Fandomii

Hi folks,
I'm running a Magura MT5e dual piston disc brake on my front wheel (ebike), it's pretty new (800km). Every since pretty early on it's been squealing, so this week I decided to finally pop it up, sand the pads with 120 grit sandpaper, and then clean them with 99% isopropyl alcohol. Same with the rotor, clean with alcohol and gentle sanding.

Then I started bedding the pads, accelerating to ~25-30km/h (15-29mph) and then braking without fully stopping, down to ~8km/h (5mph). The brakes were quiet during the bedding process, I did it about 20-25 times.

Then I started braking to a stop and they started squealing again!

Did I do something wrong? I read at Magura's instructions that I instead should have bedded them with a 30km/h to a full stop process. Is that where things went wrong, or do you have another suggestion?

TiHabanero

The wonderful world of light weight disc brakes. There ain't enough material in the calipers, pads or rotors to help dampen vibrations in a consistent manner, thus squeal. Sometimes you win, sometimes you lose.
The last two things you have left to do is have the brake mount on the frame aligned to the axle, and replace the pad set. If the squeal persists, try a different brake set.

shelbyfv

I'd definitely start by following their instructions.

mdarnton

I've been very lucky with brake squealing. I'm afraid to read threads like this for fear I'll pick up a curse.

Polaris OBark

Don't use disc brakes on tires.

Clean the rotors with isopropanol, not the pads.

Hopefully you sanded the pads, not the rotors or the tires.

Fandomii

Not sure what you mean by don’t use disc brakes on tires.

I cleaned both rotors and pads with alcohol. Isn’t that the typical process?

I sanded the rotors gently. Again isn’t that the intended process to deal with squealing disc brakes due to glazing?

Polaris OBark

I don't understand it either:

Alcohol will help to clean a contaminant from a metal surface, but not a porous surface.

Sanding the rotor will just scratch up the metal surface.

Sanding the pads will help to remove contaminants, unless they have soaked in, in which case you will have to replace them.

Fandomii

lol corrected, tire = wheel

sweeks

He means "wheel", not "tire"?

(Oops... one minute too late!)

t2p

maybe a different pad will be the cure

have you tried adjusting / moving the caliper ? this has worked for me

( the often recommended ‘loosen caliper bolts / apply brake / tighten caliper bolts’ will often result in squeal on one of my bikes ... have better results by adjusting the caliper by sight )

if nothing else works you might reduce or eliminate the squeal by removing some of the material from the rear most portion of the pads ... hit the edge first - move in a mm from that point ... ???

Schweinhund

Pad vibration can be cured by this stuff, but I've never used it on a bicycle
https://www.crcindustries.com/disc-brake-quiet-9-wt-oz/
Being that it's easily removed, I would not hesitate to try it.

CrimsonEclipse

There's anti squeal compound specifically for bicycle rotors.

Maybe try a semi metallic set of pads or just organic to see if that changes anything.

Mtracer

Sometimes you can clean pads and sometimes you can't. Try new pads and see what happens. Be sure to re-clean the rotors.

Fandomii

rear most portion as in, the part that is at the end while rotating? Or the part closest to the axle of the wheel?

TiHabanero

He is referring to the leading edge of the pad. The edge of the pad that "sees" the rotor first gets sanded down to form a bevel of about 45 degrees or less. This is seen on motorcycle pads as it helps with the high pitched squeal. Does not eliminate it in some cases, but it can help. I have done it on bicycles without success in damping squeal.

Using rubbing alcohol on pads is something I have done for years, and I find it does help to remove glaze and contaminants during the sanding process of the pads. I have also done this on automotive brakes with good results. The worst case of brake squeal I have encountered has been on a 2021 Specialized Diverge. I pulled out all the stops and all to no avail. Caliper mount to axle alignment, sanding pads, rotors, alcohol decontamination of pads and rotors, replaced pads and rotor, replaced caliper, you name it I did it. Nothing worked until I replaced the front wheel with a better one.
The wheel was the issue all along. Not sure why, but it made the difference in ending brake squeal.

cpach

If you have any doubts about the pads just replace them. They’re porous—if they’re contaminated they can be impossible to clean.