Bicycle Mechanics - Squealing rear brake on long descents

Hello,

So I know that I'm not suppose to use the rear brake on descents and this might be the reason why I have this problem, but I just wanted to sort this out. I have a tendency to use both brakes to slow down before turns and on prolong downhills, after a while, I start getting this squealing sound that sounds like a mini version of what you hear from semi truck's brakes :lol: The noise usually starts when I'm going 30 MPH and above and is sometimes accompanied with slight vibration that freaks me out at those speeds.

I've cleaned the rim and adjusted brake toe-in. This is on a stock 2010 Giant TCR Advanced 2 with Ultegra groupset and Mavic's Ksyrium Elite rims.

Any ideas what's going? Is this what happens when you brake with your rear at high speeds?

Thanks.

-- ilya

1) So I know that I'm not suppose to use the rear brake on descents...

2) The noise usually starts when I'm going 30 MPH and above and is sometimes accompanied with slight vibration that freaks me out at those speeds.

3) I've cleaned the rim and adjusted brake toe-in.

4) Ultegra groupset and Mavic's Ksyrium Elite rims.

5) Any ideas what's going? Is this what happens when you brake with your rear at high speeds?

6) Thanks.

-- ilya

I have numbered your points so I can deal with them in turn:

1) Why not? I use a combination of both front and rear braking on long descents to avoid heat build-up and the brake fade that can result. It's nothing you're doing wrong in that regard, IMHO.

2) Vibration isn't normal, at any speed. I'm not surprised it freaks you out.

3) They would have been my first suggestions, along with checking wheel true and rim wear (latter will not apply if you've only done 1200 miles). What did you use to clean the rims? Did you clean the pads too?

4) Same setup here. No such issues. Ever.

5) I would think that there is either a series of gouges in your rim (unlikely, but worth a check) or you have some sort of sticky substance on the pads. I suppose it's worth checking your frame and spokes for any annomalies, but I think you would feel more than vibration if anything was wrong there. Can you replicate any vibration on a stand? Can you feel anything if you turn the wheel with your hand with the brakes lightly applied?

6) No problem and please post as to how you get on.

:)

Thank you for your reply.

I will examine the rear rim and brake pads when I get home.

What about the center bolt, the one that mounts the brake to frame... If it's not tight enough can it cause my symptoms? I've noticed that the rear brake can be moved, at the center bolt, easier than the front brake. I'm just a little nervous about tightening it since this is a full carbon frame and I don't want to crack anything.

-- ilya

Check for loose brake arms. Are they tight? do they move?

Check for loose brake arms. Are they tight? do they move?

Well, I don't think they are loose but I did notice a wobble in the left arm, which I don't know if it is normal or not. The right arm is does not bulge but it is also thicker than the left. Here is a video I just recorded: http://www.youtube.com/watch?v=_dSB12ADJ3M

How much effort are you putting in to make that arm move like that? There's quite a bit of movement. I'll check mine tonight and post back.

Just wanted to post this to make this more informative.

I explained my problem at my LBS. The guy over there took off my pads filled them down with a file and then with a sand paper. Then he wiped them off with aceton and explained that cleaning them with a degreaser sometimes is not enough because some degreasers actually have some oil in it. He also lightly sanded down the walls of my rim. 300+ miles later still no issue.

Thanks for the update. Interesting.
(I know there have to be some products out there that leave no residue. What about things used to prep for painting?)