I'm trying to sort out pad dragging / noises in Avid BB-7 mechanical disc brakes and I'm wondering how much runout in the rotor is acceptable? Right now I have about 0.05 mm (measured with a Park DT-3i).
3
-
1I would say straight enough that it doesn't make any noise is the acceptable amount. But that's not very helpful to you, I feel.– alexAug 19, 2015 at 12:38
-
I'm not sure but I think the caliper on the BB7 moves both pads inwards, unlike some cheaper types where one pad is stationary. In my opinion, that means they can cope with a bit more runout, Five hundredths of a millimetre doesn't sound bad to me, as long as it's not a 'sudden' kink. But what @alex said.– JHCLOct 19, 2015 at 8:56
-
1Warped rotors are pretty much a fact of life, and the single most irritating aspect of disc brakes. Across 4 different disc equipped bikes i've yet to have a rotor last more than 3-4 rides without exhibiting at least a little warping. Not an answer, but my advice would be not to bother about minor dragging noises as long as they are not significantly slowing the wheel when you spin it, otherwise you'll spend as much time truing rotors as you do riding. My LBS gave me this advice 4 years ago, but it wasn't until I had another 3 bikes with discs I saw how right they were.– Andy PMar 11, 2016 at 9:41
Add a comment
|
4 Answers
0.05mm is nothing. Stop worrying about it. As long as rotor does not make noise or drags extensively on the pads, you'll be fine.
Rotors change shape every time you ride: you apply breaks a little longer than couple seconds, rotor heats up and expands, then cools down and shrinks. On extended periods of braking rotors can heat up enough to burn your fingers when you touch it (done it myself unfortunately). And when this level of temperature change is applied all sorts of transformation happens to the level of warping/runout. So you can spend all the time in the world to make rotor as true as possible, then you go for a ride and it all changed and rotor is nothing like you've left is last time when you fiddled with it.
I am truing a rotor right now and it started with enough brake rub to stop the wheel from free spinning in a stand. I didn't get a number but estimate it had over 2mm of runout.
After 15 minutes of rough tweaking, it has 0.16mm of runout. This results in slight pad rub for me. The rotor visibly wobbles when the wheel is free-spun.
Finally after another 30 minutes effort, it is down to 0.065mm of runout, and while not perfect, its good enough.
Your 0.05mm (0.004 inches) should be fine and any further issues will be in your caliper, alignment or brake pad offset.
The BB5 and BB7 are single-sided calipers, and the pad setup is notoriously twitchy.
- Set the outer pad wide open by tightening in the barrel adjusters.
- Use a T25 torx through the wheel and tweak the dial so the inside/non-moving pad is just clearing the rotor. This should be done after bedding in brake pads. When the wheel will free-spin, go back to the ouside/moving pad.
- Then twiddle the barrel adjusters to just-not rub. If you run out of adjust, you might need to move the inner cable inside its clamp.
At no point should you need to relax the caliper's mounting bolts - that will upset the parallelism of the pistons to the disk.
Truing a rotor with a cheap chinese dial indicator, on a knockoff noga stand which is magged to a cheap mill-vise, which is clamped around a tine of an old steel fork, mounted vertically in a bench vise. With some clamps added for confidence, I suspect the magnet was not doing much.
I usually replace my disk brake pads when I have 0.5 mm thickness. The reason I do that is because they can easily be smashed by the jaws to remove the braking material ( which will eventually fall apart ). After that it will result in a friction between the disk and the metal of the pads, which is very very bad.
I change the disk when it stops braking when I use a new disk brake pads. But for the thickness I should say that It depends how much are you loading them. As you say above disk with thickness of 0.05 mm is very hardcore ;). You should really think of replacing that thin disk, because it can really surprise you when it breaks.
-
3
-
2Fortunately the disc is more like 1.8 mm thick, but I'm pretty hardcore about trying to get it straight :-) For what it is worth, Avid recommends replacing the pads when they are worn to 3 mm thick (including the backing). They start at 4 mm, and 3 mm leaves about 1 mm of pad. For rotors that minimum thickness that has been quoted here is 1.6 mm. Beyond a certain point the rotor will be more likely to warp with heat – and as you suggest, to suddenly fail.– dluAug 19, 2015 at 13:50
-
Yes, that's correct. I totally get what you are trying to make now. If the rotor is bent, it will slightly touch the pads, generating an awful creaking noise. This sound is also very hated by dogs and they start to chase you ( sometimes for not particular "heard by you" reason ) Try adjusting the brake pads to the left/right. Thought I do not recommend you make it straight by force, because that will make mini-fractures inside the rotor and reduce its life. Aug 19, 2015 at 14:05
If the rotor is bent. I would recommend replacing it instead of trying to fix it. This would give you peace of mind.
-
2Fair call - often a new part fixes the problem, but not replacing parts that don't need it works out cheaper. OP has the right tools for measuring runout so I suspect he knows his way around a workshop.– Criggie ♦Mar 11, 2016 at 0:50
-
3Also, parts aren't perfect. Some amount of runout is almost certain to exist and brake designers very likely design with an acceptable amount of runout in mind – you don't want people needlessly replacing parts (nor do you want your components to have a reputation for being difficult to maintain or adjust). So what I'm hoping to learn is what is typical, so that I have some data to make an ignore/repair/replace decision with.– dluMar 11, 2016 at 2:45