Tech FAQ: Dastardly Disc Rub

Dear Lennard ,
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, to me, is pretty simple: I’ve got 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’ is racin’ – 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 Web site and they have 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), not finding much support love.

—Chris

Dear Chris,
As you probably saw on the SRAM site, Avid brakes actually do allow you to change pad spacing. As you said, though, the adjustment is at the lever. But most brakes do not have such an adjustment.

That said, some brake pistons do get sticky and not retract as far as they originally did. In that case, remove the pads and hold one piston in place with a plastic tire lever while squeezing the brake lever to push the other piston out far enough that you can clean up around it and lube it with brake fluid. Then push it in and hold it with the plastic tire lever, and do the same procedure to clean and lube the other piston. This is a common fix often required on early Shimano hydraulic models, and the pads will open further for a time, until you have to do it again.

What I recommend doing, and what works with brakes with no pad spacing adjustment, is simply ensuring that the rotors on both wheels are in the same spot, i.e., the same distance from the left dropout, and they are both true. I have dealt with this exact issue many times, as I have a lot of wheels that I move around on and off of my bikes, and, I too, hate pad rub. I get all my rotors lined up the same with a Morningstar R.O.C.-TECH tool. In fact, the two photos on the Morningstar R.O.C.-TECH page were taken in my shop, doing the same thing I’m describing.

With the truing-stand-mounted R.O.C.-TECH tool, as in the photo, the stand indexes it, so you can put one wheel after another on there and just tweak the rotor so it falls within the same range on the dial gauge. The other photo is of my older daughter’s fingers bending a rotor with Morningstar Drumstix tools to straighten it.

If you use the Morningstar R2.O.C.-TECH tool, which mounts directly to the bike, rather than a truing stand, the indexing is less likely to work. So just note which side of each rotor rubs the pad and correct it accordingly. In a very rare case of one hub being mis-manufactured so the rotor-mounting flanges are not in the same positions, you can shim one rotor with shims made of beer cans to line it up in the same spot as the other. But I’ve always been able to get my rotors all lined up the same just by truing them to the same spot.

— Lennard

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about 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.

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