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I'm setting up some Avid BB5s.

As I tried to bed in the brakes, the front one squeals quite heavily, the back does not.

The following image is of the non-drive side of the front disc rotor. (Where the outboard pad hits.)

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The following image is of the drive side of the front disc rotor (where the rotor is pressed into the inboard pad.)

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Notice the marks where the pad material has transferred onto the rotor in each picture. The marks end further inward toward the centre of the disk on the outboard pad side, than on the side where the inboard pad hits it.

Because of the way the brake works (bending the rotor at an angle as it is pushed into the stationary inboard pad), this would make sense to me as the very top of the rotor hits the inboard pad, and because of the angle, leaves a gap lower down. But is this how BB5s are supposed to be?

I say this because the while a similar thing is happening on the rear rotor, it is not quite as pronounced (ie, the inboard pad side seems to be hitting a smidge more square on, and does not squeal).

I have played around with different pad adjustments as well as caliper alignment. I've tried the sanding the rotor and pads trick. And it's new so nothing is contaminated and I've used alcohol on everything just to be sure. Nothing seems to make any difference either with trying to bring the position of the inboard pad lower as it makes contact, or the squealing itself.

I took it to my LBS, he seems to think it's an acceptable set up and just needs riding, but did not say why the rear seems to be acting differently to the front, despite setting them up in the same way multiple times. Could it be a slight manufacturing defect on the front one that causes the inboard pad to sit at a slight angle (it's not totally clear with the naked eye).

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I have a lot to say about the BB5's, so sorry for the long response.

The Avid BB5 brake pads can be difficult to get completely parallel to each other and to the rotor, due to (1) the shape of the pad and spring that holds them apart, and (2) the caliper angle is adjustable in 2 separate axes of rotation and 2 axes of translation (most brakes don't have those extra conical washers). They work well, but you may have to live with some noise, and it is not unusual to have to set and re-set and re-set them before you get it to the point you are confident they are in just the right position. If you were looking for a better/easier brake you can upgrade to a BB7, which has a wider pad and spring that wraps around the pad and is generally more secure. Either way, I'd get yourself a trusty 5mm allen wrench and get comfortable with all of the adjustments to your caliper if you really want to be able to maintain and adjust them; otherwise, your bike shop should be able to set you up and show you the periodic things you can do to tighten them as they wear.

Make sure the pads are set all the way in the caliper (they should noticeably "click" in and the tabs that stick out of the top should be even - since you reported the uneven wear on either side of the rotor, I am suspicious that the outside pad on this caliper is not quite in all the way - but I can't see it, so I don't know); also make sure the spring is set far down enough that the little bends in the end of the spring are set in the holes in the brake pad. If one of the pads is still at a wonky angle, you can pull the spring out and manually adjust the kink in the spring to try to even out the pressure on the brake pad. If they are (heavily) used brakes, sometimes brake dust will build up inside the caliper, and washing it out with soapy water or blasting it with compressed air can help the pads set evenly all the way around. Sometimes, even if you do all that, the outside pad will still sit funny, with the bottom of the pad leaning in towards the rotor. In cases like this you can keep toiling for eternity in a Sisyphean struggle for perfectly adjusted brakes, or just accept that they work but don't sound as quiet as you would like.

The rotor should not bend noticeably when you apply the brakes, so that shouldn't be why the wear is uneven on each side of the rotor. Try to get the inside pad as close to the rotor as possible without it rubbing. Even if it rubs a little bit, due to the rotor being out of true for example, that is ok. When you squeeze the brake, if you can notice the rotor bending in, you probably need to tighten the inside pad. If the rotor bends just to one side, you need to re-position the caliper by loosening the two bolts that hold it on, finding the desired position, and tightening the two bolts carefully. If you tighten one too much without tightening the other, it will twist the caliper and you will have to repeat the process. So be gentle. Tighten one a bit, tighten the other a bit, repeat.

I would check the brake pads after a while, and if it looks like the top of the pads is not being worn away, adjust the caliper/caliper mount on the frame so that the brake pads are completely over the rotor. (or, you can just live with unevenly worn brakes, they just won't work as well and you'll have to replace them sooner)

If you took it to the bike shop and they said it was good, though, you're probably ok. Riding it for a while will set the pads in and even if its slightly unevenly adjusted, it will even out as it wears.

Also, secret (not really) adjustment: there is a ~2mm hex screw in the caliper that sets the spring tension. Your brakes will be easier to pull if you loosen this bolt ... its a matter of personal preference, but can make it more comfortable especially for extended use.

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    Removed the pads for the third time. Scratched up the surfaces of the pads and rotors again. Put everything back and seemed to work (no more squeal). Not sure exactly what I did differently. – user34810 Jun 21 '18 at 19:21
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    One more trick: Sometimes with the BB5's even though the inside pad is completely parallel to the rotor, the outside pad is still kinked in at the bottom, and no amount of adjusting the spring/seating the pad seems to be able to make the outside pad sit parallel. In this case you can take a ~1-2 mm wide piece of duct tape or something similar and stick it on the arm of the outside brake pad, which is enough to make the outside pad sit slightly more parallel / it doesn't rub against the rotor. – Kris Nov 25 '18 at 21:34
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Having seen some mechanical disk brakes over my life, the Avid's 9 out 10 will emit some sort of sound.

You indeed are correct about how a mechanical disk brake works, by bending the rotor. So it's most important to set them properly. This is my procedure:

  • Take the caliper off, and check that the pads are parallel. Clean them if necessary.
  • Check if the rotor is tightly bolted to the hub.
  • Set the fine adjustment bolt on the lever and the caliper to none(all the way in)
  • Mount the caliper and bolt only using your hands
  • Bolt the cable lines.
  • Set, the static usually, adjustment to lock the rotor, then set it back half a turn.
  • Now, hold the handle with a ziptie and lock the brake.
  • Meanwhile use a hex tool to tight the caliper bolts
  • Finally, fine adjust the brake line.

Hope this will help you.

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