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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 10-29-05, 12:48 PM   #1
digger
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New MTB, never used disc brakes, need your help

I have been a roadie for 15 years now, before that I had a mountain bike, back in the time of cantilever brakes and front suspension was only for the rich and professional.

Well, just yesterday I purchased a 2005 Specialized Hardbody with disc brakes, rode the thing for the first time today and wow those disc brakes sure do stop the thing fast and I love front suspension.

I do most of my maintenance myself but I do not know anything about disc brakes. The bike has Shimano Acera mechanical discs. For example, is there a way to centre the pads so they will not rub the rotor? I have been looking for some sort of centring screw, like on my road bike dual pivots, but I don't see anything. Whenever I take the wheel off and reinstall, the brakes will rub the rotor. CAN one centre the pads?

There are 2 barrel adjusting knobs for taking up cable slack or for adjusting as pads wear - one by the brake unit itself and one up by the lever. Do I adjust one over the other or both?

I have look on Sheldon Browns website for info on disc brakes but he doesn't have anything there. The Park Tool website has something on Avid mechanical disc brakes but that's not the same I don't believe.

I'm heading on down now to pick away at them and see if I can learn anything, without breaking something (no pun intended).

Any help or websites for info would be great!

Thanks!
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Old 10-29-05, 05:18 PM   #2
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Well, I just finished tinkering with the bike (what no replys!) and using the brakes today obviously stretched the cables so much that the brake lever would almost touch the handlebar. Hence, I had to take up this slack.

Too make a long story short, these types of brakes do not have a centering adjustment screw like I am used too. Rather, I have to adjust the fixed or inboard pad then using the barrel adjusters set the outboard pad. It looks as if I will have to put up with some amount of the brake pad rubbing against the rotor in order to reduce the amount of mushiness of the brake lever, but it is still not set as I would like it.

So how about it guys, any help...anyone? :-)

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Old 10-29-05, 07:54 PM   #3
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Sounds like you have the gist of it. Each pad is independantly adjusted to center the rotor between them. Rotors often have a bit of runout (warp) so you have to take that into account.

Have you read the Shimano instructions that should have come with the brakes? If you don't have a copy, then go to Shimano's web site and download a copy. It should tell you how to adjust the brakes.
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Old 10-30-05, 01:26 AM   #4
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Almost all mechanical disc brakes have one fixed pad. When you apply the brakes the moveable pad actually warps the rotor against the fixed pad. You can fiddle with the position of the pads and the caliper, but it's difficult to impossible to get it perfect.
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Old 10-30-05, 07:59 AM   #5
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Try this website. I have found their information pretty useful. http://www.utahmountainbiking.com They have a lot of information on disc brakes and lots of tricks for adjustment. Scroll all the way down the page and look to your left until you see bike repair.
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Old 10-31-05, 03:33 AM   #6
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What happened to loosening the mounting bolts, squeezing the lever to align the brake, and tightening the bolts (without releasing the lever)? I thought that was always the starting point, then fine-tune the pad adjustment.
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Old 10-31-05, 04:17 AM   #7
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Quote:
Originally Posted by Jonny49
Try this website. I have found their information pretty useful. http://www.utahmountainbiking.com They have a lot of information on disc brakes and lots of tricks for adjustment. Scroll all the way down the page and look to your left until you see bike repair.

Very nice link.. thank you... got a nice section on first aid too
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