MTB rear brake chronic squealing - I've run out of solutions........

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open roader
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MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Sat Oct 20, 2018 6:12 pm

Long story short :-

Several weeks ago built up MTB with new XTR M9000 brake calipers / new Shimano Ice Tech disks / new pads

Front brake worked fine from the start - rear had chronic squeal from the beginning.

Just toughed it out for the first week - say 60km but that is enough bedding I figured and just replaced pads in the 2nd week.

Noise even worse with new pads - truly loud and horrid.

Have tried the following :-

Checked rotor for straightness- perfect from new.

Checked rotor bolt torques

Checked rear axle tension

Loosened off the caliper mounts / held brake lever on then re-tensioned caliper mounts.

Cleaned rotor and caliper pistons with brake cleaner - then cleaned rotor with acetone (have done this twice now)

More bedding in riding (over 50km) still horrid

Today I tried de-glazing the pads on emery paper and a block

After 6-7 weeks the brakes are still truly horrid- hugely loud - mega decibels like I've never experienced before with a shocking shudder if I pull to much lever on in frustration (carbon frame might make this sound worse as it's my first carbon MTB)

Aside from replacing the entire caliper I'm all out of ideas as the front brake works a treat straight out of the box..........

Help please! Can't ride it - feel like killing it! Very disappointing for someone who can usually nail down these sorts of things.....
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Tamiya
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Tamiya » Sat Oct 20, 2018 7:59 pm

Haven't worked on XTR M9000s but BiTD odd squealing sometimes could be solved by modding the mass of the pads - hopefully changing the vibration frequency. Work out which bits "aren't vital" :) and either file them off to lighten or add mass with lead tape (sold by proshop for tennis racquets). Sometimes might even go as far as cutting diagonal slots into the friction material. You've got spare pads = great opportunity to mod.

No idea if that's in today's SOP mechanics manual, YMMV, but that was BiTD & we've tried even dumber things before... :)

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Sat Oct 20, 2018 8:24 pm

Yep, happy to mod the original pad blocks incl. filing grooves into the resin compound - anything at this stage - thanks.

I recently completed another MTB build a few months ago - will switch the rotor (solid type ie not floating) and try it out too.
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Tamiya » Sat Oct 20, 2018 9:40 pm

rotors would be where the lead tape comes in ;) wrap around 2 or 3 spokes on the disc where it don't get caught in anything, I wouldn't want to cut anything on the disc

Did u actually spray brakecleaner onto the new pads after installing? ( I assume aerosol brakecleaner from autoparts store) Some swear by it but I find it's too harsh & doesn't really "degrease" all that fully... or maybe it's actually pulling out some component in the brake pad itself, who knows...? Could be like brakepads that have overheated once too often - they're always more squeaky & fade faster after being overcooked. These days in garage I don't use it on "new" brakes anymore, only to clean everything down before starting work.

Acetone don't do much for me either, I mainly use it for cleaning off paint & dissolving dried grease. Have isopropyl alcohol but leaves water moisture behind. Shellite/zippoFluid probably degreases better.

For pushbike degreasing I've moved onto Electrical Contact Cleaner spray, buy it from Repco/Supercheap etc ... seems to dry faster, no residue & less harsh on paintwork/skin. After working on discs & everything is bolted back, I flush the whole works with that spray - let it drip off, never wipe. Then go test ride, no need to wait for it to dry.

Whenever that can is empty I grab trigger bottle of Windex :) does fair job degreasing too.

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Sat Oct 20, 2018 10:28 pm

Ok, I get the lead tape tip now- use as counter ballast.

I used Bendix Brake Cleaner Spray on the inside of the caliper only (piston interfaces) The pads are untouched, no hand contact, no acetone, no brake cleaner.

I'll get some electrical contact cleaner and leave the acetone for other things.

There is so much contradictory advice to be read about this elsewhere online - BNA is the only place I ask questions and take advice seriously.

Tomorrow I swap out the rotor for a solid billet lightly worn unit and see if that makes any difference...........
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby trailgumby » Sat Oct 20, 2018 11:17 pm

How are you bedding in your pads? I find it takes at least a dozen good hard-braking downhill passes at speed (and rolling out of the brakes before coming to a complete stop) before you start to get consistent bite and no squealing.

And why resin? They disappear in wet weather like a box of chocolate Paddle Pops on a hot day. Metallic are the go.

Sometimes it's just the rotor. Have you thought of switching it out and seeing what happens?

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Tamiya » Sun Oct 21, 2018 12:39 am

open roader wrote:Ok, I get the lead tape tip now- use as counter ballast.


it's not a "balance" issue, if that's what you're thinking. IMHO pushbike wheel don't go fast enough for balancing to matter much

dunno if the added weight changes the harmonic buzzing outside of squealing range or if the added mass acts like a damper instead

If you've got blutac try that first ;) it'll do same, just doesn't hang on as well

I buy Wilson Lead Tape from proshop (I'm a racquet tuning geek in another life). Comes 2 strips of 50cm per pack, each strip is 20g worth. Self adhesive, snip with scissors... strangely useful in many non tennis situations. :)

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Sun Oct 21, 2018 11:03 am

trailgumby wrote:How are you bedding in your pads?


Exactly this. I have a steep, straight bitumen road leading into town (10% gradient average all the way down a 1000m section) I have ridden the new pads 8 times down this hill incl. 5 down and striaght back up on the same day - tried all sorts of modulation applications. I feel can't test my brakes down there anymore as the noise is horrendous and it's 100% residential......... and I like to stay in the local's good books..........

This morning I switched out the rotor for the rotor on the previous build which is whisper quiet - zero difference. Very dissappointing too. My previous build uses XT brakes and Avid rotors, this has to be the best disk brake set up I've ridden (and I've tried a few on mate's and hires bikes etc)

To keep my 'fix' in house the only thing left is to switch the caliper over for the XT model I have on the previous build. A complete pain in the arse to re-bleed everything, however, this is the very last variable yet to be tested (and maybe fit the XTR caliper to the previuos build to see what happens.

I just can't see how a bike shop mechanic is going to find anything more than I have using a straight method of elimination. Did I just get uber lucky with the previous build - fitting 100% new everything and getting a perfect result from the first turn of the wheel?
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Mububban » Sun Oct 21, 2018 12:34 pm

trailgumby wrote:How are you bedding in your pads? I find it takes at least a dozen good hard-braking downhill passes at speed (and rolling out of the brakes before coming to a complete stop) before you start to get consistent bite and no squealing.


OP obviously knows his stuff as well as you, but I just recently got my first disc brake MTB and followed your advice above (and in the Park Tool video below) and my new baby brakes nice and quietly now :)
This video was very helpful for a noob like me (start 1:25 in).




I've also got discs on my road bike, I didn't do any "official" bedding in or maybe it wasn't necessary as it was a test bike so had been ridden before I got on it. But man, they do shriek in the wet :shock:
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Sun Oct 21, 2018 2:52 pm

Mububban wrote:This video was very helpful


This is pretty much what I have tried. My hill is considerably steeper so I don't need to pedal to get back up to speed for another hit.

I have not tried using clean water on the disk/pad interface whilst braking - I need to find a quiet hill out of town and give it a go.

Might also gladly fork out for metal sintered pads - anything to get a rideable bike! I was under the impression that metal pads squealed worse then resin...........? You simply can't believe everything you read online............
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby trailgumby » Sun Oct 21, 2018 9:33 pm

Metal pads can squeal more, but I've rarely had that problem.

I am having it at the moment though. It's an old bike I've ridden little in the last 2 years and suspect the rotors got contaminated with spray lube while hanging in the garage, which then transferred to the pads. If so, the pads will need to be chucked and the rotors cleaned before they are replaced. That'll have to be done this week. The bike is on eBay with a scheduled close date of next Sunday.

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Mon Oct 22, 2018 8:20 pm

I'll order some sintered metal pads and start over again.

I have a pair of solid steel rotors on the way too.

One change up at a time.

Also think I need to double or triple my bedding in repetitions.......... and not clean the rotor again.......... I was rolling at 8-12km/hr down the hill for each of the bedding runs - no real speed involved as I know heat is the enemy, esp at the beginning.

Watch this space..........
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Duck! » Mon Oct 22, 2018 8:46 pm

You need some heat to help the bedding process, but what you want to avoid is light load dragging of the brakes, which is what overheats and glazes the surface. Your approach speed is also too slow; you need to get up to a decent clip, basically normal riding speed (use the hill you live on to a high percentage of its potential), and really reef on them, just short of locking up. You need to have the rotor running through the pads for long enough to get several revolutions of the rotor under significant pad pressure to get that pad material dragged off the pads and ground into the rotor. Rears are often a bit more temperamental because they do so little proper braking and tend to unload the wheel and just lock under hard application.
I had a thought, but it got run over as it crossed my mind.

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby open roader » Mon Oct 22, 2018 9:12 pm

Duck! wrote:Rears are often a bit more temperamental because they do so little proper braking and tend to unload the wheel and just lock under hard application.


Cheers Duck. I can hang off the back of the saddle a fair bit on this bike (get some weight back onto the rear end)

I'm going to get a bad rep for disturbing the peace down that hill..........
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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby Duck! » Mon Oct 22, 2018 9:42 pm

You may find every so often you'll need to pull the rear pads out & give them a scuff to deglaze them even after bedding in, and even between bedding runs. Try to leave as much dust on the pads as possible; when you reef on the brake that loose stuff will get ground into the rotor.

My understanding was that the 02 resin compound used from the M9000 series onward was a bit more user-friendly (aka less prone to glazing) than the previous 01 compound, which I'd found needed scuffing off after almost every ride. Metal pads are so much better. They don't have as much instant bite as resin, being harder, but have much better overall power, vastly superior modulation and better heat tolerance. I get the odd groan when they're really doing a heap of work, and under normal use generate a bit more scraping noise than resin, but that's a tiny trade-off for the performance benefit.

Leave the Ice Tech rotors, especially if heat will come into play on the trails. They work. If you're only doing light trail stuff there's no real difference between Ice Tech and solid stainless steel (the braking surfaces are the same, the difference is that with Ice Tech it's a thin veneer on each side of an aluminium core), but if you're working the brakes harder, they do run cooler and keep the brakes from fading.
I had a thought, but it got run over as it crossed my mind.

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Re: MTB rear brake chronic squealing - I've run out of solutions........

Postby boyracer » Wed Oct 24, 2018 4:40 pm

Checked the rear of the pad where it contacts the piston? Is it square to piston with no dags or build up? I had a squealer a while back and sanded the back of the pads (tape or glue a sheet of sandpaper to some old glass or MDF) flat and apply real thin coat of silastic sealant . Allow to dry and re assemble. Worth a try.

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