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Uneven braking on my Avids

985 Views 14 Replies 8 Participants Last post by mountain_bomber156, Aug 23, 2009

Spongebob

Discussion Starter · Jun 29, 2009

when braking, my front avid brake has uneven braking. Seems like if it were a car, the rotor was warped and would need to be re surfaced. So I took off the rotor, cleaned, sanded down, and cleaned again, re installed and torqued properly. I was good for about 1/2 mile and then started again. The rotor is as straight as it could be, but I'm out of ideas on whats causing it, unless there's an area in the rotor that just slips more than 90% off the rest of the rotor. I didn't check the pads, because they work fine on the other 90%. I don't feel anything in the lever.
Any ideas?
Thanks
Bob

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Spongebob

Discussion Starter · #2 · Jul 1, 2009

55 views but no feedback? Anybody?

:thumbsup:

mountain_bomber156

· #3 · Jul 1, 2009

Have you tried using a rotor truing fork (or at least pliers) to restraighten it?

Spongebob

Discussion Starter · #4 · Jul 1, 2009

The rotor seems true. As it turn it doesn't rub or seem to wobble any more than any others I've had.

mountain_bomber156

· #5 · Jul 1, 2009

Well, you seem to have a very strange problem there, Bob. Fluid problems?

Spongebob

Discussion Starter · #6 · Jul 2, 2009

Sorry, failed to mention they're mechanical avid discs. I have a set of new pads, maybe I'll try that, but I don't think that its that. I'll have to get out the micrometer and do a few samples around the disc, just to see if for some wierd reason, a short section is narrower, but that wouldn't make sense either. If I only had another 160mm..... Its got to be some grease or something, even though I only handle them on the insides and clean them regularly. Dag.:madman:

Howley

· #7 · Jul 2, 2009

Me too-2009 model BB7 mechanicals-20-30 miles on new brakes/rotors. Just started yesterday. Front only give a 'pulse' feel just as you described in original post.

I'll pop the pads and give a light sanding to see if that helps-

Old 2005 version of BB7 did not do this-

vk45de

· #8 · Jul 2, 2009

Is your radial alignment ok? Caliper too far in cause a pulsation sensation. It's a crazy guess cause that shouldn't go away when you sand rotor. Is it the turkey wobble-like vibration?

Spongebob

Discussion Starter · #9 · Jul 2, 2009

Its like it just doesn't brake in a small section of the rotor, giving you a feeling like there's contamination in the metal of the rotor. But I clean them with acetone which gets any oils or grease off of it. The rear is a larger rotor, but that shouldn't matter. Btw, this is a tandem and the front fork is only rated for a 160mm.
I'll try to reset the caliper set up too.

gmcttr

· #10 · Jul 2, 2009

Spongebob said:
I'll have to get out the micrometer and do a few samples around the disc, just to see if for some wierd reason, a short section is narrower, but that wouldn't make sense either.

If it is a rotor that you have been using for some time, I agree that it isn't likely to have suddenly thinned down in a small section, but do measure it.

FWIW...I have bought one rotor (not Avid) that was .004" thinner for about 1/3 of it's circumference. You could feel a definite pulsing in the lever as well as in the braking.

xcguy

· #11 · Jul 2, 2009

I've had two rotors (out of many) that exhibited the "pulsing" that you describe, on my BB7s. To make a long voyage of discovery story short it was simply that the offending rotors were machined weirdly in one part that made the pads grab more than on the rest of the rotor. I would isolate where in the rotation the pad would want to grab more (very sketchy when it would grab unevenly like that when turning on singletrack, especially in front) and attempted to lightly sand it smoother to match the rest of the rotor.

I never could get a rotor like this to not grab. The only solution was a new rotor. In my case the brakes/rotors were new so they "pulsed" like this from break-in. If yours started this "pulsing" after they'd been ok for a while I don't know what to say.

Spongebob

Discussion Starter · #12 · Jul 4, 2009

Well today I measured its thickness with a micrometer. It was fairly consistant, between 73 and 74 thousands, (if I was reading it right). I took it off again and went at it with a belt sander and 150 grit paper. Then checked it for true. Fine. Remounted after an acetone bath. Hopped back on and tested it and it seemed to be much better. Did notice that when it spun, it was slightly out of round by a mil or 2. Oh well. Only a long ride will tell. I'll report back after a fully loaded ride........

mtroy

· #13 · Aug 23, 2009

I have the pulsing brakes on both F&R on the BB7s. New brakes, they did it within a couple of rides. Sanded the pads, flat sanded the rotors. Cleaned the pads and rotors. Rotors are true.

It sucks, basically, as it is bad enough to lock up the rear wheel on low traction areas and the front is not something you can count on in a fast, decreasing radius corner.

I guess new rotors are the next thing.

Too bad cuz I really like a lot of things about the brakes other than this. I will admit that I did not follow the break-in procedures to the letter of the law. Basically I went on a couple of rides, but this is moderate speed XC stuff on a hardtail SS. Not exactly chairlift territory. If that was what screwed up the brakes, then why did all the other disc brakes I have used just worked out of the box?

Right now, between the bb7s and the Juicy3, Avid is 0 for 2 in my book.

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kuratsunade

· #14 · Aug 23, 2009

The stock disks are something...lol...

mountain_bomber156

· #15 · Aug 23, 2009

Somebody should try running Cleansweep rotors with the BB7s, see if that fixes it.

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