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
Radical Solution: I use a hammer and screwdriver. Place the tip of the screwdriver on the brake spring and lightly tap the screwdriver to center the brake. The spring is HARDENED steel and will not be damaged!
Old school technique that I first saw used by the legendary Gene Portuesi in the very early ’70s. He had a “special punch” in his shop with a groove cut in the tip so it wouldn’t slip off the brake spring when he whacked it with the mallet (not a hammer).
My take is that the brake nut should still be loose enough so that the brake can be moved with “very firm” hand pressure. I have no idea what torque this requires but it means that a simple knock won’t move the brake but a serious hit (like in a crash or when the bike otherwise gets hit hard) will move the brake rather than damaging it or the frame.
Thanks for raising the idea of leaving the brake just loose enough so that it will move and not get damaged in a crash – or get damaged less – Kerry. There’s a lot to talk about there. I’ll do that in next week’s TT. Thanks for the idea!
Jim
Ah yes, the ol’ screwdriver and hammer technique guys – taught to me by Bruce Anderson at Andy’s Bike Shop in Keene, NH in 1973. That’s a fun hack but there are a few important caveats. And, it won’t work when there are individual springs for each side and/or hidden springs. I’ll explain more next week. Thanks for bringing this classic technique up. I never had the privilege to meet him, Kerry, but it’s nice to see the name Gene Portuesi again,
Jim
Excellent Jim! Thanks.
You’re welcome, George. Thanks for the kind words!
Jim
So timely! I have been having this exact problem both front and back, tightened the fixing screw – but did NOT know to squeeze the brakes at the same time. I also have tried the centering screw (Shimano sidepulls) but doesn’t keep brake centered. I will wait for next week’s “expanded” explanation and caveats before doing anything further. Thanks!