Why do rear disc brakes hate me?

cgfw201

I love disc brakes. Singular, mostly the front ones. I've got a carbon Topstone, and a carbon SuperSixEvo. Both are great. Both have ultegra disc brakes. Both have stopped me in the dry and wet with no issues at all for a couple of years.

However, on both, the rear disc brake has always been annoyingly noisy. To the extent where I've basically stopped using the rear brake, at all, due to the noise, and zero stopping power.

It's not been an issue in a practical, slowing down, sense as the front does the job no trouble.

I've recently spent a bit of time in trying to sort the rear brake on my Topstone out, as doing some tailwhips and skids off road seems fun. First up I gave the disc rotor a proper deep clean with isopropyl alcohol and put (another) set of new pads in.

No improvement.

I then bought a new shimano disc rotor, and matched that with another set of new pads. And I had some decent rear braking for about 2 rides.

And now it's completely back to square one again. Loud, and completely ineffective.

Am I doing something wrong? I'm not putting grease on the pads or rotors, I'm cleaning and treating them exactly the same as the front pads and rotors, which are 100% absolutely fine. I've removed and replaced the caliper several times, alignment looks bang on. There's no rub and the pads engage the disc when the lever is pulled.

This is how it sounds (today and every day) - https://photos.app.goo.gl/MaQzhTbZWf2jPUEz8

photonic69

They/some do. I had terrible noises when I used some Gorilla sintered pads in my 105 brakes. Horrible. Set my teeth on edge everytime I braked. Changed them for some Shimano resin pads and now almost silent. OK performance is a little less but not that much so.


YMMV as they say.

cgfw201

photonic69 said:

They/some do. I had terrible noises when I used some Gorilla sintered pads in my 105 brakes. Horrible. Set my teeth on edge everytime I braked. Changed them for some Shimano resin pads and now almost silent. OK performance is a little less but not that much so.


YMMV as they say.

this is across two bikes and now 3 disc rotors. Have tried the shimano LO3A pads, as well as now every colour of gorillapad. Each with the exact same outcome. Done 10,000km on each bike.

mully79

My shimano rear brakes have always done this. I think a tiny misting of brake fluid escapes the pistons and contaminates the pads/discs.

If I find a steep hill and brake hard with the rear, it’s totally wooden, then squeals like a pig then starts to recover. Eventually the braking feels brand new and will usually work for the rest of the ride. Once left overnight the rear brake deteriorates back to useless.

trevor.hall12

Never had any issues with any disc brakes ,
What size disc are you using ,might be an idea to try a 160 if it's a 140 on the rear. Not that it should be noisy but just a thought .

cgfw201

trevor.hall12 said:

Never had any issues with any disc brakes ,
What size disc are you using ,might be an idea to try a 160 if it's a 140 on the rear. Not that it should be noisy but just a thought .

it's a 160

veronese68

I used to have trouble with the rear disc, I seem to have solved it by braking harder. I found the rear pads were glazing as a result of not using the brake hard enough. So the brake would become ineffective, I then made a point of braking harder with the back brake every now and then, kind of like when bedding in a new brake. Not had an issue since.

rwoofer

I have exactly the same problem. If I drag them on some steep downhills it fixes for the ride, but next time they are screaming. I think veronese68 has a point, that rear brakes are just not used as much. Even so, we shouldn't have to put with this.

veronese68

Since I've got into the habit of using the back brake a little harder, and not feathering it so much, I've not had the problem again. It used to happen on the old Parabox brakes I had, that's when I changed my technique and the problems went away. Now got 11 speed Ultegra hydraulics and still no problems.
Never an issue on my mountain bikes, possibly because I use the back brake more off road.

cgfw201

Interesting. Glad it's not just me.

Half the issue is the noise is so bad, that it's completely embarrassing to be using the brakes within half a mile of another human so I just don't.

Will find a quiet spot and give them a properly hard go and see if that does anything to help things.

woodywmb

Experiment with the caliper. A slight shift by loosening the fixing bolts can make the pads touch the rotor at a different angle. This has worked for me after changing cables, pads then scrubbing the rotor with meths - without success. No science was attached. I used feeler gauges to line up the pads with the rotor but reckon it was sheer chance I found a solution.

cgfw201

Went out today after taking the caliper off, regreasing the threads on the bolts holding it to the frame and putting it back.

Headed off down a grassy hill, slammed on the rear brake and did a massive tailslide and went arse over tit 100 metres from my house.

All good. Thought that meant i was back in business, however rest of the ride the rear brake was just a noisy sponge.

Have noticed some vibrations coming up through the saddle when the rear brake is on, will have another check see if there's anything amiss down there.

i.bhamra

I'd definitely be looking for a source of a small leak somewhere resulting in pad contamination, either fluid getting past the pistons or possibly even just the bleed screw needing nipping up. the later is the only occasion I've experienced similar issues with R8070 brakes. It is odd that yo have the same issues on two bikes though.

photonic69

cgfw201 said:

regreasing the threads on the bolts

cgfw201 said:

Headed off down a grassy hill, slammed on the rear brake and did a massive tailslide

Why are you putting grease anywhere near your calipers? They don't need it and a possible source of contamination.

Doing a massive tailslide is not going to bed your pads in. The idea is that you stop in a controlled and slow manner so that the pads bed into the disc. It need the disc to be rotating to do this.

Try this - if your pad carrier, the metal bit, has paint of a coating then try sanding this off before fitting. I found this with my afore mentioned Gorilla pads making a horrible noise. Thay had a thick paint on the back so when the pistons pushed on it you could see the marks it made in the soft paint. i removed this and they were much better.

Charlie_Croker

The noise from a disc brake is caused by vibration. We are all familiar with the wet finger running around a wine glass making a sound. It’s the same thing really.

Bicycle discs are made of thin steel and are prone to move a little. If you could get a thicker or tempered or even cast disc for a cycle I think would go quite a way in stopping this annoying trait. For what it’s worth.

MattFalle

Charlie_Croker said:

The noise from a disc brake is caused by vibration. We are all familiar with the wet finger running around a wine glass making a sound. It’s the same thing really.

Bicycle discs are made of thin steel and are prone to move a little. If you could get a thicker or tempered or even cast disc for a cycle I think would go quite a way in stopping this annoying trait. For what it’s worth.

can't you just fix a weight/s to them to balance out the vibes, man?

MattFalle

or get them mounted floating to do the same?

photonic69

Car discs with thickness of up to 21mm or more often screech, squeal, vibrate and judder. A non starter from me.

pmannion9

Crikey .. reading this thread puts me off ever buying discs if I can avoid it !!!
Im still hoping the cycling industry comes around to this new superlight weight braking systems and starts pushing rim brakes again ....