[cmc]

a friend of mine has a typical $550 Trek cross country mtb.

both his front and rear disc brakes work terrible. he says they used to work great when he first got it.

my experience is in bmx, so i don't know how to tune them up. i already re-lubed the cables, but that is not the issue. what seems to be happening is that only one of the tiny prong things is clamping down on the disc, or it is clamping later or weaker than the other one.

with 990 brakes on a bmx, this is easy to see. if you don't have your spring tension even on both sides, and if one pad hits the rim before the other one, you get a squeal and you get brakes that don't grab well.

i had the same problem briefly on my mtb. i rigged it though---i put a washer inside of one dropout to shift the wheel very slightly over. 1mm was enough, so that the disc was centered and the disc brake clamped down on both sides at the same time. also, there is a wheel adjuster on the inside of my disc brake which appears to position the internal pads closer or farther from the disc. i have been using this to fine tune the braking power. mine work great.

the local bike shop where he bought the bike has been lame. they told my friend first that he just needs to clean the disc (complete b.s.) and now they say he needs to replace the entire disc brake. i think they just don't know how to adjust it.

any ideas? sorry i don't remember what brand the brake is. i can take pictures and describe better if necessary.

thanks
cmc

 

[Bicyclist]

Adjust the caliper on the mount to line the pads up parallel to the disc. The inner pad (closer to the hub) should be almost touching the disc, or even rubbing in a spot or two so the rotor does not have to bend very much to make contact. Clean the rotor with a solvent, and oil the cable with tri-flow. That is more or less it. Also, you may have to face the mount or caliper. I know Tektro brakes often need to be faced.

If the pads have been contaminated with oil, or even if anyone has touched the rotor, clean or replace them.

 

[RimJobbed]

First off, are they hydraulic or mechanical? I'm guessing mechanical, which means the cables are either rusted/bent/no lubrication anymore. Could have to do with the pads as Bicyclist is referring to, which means follow those instructions. If they're hydraulic, then there are a number of other things that could be causing it. Hard to tell you how to fix it, when you can't explain what's happening other than they have poor performance. It could be worn down pads, lack of fluid, un-aligned pads or an un-aligned rotor, a bent rotor, an oily rotor. The list goes on, but all bike parts (including hydraulic/mechincal discs) can be tuned back to working properly, unless of course they have obvious broken parts.

 

[Bicyclist]

They're mechs, he already said he oiled the cable.

 

[opjones]

First off the pads could be old and just plain ****ty. If you realign and lube everything up with still no luck with the stopping power, then replace the pads.

This is an easy way I've found to realign the caliper. Slightly loosen up your caliper on the mount. Check to make sure the inboard pad is sticking out far enough inside the caliper so it will make good contact with the disc. Next take a business card, slide it in between the inboard pad and disc. Lightly push the caliper up against the card and snug up the caliper mounting bolts. Take the card out and spin the wheel to make sure your disc isn’t going to be rubbing on your inboard pad because of a bent disc. If everything looks good tighten up your mounting bolts.

As for the outboard pad, loosen up the bolt that holds the cable. Slide your business card in between the outboard pad and disc. Then manually actuate the brake so the outboard pad snugs up against the business card. Once you’re set, tighten the cable bolt up. Spin your wheel again and make any fine tune adjustments you need.


Hope that helps out.

 

[cmc]

thanks, yall.

i'm pretty sure there's an alignment problem, because when you pull the brakes you can see the disc get pushed/bent inward. if both pads were clamping down equally this wouldn't happen.

 

[urbaindk]

thanks, yall.

i'm pretty sure there's an alignment problem, because when you pull the brakes you can see the disc get pushed/bent inward. if both pads were clamping down equally this wouldn't happen.

A little of this is not a bad thing. It gives you some modulation (better feel at the lever). Too much would be a bad thing. I don't know what brakes you have, but on avid bb7's typically you turn the adjusters in all the way so that you are grabbing the disk, then back out 1 click on the outside, 2 clicks on the inside until you don't get any rubbing from the rotor.

If you don't have any adjusters you have to do it via the cable as opjones mentioned above. You can play around with different spacings, 2 business cards inside, one outside, might give you a good feel.

 

[scag]

thanks, yall.

i'm pretty sure there's an alignment problem, because when you pull the brakes you can see the disc get pushed/bent inward. if both pads were clamping down equally this wouldn't happen.

both pads are not supposed to clamp down equally. The outer pad (the one farthest from the hub) is supposed to do most of the work, actually pushing the rotor into the inner pad.

 

[Bicyclist]

The inner pad doesn't move, it's not doing any clamping on a mechanical setup. You want the rotor to be closer to it.