Tech: MTB Q&A with Lennard Zinn

Published November 3, 2009 02:49PM

While I’ve been on or around bikes for the vast majority of my life, I’m struggling with disc brakes on my mountain bike. I’ve had a couple different models, now working with Avid Juicy Ultimate.

My question is pretty simple: I have different rear wheels. What can I do to ensure that each of them will work well with the calipers? It seems that I’m constantly rubbing one or both pads. According to “Days of Thunder,” rubbin’s racing, but for someone like me I need all the help I can get (up hills).

What can I do to maximize the space between the pads? It seemed so easy on my tri bike – loosen cable a bit, center the calipers, etc. I’ve checked with the SRAM website and they’ve got tech docs, but unless I want to change the setup on the lever (it’s fine for now) or bleed my brakes (everyone says this is a nightmare). I’m not finding much support love.

First off, the rotors must be true. This is the tool for that, the Morninstar Rotors on Center 2 tool. At under a hundred bucks, it’s a tool any hard rider avid about maintaining the bike can own. The dial indicators allow you to get it perfectly true, and a rotor is easy to bend with your thumbs or with Paul Morningstar’s Drumstix.

Having your rotors true will only be useful if they are located the same distance from the axle end face so they line up in the caliper slot at the same place. Otherwise, one will run centered in the caliper and the next will be leaned against one pad or the other.

I run into this issue myself all of the time when switching wheels around. I actually have a Morningstar Roc Tech tool for truing stands. In fact, I just noticed that the photo on this page is one I took of my stand in my shop many years ago. With the tool mounted to a fixed truing stand, the dial indicator is always set up the same, so that I can ensure that the rotor on every wheel is at the same distance from the hub axle end. But you can do the same with ROC Tech tool mounted to your frame or fork; you just need to run through all of your wheels on it in one sitting before removing it.

Clean the rotor with rubbing alcohol after touching it and especially after getting grease or other contaminants on it.

After you have those first two dimensions — rotor trueness and centered rotor — dialed in, that only leaves pad spacing. Obviously, more spacing between pads means less pad rub. I’m sure you’re aware of the dangers of pulling your brake lever when there is no rotor between the pads. This will push the pistons far out of their cylinders and you’ll need to push them back in somehow. But they can become overly extended for a number of other reasons as well. To retract a Hayes brake, you generally pull out the pad and push the piston back in with the box end of a 10mm wrench (with the round “box” surrounding the pin that the wire catch on the back of the pad engages). On a Shimano brake, you slide in first a flat blade or credit card and pry apart until you can fit in a plastic tire lever to pry them fully apart. This method can also be useful on an Avid as well. However yours work, get the pistons to retract fully.

Some brakes have an adjustment for pad distance to rotor, so investigate this in your manual.

In case you were wondering, bleeding is not generally going to give you more pad spacing. What pulls the pad back is the return spring, or, in the case of Hayes, an O-ring seal around the piston whose cross section is square and which tries to untwist back, pulling the piston with it, once the brakes are applied and the inward-moving piston twists the seal.

Bleeding is the process of removing air from the hydraulic fluid within the brake lines and cylinders. It is indicated when your brakes are soft, don’t stop the bike completely or repeated squeezing of the lever pumps them up.

There is actually one other item, and that is frame (or fork or hub) flex. If the swingarm (or fork or hub) flexes a lot, then the rotor can move back and forth, rubbing one pad or the other, as your riding produces lateral forces acting on the wheels. You can tighten up a swingarm by replacing worn bushings and bearings as needed to eliminate side play. If that doesn’t do it, or it’s in the fork or the hub shell, there is not much you can do here besides replace the flexing culprits.

Zinn’s regular column is devoted to addressing readers’ technical questions about mountain bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.