Jim’s Tech Talk
By Jim Langley
A few Tech Talks back, we showed a cool new tool from Hayes for dealing with disc brake rubbing and included some tips. And even before that we went over truing bent rotors – another cause of rubbing. Here are links in case your discs are dragging: Cool Tool for Fixing Rubbing Disc Brakes and How to Straighten Bent Disc Road or Mountain Bike Brake Rotors.
Off-center Rim Brakes
This week, we’re looking at something that can cause rim brake rubbing. It’s when a brake becomes off center, or shifted to one side. When this happens, one brake pad gets closer to the rim than the other and it can also rub. Reader Larry English asked for help with his brake. He wrote,
“Why do road bike brakes get off-centered? It seems like I have to re-center mine about every other month. What is it that makes them move?“
Larry asked what makes rim brakes move? The answer is usually that they got bumped somehow and the impact knocked them off center. This happens because of the brake’s design.
About Sidepull Brakes
The standard caliper rim brakes on most road bicycles are called sidepull brakes. They’re called sidepull, because the cable is routed to and pulls on one side of the brake. There are also rim brakes with cables centered and pulling from the top. They’re called centerpull brakes. But, sidepulls are the most common type of rim brakes on road bicycles today. You may hear them called “dual pivot,” but the generic name is still sidepull.
Back to the brake getting bumped and moving, this can happen because these sidepull brakes are mounted to the frame with a single centered bolt. On the front, this bolt passes through the fork. On the rear, the bolt passes through the center of the frame – there are different ways depending on the frame design.
But because there is a single bolt, if you lean your bike in such a way that the brake bumps into something or if your friend leans theirs and it knocks into your brake, it’s possible that hit can shift the brake over. The brake pivots on the bolt making one side closer.
Rim Brakes Should Self Center
Here’s the thing, though: if a sidepull brake is properly installed, it doesn’t matter if it gets bumped and knocked to one side. Because there are centering springs on all brakes that will re-center the brake the first time you hit the brakes.
So, the fact that Larry’s brakes keep going off center and that he says he has to re-center them, tells me what the problem is.
The Culprit is a Loose Brake
When a brake actually moves from a bump like Larry’s does, it’s because the brake is not tight on the frame. When this is the case, any knock doesn’t just temporarily shift one pad closer, which the spring would auto fix when you brake.
Instead, the knock actually pivots the entire brake to the side. The center bolt rotates inside the frame. When this happens, the spring gets rotated off center, too. And off center like this, the spring cannot self center the brake pads when you hit the brakes.
The Solution: Re-center and Tighten the Brake in the Frame
To fix Larry’s brake so that it says centered and self centers should it get bumped, is as easy as tightening the brake to the fork/frame properly so it cannot move.
To do this, find the nut on the back of the fork for the front brake or frame for the rear. Usually, it’s a recessed 5mm allen nut. It may be recessed so far inside that you can’t see it. But, you should be able to find it by putting the 5mm allen wrench in the hole in the hole until it fits in the hole in the nut. Or, use a flashlight.
Once you’ve got the allen in place in the nut on the brake, turn the wrench and nut counterclockwise (to the left), to loosen the brake more. Test if the brake’s loose by trying to move it in the fork/frame. It should be loose enough that you can move the entire brake side to side.
Leave the wrench on the nut. Now to finish the job, grab that brake’s lever, squeeze it and keep it squeezed hard to hold on the brake fully. Now tighten the brake’s attaching nut by turning it clockwise (to the right). If you have a torque wrench, the nut should be set to 8-9Nm.
Now that the brake is tightly fastened to the frame, it will self center itself should it get bumped and knocked off. And that should fix Larry’s bike and keep the brakes from moving in the frame, going off center and having to be re-centered.
Ride total: 9,480
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Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s streak of consecutive cycling days has reached more than 8,000. Click to read Jim’s full bio.