WTF's going on with my front brake.

A change of pad material is best accompanied with a clean rotor. As you use a pad, it deposits a thin layer of brake material on the rotor. If it can't do that due to the rotor being coated by a different pad material then squeals are more likely. Look up "bedding in'.

However, it could also be a combination of cold/damp and/or contamination from the road.

Well I'll clean up the rotor with isopropyl alcohol and perhaps sand the pads and have another go if there's no obvious sign of leakage and see if that helps.

You should check your brake pads for wornness. Also if your brake pads, calipers, or slider pins are not properly lubricated, they may cause vibrations and noises.

Also make sure the caliper is aligned correctly. If it's just sort of catching an edge on the pad it may be causing vibration which you can hear as the squeal.

I have a theory that in the cold pads and rotor don't get hot enough and glaze which also causes that squeal. I'm  probably wrong but I notice on my winter bike which is ridden in really muddy conditions all the time, and never really gets up to any speed it always has a squeal. A brake pad change and rotor clean solves it for a while but it normally comes back.

Summer bike which has long downhills with much fear led braking by me never squeals

Right, what's going on here then? Bare in mind that I've never had an issue with Shimano pads and I may have bought a duff caliper, but I took the pads out and found this:-

I cleaned the rotor thoroughly with isopropyl alcohol, sanded the pads down (removing all the black) and tried again. After 6 or so hard stops, more squealing and this:

I found another set of pads (unknown brand) with the same effect.

The rotor is true and the calliper's aligned, and I've been running the same setup with Shimano pads for years. Is the new calliper duff or is something wrong with the rotor?

Show us your rotors...

That’s classic George Foreman grill syndrome… sorry the bikes knackered… scrap it.

The rotor is a 180mm SRAM Centreline (single piece, not floating) and is true. It looks perfectly good and I can't feel any discernible lip suggesting that it's overly worn, but perhaps that's the next thing to try.

I have an old pair of Shimano RT66 rotors that have been lightly used, so I could clean these up and try them.

I'm off to work soon, but I'll have a try tomorrow.

Looks like the rotor surface is concave, so only touching top and bottom of the pad...

rotor surface has been worn into a concave shape, time for a new rotor

Yes surely a worn rotor

If you just keep riding they might match up

I'll try some different rotors tomorrow.

Out of interest, why has the braking surface worn concavely rather than just thinning? As it & the pads are flat I'd have assumed (obviously incorrectly) that there'd be even friction across it.

I expect it's maths 🙄

My assumption is this. The force from the pistons is in the centre of the pads. Because the pads flex, very slightly, the pressure applied is greater in the centre than the edge.

Out of interest, why has the braking surface worn concavely rather than just thinning? As it & the pads are flat I’d have assumed (obviously incorrectly) that there’d be even friction across it.

Some manufacturers seem to have forgotten you need uniform material surface across the braking surface, i've not looked at a centreline, so i'm not sure what they look like. If there is less material to press on, it'll wear faster.
The outer edge of the disc tends to wear (slightly) faster as well.
You may have, in the past, had a different alignment between disc and pad (low/high on the braking surface).

Worn rotors and pads?
New bike time.

PJay
Free Member

Out of interest, why has the braking surface worn concavely rather than just thinning?

Some rotors just do it. The old rubbish Formula drilled rotors were legendary for it frinstance, you always ended up with discs like a little wave and pads worn to match, left unchecked you could literally wear through the middle and cut the outer rim of the disc off

Centrelines are in a lot of ways like 2 narrow rotors attached together in 12 little spots, most of the time the centre of the pad is touching nothing at all. The actual effect that has is way more complicated than it seems- you've got varying heat loads, different bits experience more duration of friction, but others see more leading edges as you go from air back to metal which can be quite wearing, and there's a natural self-correction of uneven pad wear until teh rotor also wears to reflect that shape. I'm not going to pretend like I can predict how a rotor/pad combo will wear by looking at it...

But centrelines definitely look like they could wear funkily. Also, not sure if it's relevant but there are absolutely tons of convincing fake centrelines out there. No idea if they're better or worse than genuine... But I think it's easier to buy a fake than a real one now.

Crickey I didn't realise just how worn they were. I'd been keeping an eye on the edge of the rotor, which looked fine but you really can feel the dip.

Putting replacement RT66 rotors on not only solved the squealing but also removed quite a bit of lever travel before biting. I'm lucky that they didn't disintegrate!

I shall be more attentive in future.

I had exactly the same with Shimano calipers on old Avid 183 rotors. They wore concave, so much so the edges of the pads were down to the thickness of the spreading spring but the middles looked like new!!

Can't believe how well they still worked like that but after a change to Shimano discs I'm much happier