Grabby Disc Brake
 

Since I put new rotors on my CX bike (replaced the Deore-level rotors with XT ones), the rear one is not quite right.

It "grabs" at a regular point every rotation...either that, or a point opposite is "slipping"...hard to tell. Whatever the case may be, it creates a grabbing/pulsing effect.

I've sanded it, and cleaned with rubbing alcohol. The rotor appears to be true as well.

It's "bedding in", so it is starting to show marks/wear from the pad, but I can't visually detect any irregularities.

The brake is a mechanical Avid BB7.

I'm at a bit of a loss now. It certainly doesn't stop me from riding, but it is irritating. And it has diminished the usual advantage this bike has in wet weather. I was coming to a stop the other day, the front brake was modulating normally, and the rear brake "grabbed" at that point in the rotation, causing the tire to momentarily lock and make the back end go squirrely.

Thoughts?

How did you torque it on the hub, using the correct tightening pattern?

Are you using an adapter for the Avid 185mm rotor or one for the 180mm Shimano rotor? You sure the interface between caliper and rotor is ideal? The other poster may have something in going thru the installation of not only rotor, but caliper, too.

On motorcycles a grabby disc is a sure sign of a kink or warp in the rotor or a thickness variation that in some way tries to wedge the pads apart moreso than the other portions. The amount of variation that can result in this sort of pulsating braking action is so slight that it cannot be seen with the naked eye in any but the most gross of conditions. It requires a machinist's dial gauge capable of reading in thousandths of an inch to measure runout and thickness for signs of whatever damage is causing the issue.

That makes sense. Banzai, you may just have a defective rotor. Just curious, but why are you running Shimano rotors with Avid brake calipers? My BB7's came complete with Avid rotors.

I'm running Avid rotors with Shimano calipers.

So here's the story...

I had two bikes; Apollo HT and a Giant HT. The Apollo had BB7s, the Giant had 486s. I came across a great deal on an '04 Giant NRS, but it came without wheels and chain, and needed a bit of other work.

So the wheels for the NRS came from the Apollo, rotors attached obviously.

I then got some SLX brakes for the HT, and the 486s replaced the Hayes Nines on the NRS. :D

I don't think there's necessarily anything wrong with it, I was just asking Banzai what his reason(s) for doing it are.

Rotor to caliper mismatches are fine as long as the pads are fully contacting the rotors.

So you need to check the rotors for any kinks or localized warping. One way to do this without a dial guage is a good quality 6 inch machinist's ruler. Offer the straight edge up to both faces of the rotor and look for signs of any see saw like movement in the ruler. With care you can easily detect a localized warp, kink or other damage that will show up while riding. With the ruler on the concave side of the damage it'll seem fine because it's bridging the area. When on the opposite side it'll pivot on the high point. It's the pivoting that you'll feel so it's important to check both sides with the ruler aligned in a lot of different angles and on a lot of spots. Light pressure should be used since a rotor is easily flexed.

Another form of damage that'll catch the pads and cause some grabbiness is a dent that raises a burr along the edge or in one of the ventilation holes. Again you can use the edge of the ruler to check for that by lighly dragging it over the swept faces of the rotor and feel for any snags. If you get one it's a simple matter to dress the spot down with a file or Dremel.

Swap rotors and see if the problem follows the rotor.

Thanks guys.

I am "mismatching" the rotors with the calipers because my hubs are Shimano centerlock. So, I don't have the problems with correct tightening pattern.

When I thought it might be a contaminant, I was willing to work with this. Given that the rotor is brand new, and from Nashbar, I'm just going to order another, then return this one. Maybe I'm being wasteful, but while I'm willing to do some work on a used rotor, I've NEVER had one right out of the package need attention on the order of, say, dremel tools.

The calipers were working perfectly with my "Deore" level rotors, so I'm just going to put one of the old ones back on. Replacement for this one already ordered.

I love to tinker and repair, but I have little patience for "out of the box" failures. You guys have persuaded me that this is very likely a shape/structure flaw in the rotor itself.

So you have matched adapters to your rotor size?

I believe that I have the same combination of rotor (SM-RT78 from Nashbar) and brake (Avid BB7 road). I also had problems with weak, pulsating brakes. If you look closely, the brake track on the XT rotor is narrower than is typical. This means that your pads are probably hitting the arms, which causes the pulsing sound.

Using the XT rotor, the stopping power is about the same as the cantilevers that came on my Crosscheck-- not terrible but not as good as I'd like. The pulsing goes away and I have better stopping power when using an Avid Roundagon or a Clean Sweep G3. This solution works for both 6-bolt hubs and Centerlock hubs using a Shimano adapter (SM-RTAD10). A cheaper solution that doesn't work as well is to shim the brake mount out by one millimeter, so that less of the pad is hitting the arms.

Yes. That's not the problem.

The braking isn't weak, and it isn't a "pulsing"...nor is it rapid enough to be paired with the phenomenon you are describing. Rather, there is a definite "grab" at a particular, and regular, point of the wheel rotation. Having done my best to clean the rotor, I feel I have to accept the theories offered above, that the rotor is quite possibly thicker in that location, or that there is some form of localized distortion that has eluded eyeball detection so far.

Did you pull the pads and inspect them for any unusual wear indication? There is a braking track difference with the Shimano vs Avid rotors, so still wonder if something along those lines (other than the slight diameter difference). If you took calipers to the rotor you might verify the thickness issue which sure sounds like the cause if nothing else checks out.

Give that man a cigar. Yes, the XT brake surface is different than the Avid, so much so that given certain configurations the brakes are actualy hitting the center web of the rotor.

And sanding them was certainly not a good idea. Rotors are blanchard ground to assure a consistent surface finish.

I have the same problem on the rear of my bike, Shimano rotors and calipers, giant and the bike shop think theres "nothing wrong" :rolleyes:
I was thinking it was a difference in thickness in one spot of the rotor aswell.

The front brake generates a sound due to the pad hitting the center web. The rear brake "grab" is at a regular point of wheel rotation, much lower frequency than the faint pulsing noise on the front that corresponds with the 8 supports on the center web.

Sanding was my cleaning device of last resort, since no solvent had done the trick. Either way, the sanding/alcohol cleaning produced absolutely zero change.

There are harmonics you can setup with any brake, too. I just revived an old frame with some v-brakes and the rears howl pretty bad even though they look good, I'll experiment more tomorrow. You can only eliminate possibilites by going thru them just like so many noises with bikes...

Banzai, just wondering if you solved the problem yet and if so how.