Author Topic: Disc brake fettling  (Read 682 times)

Disc brake fettling
« on: February 17, 2017, 02:39:08 pm »
I have a set of Shimano M575 brakes fitted to my commuting bike.
I find that there is no way to get the pads to align so that they wear evenly. There is always a small triangle that doesn't contact the pad and which eventually stops the brakes working effectively.
I pull the pads out, sand this triangle flat and reassemble the brakes and get a few more weeks wear out of them.

Firstly, is this common to all discs?

Secondly, I find it annoying that I can never really get full life out of the pads. Typically they get replaced when they're really only about half worn through. The calipers themselves (even when freshly bled and refilled) never seem to be able to completely close when the pads are worn.

Maybe my disc is worn too much, but it seems OK, worn, but no excessively so. I know there are fancier levers out there with bite point adjust. Do they solve this problem?

I had wondered about replacing the levers with SLX for example, but it's not really worth it since the pads only cost about £5 a set anyway.

Re: Disc brake fettling
« Reply #1 on: February 17, 2017, 03:10:37 pm »
It sounds to me that the caliper is out of alignment to the disc or the pads are the wrong type?  I seem to remember that there can be some variation in the size of pads that ares sold as compatible eg B01S E01S which could also lead to uneven wearing?

Pads that have worn to .5mm or less should be replaced, is this less than the wear than your are seeing?

It is possible to increase the biting point by removing the wheel and lightly pump the brake lever to force the pistons closer together, but not so close that you can't refit the wheel or so close that it causes the brakes to bind.  You need to watch the pistons move as you're pumping the lever to get a feel for how much pressure to apply and not end up forcing the pistons right out of the caliper.
Most of the stuff I say is true because I saw it in a dream and I don't have the presence of mind to make up lies when I'm asleep.   Bryan Andreas

Re: Disc brake fettling
« Reply #2 on: February 17, 2017, 03:36:42 pm »
I can't visualise the unworn triangle, nor can I see how it would stop the brakes working - unless not all the pad is contacting the disc?  Even then I don't see how it woud cause braking issues. Anyway, when putting in new pads it's a a few minutes work to loosen off the caliper mounting bolts, squeeze the brake tight, then retighten the caliper bolts, in order to make sure alignment is correct.
We are making a New World (Paul Nash, 1918)

PaulF

  • "World's Scariest Barman"
  • It's only impossible if you stop to think about it
Re: Disc brake fettling
« Reply #3 on: February 17, 2017, 04:14:23 pm »
If the unworn piece is where the pad is not contacting the disc then be very careful!

At some point the 2 unworn portions will make contact when you apply the brakes and the wheel will keep spinning.

Either the disc is too small or the calliper wrongly fitted

Re: Disc brake fettling
« Reply #4 on: February 17, 2017, 04:27:39 pm »
It's exactly as PaulF describes. The unworn triangles contact each other and prevent the brake from working fully. In fact it's not catastrophic as they act as a pivot point and the brakes still work, just not as well.
There's usually about 1mm of unworn pad when I typically crack and finally replace the pads, it just bugs me because there's only about 2mm when they're new.

Pads are identical in shape to those originally fitted.
 
There is no freedom to move the caliper into a position where this doesn't happen, so maybe the disc is too small. They're 160mm dia front and rear, so not so small.

This is an OEM set up on a Ridgeback Nemesis. It's the same at the rear as well, but I use the front brake far more frequently.

Aunt Maud

  • Le Flâneur.
Re: Disc brake fettling
« Reply #5 on: February 17, 2017, 04:28:41 pm »
Are you replacing them with Shimano pads or some other pad manufacturers' copy ?

PaulF

  • "World's Scariest Barman"
  • It's only impossible if you stop to think about it
Re: Disc brake fettling
« Reply #6 on: February 17, 2017, 05:46:46 pm »
Might be that you just need the brake mounts on the frame/forks faced?

That would bring them in a little and could get rid of the problem. 

Re: Disc brake fettling
« Reply #7 on: February 17, 2017, 06:56:59 pm »
probably the disc mounts are not quite right on the frame (so the caliper isn't centred over the disc, and is (say) mounted too far out). Another possibility is that the disc isn't the exact right model to match your brake.

You might be able to fix the issue by using (say) a 165mm disc instead of a 160mm disc.

If you want to carry on with the present setup, you may as well grind the pads down in the areas where they don't wear (e.g. using an angle grinder) when the pads are new.

BTW if you read the instructions for most disc brakes, they recommend that you change the pads when the friction material is about half-worn. Most pads come ~4mm total thickness, with 1.6mm backings and 2.4mm of friction material.

If the pads wear in a tapered fashion, and this limits the life, you can often get a longer life from them by swapping left for right. Obviously you can only do this if the pad is symmetrical; fortunately the BR-M575 pad is indeed symmetrical.

cheers

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: Disc brake fettling
« Reply #8 on: March 03, 2017, 01:34:10 am »
To all of you out there running those TRP dual arm mechanical numbers, have them serviced. I had the misfortune today to have to fettle a pair fitted to a Norco cross bike. Less than 12 months old, it had already chewed its way thru a set of front pads, both cables (internal routing) were seized, and the pad adjusters were locked solid. Nothing for it but to strip both calipers, copious quantities of brake cleaner, WD and anti-seize. reassemble and fettle. New cables, use the linisher table to deglaze the pads, wire wool & brake cleaner to get the sh!te off the discs. Thank $deity the skog flushed out of the outer cables**. I wasn't up to trying to route new outer thru the frame, not today.

Fettling tip. Once you have the adjusters moving and you've cleaned the inside of the caliper, wind them in until there's a visible gap then fill the void behind the adjuster with aerosol anti-seize, then wind them back fully before coating the inside of the caliper. Then centre the caliper before fastening the cable.

**Who the fuck thought galvanised cables were a good idea on a cross bike? RRP ITRO of 1200 quid and the bloody thing uses the cheapest cables China can make!
At least it stops now...
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Re: Disc brake fettling
« Reply #9 on: March 03, 2017, 07:02:12 am »
FWIW I agree; I have overhauled a set of relatively lightly-used Spyre brakes and

a) I was similarly 100% unimpressed with the quality of the cables (galvanised inners, no discernable lube = very draggy after a short time)

b) the caliper itself is very vulnerable to crud ingress, corrosion etc.

On the latter point, buried deep within the caliper body there are the bearings. As well as the usual 'three balls on ramps' (but on both sides) there are thrust bearings in the depths of each caliper half. These use unfeasibly tiny ball bearings (about 1.5mm dia or something like that) which means that the slightest amount of corrosion will make the braking action feel as rough as a rough thing. There are no seals worth speaking of, and clearly the calipers leave the factory with little grease in the bearings, so I think all-weather use/bike washing is likely to soon wreck these calipers unless they are maintained. Stripping the calipers to this extent is a fair-to-moderate time-consuming PITA and probably not worth it at workshop rates.

BTW if the pad adjusters in these brakes are completely free to move, they may adjust themselves as the bike is ridden. I have heard of several users complaining of this.   The adjusters come from the factory mounted using soft threadlock, which ought to allow some scope for repositioning (not more than a few times though) whilst retaining some locking action. I am presently experimenting with a generous wind of PTFE tape on the adjuster threads, so that they are quite stiff to turn. Only time will tell if this is a good/better scheme or not.

cheers

Re: Disc brake fettling
« Reply #10 on: March 03, 2017, 09:56:54 am »
There is one quick and easy thing to try (if it hasn't been done already) which is to loosen the mounting bolts of the caliper, get someone to pull the lever back hard (which should make the caliper centre itself) and retighten the bolts.

I used that as step one in getting a disc brake to work on a Thorn bike. When that did not completely fix the problem I filed the uneven paint of the stainless steel mounting points on the frame, then did a bit more careful filing until the caliper would centre perfectly.

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: Disc brake fettling
« Reply #11 on: March 03, 2017, 04:22:08 pm »
I do that but on my own.

Regards the TRP brakes I charged the guy 25 quid plus cable replacement on top of the normal service cost. He was quite sour-faced about it & thought it should have been included in the service charge. I pointed out it was an extra 2 hours work. I thought it was quite reasonable...
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Re: Disc brake fettling
« Reply #12 on: March 04, 2017, 01:27:18 pm »
I do that but on my own.

Regards the TRP brakes I charged the guy 25 quid plus cable replacement on top of the normal service cost. He was quite sour-faced about it & thought it should have been included in the service charge. I pointed out it was an extra 2 hours work. I thought it was quite reasonable...

If someone does not want to pay a reasonable amount to have a job done for them they should learn how to do it themselves.