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Old 03-09-14, 06:21 PM   #1
Sprayman
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Disc Brakes Newbie

Hello;

A bike with disc brakes was recently given to me. The rear brake has a slight "scrape" against what I'm assuming is the pad. It's not a constant scrape; it happens in the same spot on the revolution, kind of like an un-trued rim would scrape against a brake pad. I'm assuming the disc has a slight warp in it.

So, my question is, can this be easily repaired, or is it more a problem for my LBS? I've never owned or worked on disc brakes before.

Thanks
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Old 03-09-14, 06:59 PM   #2
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Probably a simple adjustment. The disc rotors are never perfect, and all have a bit of wobble. The brake is adjusted to accommodate this. OTOH if the wobble is excessive, the disc can usually be trued somewhat.
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Old 03-09-14, 09:13 PM   #3
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Disc brakes don't have a lot of adjustment available. You center the rotor in the caliper gap but that's about all the adjustment you can do. It is also important that the pads are parallel to the rotor and they hit the rotor that way. There is also the issue of there's not much room in the caliper gap for any adjustment. Basically, you need to have the rotor hyper-straight.

To achieve this you need to center the caliper first. This doesn't sound like that's the problem so I would check to see that the rotor is centered before you do any adjustment. You can sight between the pads and the rotor from in front or behind the caliper or the top of the caliper. Spin the rotor slowly and see if there is a wobble in the rotor. Make a note of where the wobble is and move it to where you can bend it a little. You can bend it by hand or you can use a rotor truing tool or you can use a small adjustable wrench to gently bend it in the direction that you need to go to take out the wobble.

I would suggest using a tool because the grease from your hands can contaminate the pads and lead to squealing brakes.
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Old 03-11-14, 03:31 PM   #4
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The rotor discs can become bent if they take a whack it is easy enough to bend back just fine adjustment.
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Old 03-11-14, 04:11 PM   #5
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if you know the brand there are online copies of the owners and service manuals ..
via those manufacturers websites.
paper copies can be found of current stuff at bike shops.. they come in the cartons,
but rarely is all that paperwork taken with the new bike by the buyer, so there are extras.

mechanical disc brakes friction pads have to be wear compensated by you and your hands on tools or knobs.
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Old 03-11-14, 08:43 PM   #6
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Thanks for all of the advice, guys. I'll give it a closer look. I may just take the wimpy way out and take the bike to my LBS, as I've got 6 other bikes that need my attention
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Old 03-11-14, 09:21 PM   #7
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Quote:
Originally Posted by Sprayman View Post
So, my question is, can this be easily repaired.....
Most likely, yes. It sounds like your rotor is a little out of true and it's rubbing against one of the brake pads where it wobbles. A very common problem, and something you'll have to deal with from time to time with disc brakes. In the shop, I use a tool that's designed to take hold of the rotor and allows bending it back to true. This sounds more elaborate than it is....the same thing can be done with your hand, a crescent wrench, etc. Mainly, you just need to get the hang of eyeballing the place on the rotor that's out of true, and straightening it. It's really not a big deal. You can then listen for rotor rub as you spin the wheel and tell if you've gotten the rotor true enough.
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Old 03-12-14, 06:06 AM   #8
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Rotor truing: https://www.youtube.com/watch?v=0fosSuyiCDE
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Old 03-12-14, 06:53 AM   #9
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Yes. True the rotor and ensure the caliper is mounted properly and not at an angle with respect to the rotor. Also, are they hydraulic or mechanical? If hydro, the caliper pistons may need to be reset so that the seals properly pull the pads away from the rotor. This is done by pushing pushing the pistons all the way apart and then applying the brakes until they pads contact the rotor. Also, dirt and grime can get between the pistons and bores making the pistons hang up. They many need to be cleaned.

The joys of hydro...
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Old 03-12-14, 07:10 AM   #10
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Before trying to do any rotor "truing" I might suggest taking loose all the bolts on the rotor. Then retighten the bolts like you would a car wheel. That means tighten them in a star pattern by doing them by tightening every other bolt. You might tighten them in a couple of stages too.
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Old 03-12-14, 05:10 PM   #11
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If the brake is functioning well but just rubbing at one spot, no need to mess with the brake itself. Rotors are easily bent and need to be incredibly straight to prevent rubbing due to the very tight clearances in the disc brake system. Just use your hand or a wrench to GENTLY bend it back. It doesn't take much. Realize this will happen from time to time, and IMO it's one of the easiest jobs you could do. Easier than changing a flat tube. So you might as well do it yourself.
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