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I got a new bike a few months back. Its a cheap decathlon Rockrider mountain bike so I am not expecting anything too special. However I have noticed that going down steep descents, even slowly, the back disc brake makes a sort of knocking sound, tap tap tap all the while I hold it down. Even if I am going slowly it happens if its steep enough. Going fast and using it on a flat surface it doesn't seem to happen. It's my first bike with disc brakes. Any ideas what's going on?

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  • Could you get a video of it? Usually brake noises manifest as screeches or scraping noises, not thunk thunk thunk impacts. – MaplePanda Jun 20 '20 at 23:57
  • Clarify please "while I hold it (the brake lever) down" does that mean you're riding the brake for longer periods ? Does the noise go away if you brake harder, as in you're trying to stop on the downhill? Do please be safe while testing. – Criggie Jun 21 '20 at 12:26
  • What kind of disk brakes do you have? I'm expecting cable-operated disks, but what calipers? Brand and model if you can. – Criggie Jun 21 '20 at 12:27
  • Thanks for the feedback. The brakes are cable operated. The model is 160mm TEKTRO TKD 68 according to the website. The noise doesn't go away as I brake harder. I wasn't braking for long when it started. It's pretty reproducible - I'll try get a video next time I'm out. – steprobe Jun 22 '20 at 13:08
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Possibly calliper alignment. Manufacturing variation in hub or badly faced rotor mounts can move the rotor plane in or out a crucial tiny amount which combined with a odd calliper shape can result in some of the arms of the rotor bonking the calliper under the right conditions. Hence the thunk, thunk sound. Not dangerous but annoying.

Moving the callipers a touch to one side (e.g use a paper width as a gauge) might fix the issue without significantly affecting the centering.

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