Author Topic: Disc brake fettling  (Read 270 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.

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