front disc brakes loose and squeak

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brettpringle
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front disc brakes loose and squeak

Postby brettpringle » Tue Apr 23, 2013 14:55 pm

Hi
The bike I am using is a Dawes MTB with Shimano disc brakes
I have used it to commute into london for the last few weeks and slowly my front brakes seem to have come fairly loose.
I have taken it to Cycle Surgery for a brake service, both front and back were both tighter and more responsive when they returned the bike. however after my first cycle the front brake was again loose and squeaking when I apply the brakes.
is there a way i can tighten them and reduce the squeaking without bleeding/replacing the pads?
thanks,
BP

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The Rookie
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Re: front disc brakes loose and squeak

Postby The Rookie » Tue Apr 23, 2013 14:56 pm

What does 'loose' mean in English?

If it's loose, tighten the retaining bolt.

brettpringle
Posts: 4
Joined: Tue Apr 23, 2013 14:43 pm

Re: front disc brakes loose and squeak

Postby brettpringle » Wed Apr 24, 2013 10:07 am

when I pull back on the brake lever it doesn't grip until almost right against the handlebar, and even then it doesnt stop the tire moving easily and squeaks.
i think they are mechanical disc brakes as opposed to hydraulic ones.
where is the retaining bolt?
apologies, complete novice with these brakes!
Last edited by brettpringle on Wed Apr 24, 2013 14:51 pm, edited 1 time in total.

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cooldad
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Re: front disc brakes loose and squeak

Postby cooldad » Wed Apr 24, 2013 12:57 pm

Well you can't bleed cable discs as there is nothing to bleed.
Either adjust the cable, or advance the pads.
Or get new ones if they're worn.
Clean the rotor if it's dirty.
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bluechair84
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Re: front disc brakes loose and squeak

Postby bluechair84 » Wed Apr 24, 2013 14:38 pm

There is probably a barrel adjuster at the lever which, if wound out, takes up some of the slack in the cable. If you can't wind it out far enough to take up the slack, you need to undo the cable at the caliper and pull some thorugh - of course, wind the barrel in before you do this so you can adjust it later.

brettpringle
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Joined: Tue Apr 23, 2013 14:43 pm

Re: front disc brakes loose and squeak

Postby brettpringle » Wed Apr 24, 2013 14:51 pm

fantastic thanks guys will have a look! :) great help!

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Ouija
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Re: front disc brakes loose and squeak

Postby Ouija » Wed Apr 24, 2013 15:34 pm

The key to getting the best out of mechanicals is to understand that a lot of the pressure you apply to the lever can get lost in cable flex, outer cable compression and rotor flex. One or all three of these things will mean that you have to pull the lever a lot further back before any real clamping force is being applied to the rotor. A checklist for improving you mechanicals perfomance is:

1) Do the outer cables "twitch" and move when i pull the levers.

If yes the solution is to try and shorten the outer cable so that they are straight as a ruler with no curves in them and as short as they can be made to be. You can't do much about the cable that comes straight out of the lever to the frame, but you can do something about the cable from there to the back of the frame (shorten the outer cable that starts at the seatpost so that it's as near to a straight pull as possible).

Remember that some of the lever pressure being applied is being lost in the inner cable just straightening up before it starts to apply any real clamping force to the pads, even if you've got the cable pulled fairly tight before touching the levers.

2) Am i using too much outer cable.

Tricky one this. Outer cable compresses and acts like a sponge. As you squeeze the lever, some of that pressure is going to the pads, but some is being diverted into simply compressing the outer cable. It's therefore a very bad idea to use a completely unbroken outer cable all the way from the levers to the back brake (some people do). There's a reason most bikes come with cable stops welded to the frame. The more cable outer you can get rid of the less clamping force is going to get diverted into compressing the outers.

3) Make sure the static pad is only one or two clicks away from the rotor.

This is the big one. The further away from the rotor the static brake pad is, the more the rotor has to be flexed to make contact with the pad. The only problem is that rotors don't want to be flexed and a certain amount of force is lost in making it do so (more so the further away it gets). Or put it another way, some of the clamping pressure being applied at the lever gets diverted into flexing the rotor and therefore isn't being used as clamping force ON the rotor. So, if you applied 80 pounds of pressure at the lever, maybe only half as much of that pressure is being converted to clamping force on the rotor depending on how far your trying to flex the rotor. As said, it get worse the further away the static pad gets.

Larger rotors flex easier than shorter ones. It's one of the reasons a large (200mm ish) rotor will give better braking performance than a 160mm rotor). And rotors with a lot of holes drilled in them tend to flex easier than identical sized ones with fewer holes drilled in them (which is why some of my swiss cheesed 160mm rotors tend to perform better than my solid ones, despite having less surface area actually touching the pads).

If your sure that your rotors aren't bent but you still have to back the static pad off more than one or two clicks to stop the rotor rubbing against it then it's probably down to the way your tightening up the post mount screws after aligning the calipers. A lot of people attack these with an allen key, tightening them up without realizing that the process of just "tightening each bolt up, one at a time" actually causes the caliper to move, so that it's now no longer properly aligned with the rotor.. hence the need to back the static pad more than one or two clicks to prevent it rubbing and scuffing against the rotor.

My preferred method is to loosen the mounting bolts, screw both pads in to clamp on the rotor so that it sits perfectly in the middle of the calipers entry slot (on some models of mechanical disk brakes, you can't adjust the non-static pad and need to pull the brake lever or cable to get both pads to clamp on to the rotor). Having applied a little bit of grease to the underside of the bolts head (to reduce it's friction on the post mounts) i tighten each bolt up 1mm at a time, taking it in turns with each bolt, applying no more than the slightest finger pressure. This stops the bolt from moving the caliper and bending the rotor slightly when tightening them up. After that, i simply move the static pad well away from the rotor and spin the wheel.

If there is a slight constant rubbing noise, back the static pad away from the rotor by one click and spin the wheel again. If there is still a constant rub, back it away one more click and that should do the job. If not, then the caliper moved as you where tightening up the mounts bolts. And if you get a variable rub (like a helicopter going "whoosh, whoosh, whoosh" then your rotors bent and there's not much you can do about that other than move the pads even further away from it, guaranteeing that clamping pressure at the brake lever is going to get lost in rotor flex (oh well!).



A good sign that a mechanical system is suffering from all three of the above problems is when you have to pull the lever all the way back before you feel like it's doing anything. I don't mean that you have to pull the lever all the way back before the pads even touch the rotor, i mean the pads touch the rotor as soon as you start to pull the lever but there appears to be no braking occurring until the levers all the way back to the handle bar. This is a sign that most of the clamping force being applied to the lever through at least half of its travel is being diverted into inner cable flex, outer cable compression and rotor flex. It's that experience of squeezing the levers for all their worth and having very little of that force being turned into clamping force on the rotor.

Of course, hydraulics don't suffer from those problems which is why they tend to be a bit more idiot proof (unless you get air bubbles in the hose).

brettpringle
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Joined: Tue Apr 23, 2013 14:43 pm

Re: front disc brakes loose and squeak

Postby brettpringle » Wed May 01, 2013 10:11 am

turns out the brake cables needed replacing and now the chain and cassette also require change, fun times!


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