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Bicycle Type: Gary Fisher Marlin Mountain Bike with 3x9 drive train.

Issue Description: When pedaling, there's a clunking noise emanating from what sounds like the cassette & hub interface but only at higher pedaling RPM. The clunking noise doesn't occur when pedaling at lower RPM. Proper cassette lock ring tightness has been verified with a torque wrench, so the cassette isn't loose. I have a video of the noise occurring only as pedal RPM increases to a certain speed: Video Displaying Issue

Background Information: I think this clunking issue started after changing the tires on my bike. I went from a mountain bike tire (2.1" wide?) to more road friendly tires that are 1.9" wide. Could the weight difference of the tires be causing the slack in the drive train and clunking? I verified that the chain isn't stretched with a chain stretch tool. The bottom bracket bearings/cranks are tight and quiet as well.

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    In the video it appears that your crank speed is not consistent through the circle so the wheel freewheels briefly faster than than cassette is being turned. When the pedal speed increases again, the free hub engages with a clunk
    – Andrew
    Commented Jul 12, 2020 at 1:23
  • Does it happen in all gears ?
    – Dan K
    Commented Jul 13, 2020 at 11:40
  • Yes, the clunking happens in all gears. Yes, the clunking sound emanates from the cassette and free hub interface making contact whether due to a rotational speed differential or slack in chain tension.
    – J99
    Commented Jul 19, 2020 at 20:38
  • Sounds like the crank arm is hitting the chain stay, though I don't see the marks I'd expect to see if this were the case.
    – Daniel R Hicks
    Commented Jul 19, 2020 at 21:31
  • The suggestion from @DanielRHicks can be easily verified: switch the front gear to 1, then take the chain off the cog (as if it had fallen off, onto the bottom bracket, due to poor gear adjustment). Then try spinning the cranks again, with the chain and wheel staying stationary. Does the sound stay or go?
    – pateksan
    Commented Aug 18, 2020 at 23:00

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Given that the cassette lock ring was torqued properly and the play between the cassette and free hub was similar to that found on friends bikes, I moved on to checking other parts. I removed, cleaned and lubed both rear derailleur pulleys. This reduced the clunking sound about 90%. The pulleys currently installed are original and I'd guess have about 700 miles on them. In any event, I plan on replacing both pulleys with new ones with the hope that this completely eliminates the clunking sound.

My guess is the old pulleys weren't turning as freely as they should and this resulted in less tension on the chain, which created the slack between the cassette and hub that enabled the clunking sound. My thinking is the new pulleys will have a slightly larger diameter that will allow the rear derailleur spring to provide just a tad more tension on the chain thus completely eliminating the clunk. The reduction in chain tension may have resulted from a combination of pulley wear and slight chain stretch (but not enough to register on a chain stretch tool). New derailleur pulleys will do the trick. Thanks for taking the time to read this.

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    I don't seen how problems with the RD idler pulleys could cause such a loud clanking sound. And the clank is in sync with the crank, meaning there is something amiss with the crank assembly.
    – Daniel R Hicks
    Commented Jul 19, 2020 at 21:35

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