Thursday, November 13, 2008

Occassional Rubbing of Disc Brakes

Q: occasionally I'm experiencing rubbing with my Avid BB7s - how do I fix it?

If you read the Avid manual there is a procedure to align your caliper and rotor as well as to set your pads up at the right distance from the rotor.

Four things that can cause an occasional rubbing even though your brake is setup correctly are:

- a disc brake on the back of a fork is trying to force the wheel down and out of the front dropout when you brake. This force is resisted by the clamping friction of your QR and the lawyer lips on the ends of your drop out. If the QR isn't tight the axle can move slightly under braking and this will cause some rubbing that will go away on it's own when you hit a bump or reseat the QR. If you experience this occasional rubbing stop the bike and open/close the QR with some downward pressure on the bars. If the rubbing goes away this was your problem.

Sheldon Brown acknowledges this issue and points to this link for more details.

BTW - do not ride a bike with a front disc and no lawyer lips on the fork. If your QR is loose and you brake hard you could have a gnarly accident.

- Another problem is your disc rotor can be slightly bent causing occasional rubbing. This may go away after some braking if it is only slightly out of true. You can check this by doing the test above when the rubbing starts - if it doesn't go away lift the wheel up and rotate the wheel by hand if this is the problem it will happen at the same spot once per rotation. You can gently bend the rotor back into shape by hand [wear clean gloves so you don't contaminate the rotor with oil]. There is a specific rotor truing tool if you want to get a bit more precision in your adjustments.

- Another cause of occasional rubbing is dirt in your caliper. Clearances are fairly tight between the rotor and the pads. If you get some grit in there it can reduce the clearance so that the pads rub anywhere the rotor isn't totally true [no rotors are perfectly true]. This will take care of it self as the grit get cleaned out. You can check for this by inspecting the rotor/pads for dirt.

- The last cause of occasional disc brake rubbing is a flexy fork. It doesn't take much side to side movement of the fork to cause the rotor to rub. You can test for this by playing around with your bike - lean it from side to side, turn hard - if this causes the rubbing to start you'll know this is at least part of your problem.

Keep in mind you may be experiencing occasional rubbing due to one or all of the causes noted above. Assuming the rubbing is light and only occassional it isn't something I'd worry too much about. Just make sure your front QR is tight.

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