Sanding disc brake rotors -- terrible squeal?!

esuhl

Strider590 wrote: »

Sanding the discs was probably a bad idea, when bedding in new discs+pads your basically transferring pad material onto the disc surface, so what you've done is removed that layer. If you've not taken time to bed them in again, the pads will likely glaze over and cause the problem you describe.

Ah, right -- I never really understood what "bedding in" meant!

But what causes the pads to glaze over on clean/unbedded rotors? Do the rotors get too hot, causing a layer of the pads to melt into a smooth surface...? If the pads became glazed, would sanding a layer off of them fix the problem?

Nasqueron wrote: »

It's not necessarily about bedding in, contaminants on the disc or pad e.g. cleaning fluid from a bike wash can cause the squeal

Cheers for the links! I thought I'd read about sanding rotors somewhere.

But yes -- I think the rotors and pads were contaminated. I haven't properly cleaned them for many months. I don't know if it was just road grime, but a lot of black sludge came off the rotors, and the old pads were filthy.

Anyway, the brakes are fine now they've been properly cleaned, so it doesn't look like I've caused any lasting damage. Phew!

esuhl

I went out in the rain today, and the terrible squeal returned (although it was much quieter than before). Hopefully the noise will continue to fade as the discs get re-bedded-in.

So I think the moral of the story is essentially this:

Strider590 wrote: »

Sanding the discs was probably a bad idea...

Nasqueron

esuhl wrote: »

I went out in the rain today, and the terrible squeal returned (although it was much quieter than before). Hopefully the noise will continue to fade as the discs get re-bedded-in.

So I think the moral of the story is essentially this:

It really isn't a bad idea, it's considered a perfectly legitimate technique as per those articles I linked, you're falling into the correlation = causation fallacy. Properly clean the disks and pads (take them apart) using the proper cleaning fluid and carefully put them back on then go for a ride with lots of speed/sharp braking (no skid) to clean off the residue. Check pads for wear as well

Could well be picking up contamination from the road with the rain e.g. oil or fuel

esuhl

Nasqueron wrote: »

Properly clean the disks and pads (take them apart) using the proper cleaning fluid and carefully put them back on then go for a ride with lots of speed/sharp braking (no skid) to clean off the residue.

What... again?! Sigh... Will I have to repeatedly clean them every 5 or 10 miles until they stop squealing? How many times will I have to do that before the squealing disappears...? :-(

Nasqueron wrote: »

Check pads for wear as well

They can't have worn out that quickly -- it's only been a fortnight. The last ones I had lasted about 18 months.

Nasqueron wrote: »

Could well be picking up contamination from the road with the rain e.g. oil or fuel

Yeah -- quite possibly. But they never used to squeal like they do now.

Richard53

I have two bikes with discs and they don't squeal, ever. No matter how much water or crud gets on the discs or pads, they just keep working, silently except for a slight 'whizz' noise which is the vent holes passing under the pads. Cleaning the discs every pad change is a good idea (use brake cleaner only), but other than that they should be maintenance-free. I'd suspect either a problem with alignment (the pads aren't hitting the discs square) or a vibration in the pads caused my a missing shim or spring.


Sanding the discs is a really bad idea, because you are introducing an abrasive medium into the metal of the discs at a microscopic level. For maximum braking effort, you need both the discs and pads to be perfectly flat and free of contaminants. You wouldn't sandpaper the discs on your car.

coffeehound

OP, what grade of abrasive did you use?

esuhl

I cleaned the rotor and pads today, and they were filthy! I've only been on a few rides since I last cleaned them!

Anyway, the visible abrasions from the sandpaper have disappeared, and the squeal seems to have gone again. I didn't get a chance to go for a ride as I broke the rear axle/skewer. :-/

esuhl

coffeehound wrote: »

OP, what grade of abrasive did you use?

I'm not sure. It has two stamps on the back: "Waterproof silicon carbide paper 166" and "P180C" (which is in a bigger typeface).

coffeehound

The P180 is apparently the grade and that is pretty fine so isn't too coarse, which is what I was thinking could be the problem. I'll !!!!!! now

esuhl

coffeehound wrote: »

The P180 is apparently the grade and that is pretty fine so isn't too coarse, which is what I was thinking could be the problem. I'll !!!!!! now

Ha ha! No that's fine -- I appreciate you taking the time to post. It sounds like you know more about this than I do!