Crazy Brake Vibration

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flag Nichtj
Posted: May 29, 2023 at 19:02 Quote
https://youtube.com/shorts/T5xo1mgn0Sg?feature=share


k. Need some help. I have a terrible rear brake vibration on my bike. Video is of moderate braking with cold brakes. It does it at light and moderate braking. Under heavy braking it will not. After four or five moderate braking and things get heated up, it almost goes away.

I am running Sram DB8 levers and calipers. I have tried two different pairs of pads (Sram Sintered and Galfer standards). First tried both pads with Chinese rotors. Then tried both pads with Hayes Dominion rotor. Followed bed in procedure each time. Sanded pads with medium grit sandpaper between rotors. I’m at a loss! Any ideas or trick?!

I have brand new sram centreline rotors and organic pads (PN- 00.5315.023.030) on the way in hopes that will fix it but…..not holding my breath at this point.
flag Nichtj
Posted: Jun 2, 2023 at 6:19 Quote
It’s a custom build. Came off same assembly line as Trek apparently. China brand. Full aluminum dual suspension e-bike.
flag BenLow2019
Posted: Jun 2, 2023 at 7:02 Quote
Here's things to check:
(1) rotor not contaminated, not rusty or corroded
(2) brake pads not contaminated
(3) brake pads not deformed, nor chipped and pad surface flat
(4) Rotor aligned and straight, not deformed
(5) Caliper aligned and straight
(6) Caliper pistons both operating smoothly
flag Nichtj
Posted: Jun 2, 2023 at 7:06 Quote
Thanks Ben. As I mentioned, have tried two different rotors. Both were brand new. Have tried two different pairs of pads. Always same results. There’s no rubbing of pads at any point during rotation so caliper is straight. I check caliper pistons; running 4-piston DB8 that were installed brand new so should be fine but that’s on my to-do today for it.
flag BenLow2019
Posted: Jun 2, 2023 at 8:35 Quote
Watched again.
Seems like it could be any of the things I mentioned.
Double check the caliper alignment - look at the space between the pads and the rotor as you spin the wheel - if you see the rotor stay in the exact same place all the way around then good, but if it is drifting back and forth you have a very slight bend in the rotor. You'll have to straighten it. You can get a wobble just by uneven or over torqueing a rotor bolt. It doesn't take much.
And you should have EVEN SPACE on both sides of the rotor during complete rotation which is why caliper alignment is crucial. Just because it isn't touching doesn't make it aligned - if you have more space on one side then you will have issues.
And finally the sticky piston - make sure your pistons are operating smoothly. Some would say that piston lubrication is a myth and others will swear by it. I say check and resolve any issue you discover - a little piston lubrication can't hurt.

Good luck.
flag HaggeredShins
Posted: Jun 2, 2023 at 10:12 Quote
Sounds like what happens when the rotor is contacting something that's not pad. Check that the vertical spacing of the caliper is sufficient so the top of the rotor isn't contacting the pad spring -- this noise is classic of that.

Of course also check pad spacing as stated and make sure the rotor isn't getting pushed into the caliper body or adapter.
flag mior
Mod
Posted: Jun 2, 2023 at 11:38 Quote
I had this and got the classic "you try cleaning it??" from the bike shop. sram DB8's have cottered brake pads, and the noise is from the pads and cotter shifting. my pads and rotor are still not as quiet as my mt5's, but i think some combos are louder due to resonant frequencies and stuff. i bent the cotter pin so far that it doesnt move and that fixed it, probably because it restricts the pads
flag Nichtj
Posted: Jun 2, 2023 at 12:02 Quote
mior wrote:
I had this and got the classic "you try cleaning it??" from the bike shop. sram DB8's have cottered brake pads, and the noise is from the pads and cotter shifting. my pads and rotor are still not as quiet as my mt5's, but i think some combos are louder due to resonant frequencies and stuff. i bent the cotter pin so far that it doesnt move and that fixed it, probably because it restricts the pads

My DB8s are not cotter pin. They are the Allen screw with with circlip safety.
flag Nichtj
Posted: Jun 2, 2023 at 12:04 Quote
HaggeredShins wrote:
Sounds like what happens when the rotor is contacting something that's not pad. Check that the vertical spacing of the caliper is sufficient so the top of the rotor isn't contacting the pad spring -- this noise is classic of that.

Of course also check pad spacing as stated and make sure the rotor isn't getting pushed into the caliper body or adapter.

Thanks. I’ll have a very close look at it all. I know it’s not touching the pad spring but potentially pushed into caliper. I’ll check that when going through calipers and pistons later on.
flag mior
Mod
Posted: Jun 2, 2023 at 12:21 Quote
Nichtj wrote:
mior wrote:
I had this and got the classic "you try cleaning it??" from the bike shop. sram DB8's have cottered brake pads, and the noise is from the pads and cotter shifting. my pads and rotor are still not as quiet as my mt5's, but i think some combos are louder due to resonant frequencies and stuff. i bent the cotter pin so far that it doesnt move and that fixed it, probably because it restricts the pads

My DB8s are not cotter pin. They are the Allen screw with with circlip safety.
try cranking it down or a quick ride with no circlip or extra loctite or something
flag WrecklessREX
Posted: Jul 25, 2023 at 3:16 Quote
I have done this exact procedure and had good results.

https://youtube.com/shorts/GomGfiAnRqY?feature=share

Followed by bedding the the pads in again

Coincidentally this video was the first one under your video
flag Daniel092
Posted: Jul 26, 2023 at 12:40 Quote
Check the hub bearing and make sure the freehub isn't seizing either. If you've done all the "normal" stuff around the brakes and it didn't fix it, this is where you need to look next.
flag dood
Posted: Jul 28, 2023 at 0:02 Quote
Didja check to see that the hub face for rotor mount isn't out of true? Wouldn't take much on runout to completely screw up a bolted connection.......maybe a few thou......

Contrary to most thoughts on the interweb, machines still screw up once in a while. Production is difficult - and mounting a small part perfectly in a lathe, or CNC, is a big cause of failure.

One wacky bolt face, out a few thou, could easily turn into the ol' Shakira routine. And let's be honest here - hips don't lie.
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