Sanding disc brake rotors -- terrible squeal?!

esuhl

My mountain bike is about 5 years old, so this time when I bled the brakes and replaced the pads, I lightly sanded the rear disc rotor.

Now, whenever I touch the brakes, the whole frame vibrates, emitting a loud screech! The brakes work fine, but... it sounds like I'm using an air horn every time I brake!

Have I been too heavy-handed with the sanding? Is this normal? Will the screeching subside after some time? What have I done?!

Dan83

Just out of interest, why did you sand the rota?

Take your brake pads out and put some copper grease on the back, the replace them.

You will only need abit of copper grease, so no need to buy a big tub of it, it's not dear.

Put the grease on the back of the brake pad, the part that goes into the calliper, not the brake rota. Don't put to much on that it squish's out and goes on the front of the pad.

That's an old trick to stop car brakes squealing, I can't think of any reason why it won't work on a push bike.

coffeehound

Probably down to the grade of abrasive on your sandpaper. A rough grade will cause more noise and probably eat through your pads, too. Perhaps worth working down to a fine grade so the disc looks smoother.

.

Retrogamer

What compound of pads did you have originally and what did you replace them with?

esuhl

Dan83 wrote: »

Just out of interest, why did you sand the rota?

To improve braking performance. Apparently the rotors become smooth over time and should be periodically sanded.

Dan83 wrote: »

Take your brake pads out and put some copper grease on the back, the replace them.

Thanks -- I'll look into that.

Retrogamer wrote: »

What compound of pads did you have originally and what did you replace them with?

I replaced the pads with identical ones -- both Clarks brand sintered pads.

Johnmcl7

I've not heard of sanding a rotor before as normally it's just to roughen up the pads a bit if they've been sitting around, does the surface of the rotor look marked?

Retrogamer

sintered pads are known to be really noisy as well.

I've sanded down discs before with Kevlar and resin pads before and no issues at all with vibrations or the noise

esuhl

Johnmcl7 wrote: »

I've not heard of sanding a rotor before as normally it's just to roughen up the pads a bit if they've been sitting around...

I thought it was a common procedure. I remember seeing several "how to" videos on YouTube (which is where I've learnt everything I know about bike maintenance!). I had another look, and I can only find the one video below. Maybe I've been misled in thinking that this was a normal thing to do...?

https://www.youtube.com/watch?v=NsJGvP--3UI

Johnmcl7 wrote: »

...does the surface of the rotor look marked?

Yeah... there are visible scratches where I used the sandpaper. :-/

Retrogamer wrote: »

sintered pads are known to be really noisy as well.

I've sanded down discs before with Kevlar and resin pads before and no issues at all with vibrations or the noise

When I got the bike (new), the rear brakes often made a juddering sound, vibrating the frame. But it disappeared after several months. I've always used sintered pads (I vaguely remember them being cheaper and more hard-wearing than organic pads).

I cleaned the brake rotor with Muc-Off Brake Disc Cleaner before I sanded it. Since then I saw some silver reside on them (probably from the metal dust after sanding). I'll clean the rotor again, and sand off the contaminant on the pads. Hopefully that will improve things.

I've only ridden the bike for a mile or two since the noise started, and I've already noticed it getting quieter. So hopefully cleaning everything up again will sort it out... (*fingers crossed*).

glasgowdan

No need to sand rotors on bikes.

Best with organic pads too. Also check the caliper is aligned perfectly over the disc. And that the bolts are tight.

Use isopropyl alcohol to clean the disc. If the pads get contaminated it's easiest to just replace them.

esuhl

Well, I cleaned the rotor and pads thoroughly with brake-disc cleaner, sanded the remaining layer of grime of the pads (and cleaned with disc cleaner again), and... the brakes are the best they've been in ages! No squeal, and great stopping power!

Phew! Cheers, all! :T

glasgowdan wrote: »

Best with organic pads too.

Why's that? I read up on the differences, and thought that sintered pads seemed ideal for my purposes (although I forget why).

Retrogamer

Sintered pads last longer in wet / muddy rides and when they do heat up they perform better but they cause the rotor to wear quicker and can be noisy.

If you ride in the mud / grime a lot or you're a heavy rider or are very aggressive with braking these are best.



Organic pads perform better when cold and don't cause as much wear on the rotor and are (usually) much quieter. They wear rapidly in the muddy / wet conditions though and they can become glazed a bit easier with lots of extreme braking / downhill.


I bought some alcohol cleaner for my rotors. On my hybrid i have always been really careful when applying chain lube and cleaning the cassette and chain to avoid contamination the rotor or pads. After cleaning both the front and rear rotors i found that the rear one was about 4 x as grimey as the front. I imagine at speed some spray must get onto it.

I had the same issue with contaminated pads a while back.