So cyclocross season is upon us and like any good racer you've got a quiver of 4 sets of wheels... One to train on, a set of muds, mid's and drys... Oh, and since it's 2017 and you're tech savy, they're disc as well. And... you're either broke or thrifty so you by whatever you can find on sale. Or perhaps you're on the "if my spouse finds out how much I spent I'm screwed" budget. Either way, you may have ended up with three or four different brands of hubs, disc mount standards and maybe even different disc brands. No big deal, right??? Nope! Not until you realize that you can't change wheels without readjusting your brakes. What a pain! No fear, let's talk quick about managing this issue!
Ok, I know that most people don't care about the "why" but would be remiss to not quickly jump into what could cause all the rotors to be in different positions. It boils down to manufacturing tolerances. A quick poll of hub manufactures revealed to me that most hubs are machined to have a disc mount position of =/- .005". So right there, your disc can be in any position within a .010" range. Add to that, the tolerance of the rotor width and the tolerance of the disc brake mount on the frame and you could be talking in the range of .030-.040" ( about 1 mm if that helps anyone). That's huge when you consider the pads only sit about .007" from the rotor (and yes, I measured it).
Great, so what does this all mean and how do I fix it? Well the bad news is, your pads most likely rub on your rotors when you change wheels... at best, annoying you, at worst slowing you down, Your challenge should you choose to accept it, is to get all 4 rotors to fall in the same position relative to the brake. It's easier than it sounds too! The only additional part you're going to need is a rotor shim. They can be ordered through any bike shop if you have traditional 6 bolt rotors. If you have centerlock rotors things get a bit trickier as I've been unable to find commercially made centerlock rotor shims. Have no fear though, I custom machined some and I'm offering them for sale!
Step one. Identify the wheel/rotor combination that falls closest to the dropout (furthest outboard). Adjust your brake caliper to this position. Ensure that you've done this brake perfectly as you're going to shim the other three sets of wheels so that their rotor position matches this one!
Step two. Remove that perfectly centered wheel and replace it with your next wheel.
Step three. Remove the rotor and add the appropriate number of spacers so that the rotor now falls in the same location as the last!
Step four. Repeat with as many wheels as you have!
Not too hard, right!!! Now you can change wheels quickly without having to readjust your brake! Feel free to contact me if you need help with this or if you need to purchase some shims (or if you're lazy or perhaps mechanically declined and just want me to do it for you!)
David is either found riding his bike or in his workshop working on them!