potential for bolt stress (and failure) just increases.
Always install brake caliper fixing bolts and rotor discs with a bit of blue loctite (NOT grease or any other lubricant) to ensure they don't work themselves loose. Check them FREQUENTLY for proper torque.
