Squealing disc brakes

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rjb
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Squealing disc brakes

Post by rjb »

Ok I've been giving my new purchase some much needed care and attention.viewtopic.php?t=149442
I've given it a thorough clean and lubed everything that moves. Straightened out the bent gear hanger and turned the bars and stem around. :lol:
It's my first foray with disc brakes so I need some advice. The brakes are cable operated Shimano discs. The front with its 180 mm rotor is very good but the rear 160mm rotor is not great. The issue is the rear doesn't feel powerful enough. I expected to be able to lock the rear wheel when the lever is pulled hard. It slows me down but I feel it should be better like the front. It squeals just before it comes to a stop. I've cleaned the rotor with IPA and removed the inner cable and lubed it from end to end so it slides nicely in the full length outer. That reduced the squealing but it still does this just as it comes to a stop. So should you be able to lock up the rear wheel? I'm only a 9.5 stone rider so not a lot of momentum involved. I'm baffled. Could it be the brake pads are contaminated? Can you clean them or service them?
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
PH
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Re: Squealing disc brakes

Post by PH »

Could be a number of things. I'd try and narrow it down by swapping pads front and rear and seeing if the problem follows the pads or stays with the brake - assuming they're the same brakes of course...
Good compressionless cables help, but even without I've had no trouble locking a back wheel with any cable disk brake I've tried, from BB5's up.
mattsccm
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Joined: 28 Nov 2009, 9:44pm

Re: Squealing disc brakes

Post by mattsccm »

Just check cabling. There is more to "stretch" etc so they can be less effective. Should be fine with a decent set up.
mattsccm
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Re: Squealing disc brakes

Post by mattsccm »

Swapping pads may not work. Different size discs may wear differently.
rmurphy195
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Re: Squealing disc brakes

Post by rmurphy195 »

Maybe the calipers aren't properly aligned with the rotor? Check any instructions you have for making sure the calipers are properly aligned, this is likely to mean putting new pads in, slackening the caliper mounting bolts, then holding the brakes hard on while tightening up the caliper mounting bolts. If they are TRP spyre then there's a number of excellent videos on youtube produced by the manufacturer.
Brompton, Condor Heritage, creaky joints and thinning white (formerly grey) hair
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hemo
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Re: Squealing disc brakes

Post by hemo »

How many miles done ?
They may need a bit more bedding in.
slowster
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Re: Squealing disc brakes

Post by slowster »

rjb wrote: 24 Jan 2022, 7:13pm It's my first foray with disc brakes
I recommend you watch the Park Tool video on mechanical disc brake adjustment, if you have not already done so.

https://www.parktool.com/blog/repair-he ... -alignment

I would compare front and rear pads. Given that the previous owner had the forks on the wrong way round, I doubt they performed the sort of hard braking required for initial bedding in of resin pads, and it might be that the rear pads are now 'glazed' and need to be sanded and then bedded in.

Brucey's explanation of what bedding in does to the pads is here: viewtopic.php?p=1388427#p1388427.
bgnukem
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Joined: 20 Dec 2010, 5:21pm

Re: Squealing disc brakes

Post by bgnukem »

Pads could be contaminated, if squealing. Try removing them and abrading slightly on a flat surface, with some emery paper or against a large flat file. Then be prepare to run them for a while until the pads wear into the disc surface.

Also do both pads in the brake caliper move towards the disc when the lever is pulled? If so, ensure the caliper is centered so the movement of each pad is approximately similar, or you could be wasting lever effort bending the disc rotor sideways towards the more distant pad. If only one pad moves, set the rotor close to the static pad (using the pad adjuster) so the disc rotor does not have to bend much before it makes contact when the moving pad pushes it across into the static pad.

Also check the outer cable routing for tight bends / excessive length.

Of course, a 160mm brake will be less powerful than 180mm (based on radius, around 88%) but that's not a bad thing, as you don't want the rear wheel locking up in panic braking situations.
TheBomber
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Re: Squealing disc brakes

Post by TheBomber »

rjb wrote: 24 Jan 2022, 7:13pm It's my first foray with disc brakes so I need some advice.
The likely poor performance issues look to be well covered above - but as you're new to disks I would add a caution about the use of spray lubes on the bike as it is easy to contaminate disk pads and not always possible to restore them. Even if you are careful using a spray lube it is not obvious how much of a plume of spray is produced. If you stand back from the bike and watch while someone else uses the spray, the size of the plume may surprise you. Generally I only use a spray when the wheels are out, but I have made a couple of u-shaped cardboard cutouts that can be interlocked around the rotors, providing some protection for occasions when the GT85 is out but the wheels are in.

HTH
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kylecycler
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Re: Squealing disc brakes

Post by kylecycler »

One of my pals puts shower caps over his disc brakes whenever he's using spray lube or washing his bike - keep meaning to try that, or something similar, right enough. He's still at the obsessive stage, though, after getting his first disc braked bike, but he's probably right.

Definitely, as others have suggested, make sure the rotors are centred in the calipers, and make sure the inner pad, if it's static, is close enough that the rotor doesn't bend as the brake is applied. When my Subway came out of the box (I had it delivered rather than have Halfords build it up), the brakes weren't properly adjusted, although the gears weren't far away. It was my first experience of discs too and it took me a while to learn the tricks.

My Subway's brakes (mine's a year or two newer than yours) are Tektro Novela with 600 mm rotors F&R. Don't know how yours are Shimano with a 180 mm rotor at the front, but that obviously shouldn't be a bad thing - I'm jealous! :)

The rear (mechanical, not hydraulic) discs went, over time, from working better than the front to being just a token gesture - not working at all. Rear pads are far more prone to getting contaminated than front pads, for obvious reasons; they also have a far longer cable run, especially if you have full housing. Replacing the rear cable with a well lubed new one made a huge difference (although I reckon I could just have lubed the old one), but you've tried that so it's not the answer in your case.

I also took the pads out, abraded them on a flat surface to remove the glazing as others have suggested, then sprayed some electrical contact cleaner (more powerful than brake cleaner and curiously cheaper) into a jar and soaked them in that, which also helped.

Trouble was, first run out, a set of traffic lights changed on me as I approached downhill in the wet, I pulled on the back brake as I'd been used to doing for far too long before I sorted it, the rear wheel locked and the bike went to swap ends - I'd to release the rear brake or I'd have been off! :shock:

You should also bed in the discs and pads - you want a deposit of pad material on the disc - ten light stops followed by ten hard stops, one brake at a time, releasing the brake as you stop so as not to localise the deposit (just don't do what I did and feel the disc to see how hot it's got! :lol:)

Also make sure your cables are compressionless - longitudinal, not spiral, cable strands. My Tektro brakes' instructions insist on compressionless cables being fitted but they came out of the factory with spiral cables. I keep meaning to replace them but tbh they work ok as they are - the limit is always whether you can lock a wheel; anything beyond that is overkill.

Only other thing is, make sure the rear rotor isn't warped - it's not unusual for a rotor to be even slightly warped. If it is, adjust the pads one at a time so they're close to the rotor, listen for where the rotor touches the pad (where it's warped) then simply use your fingers and thumbs to manipulate it straight and true. You can use an adjustable spanner instead to grip the rotor and bend it, or a purpose made tool, but your fingers can do the job (wear gloves to prevent contaminating the rotor with oil from your fingers, and clean the rotor again afterwards anyway).

That's all I can think of. It's like what Brucey used to say about getting cantis to work - a lot of little things but they all add up.

Incidentally, re. mechanical vs hydraulic disc brakes, I've always thought, with mechanical discs you tend to have to make an effort to lock a wheel, whereas with hydraulics you have to make an effort not to - in an emergency, at least, or if it's slippery - and the former is safer than the latter.
ElCani
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Re: Squealing disc brakes

Post by ElCani »

Lots of good advice given already, but I will just add that I recently had a pair of new Shimano resin pads that started squealing horrifically and didn’t perform particularly well either. No amount of aligning, sanding, cleaning or bedding in made any difference and the problem followed the pads from front to back when I swapped them. No idea how they got contaminated (or if they were just faulty) as I am pretty careful and the bike isn’t used much, but I ditched them and the problem disappeared.
rjb
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Re: Squealing disc brakes

Post by rjb »

Took the pads out and wire brushed them. Wiped the disc with Emery paper, note I had also cleaned this with IPA previously having then drunk the rest of the bottle :lol:
No more squealing and a lot better stopping power but still can't lock the rear wheel. Thanks to everyone who contributed here. :D
As it was a 2nd hand bike ive no idea of the use it's had but the front tyre still has remnants of the moulding in the centre so not a lot.
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
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