Folks, I need a hand. Does anyone know of a bike mechanic that can diagnose problems via FaceTime or another audio/video app?

I ride strictly indoors (I’m in Iraq) and there’s a knocking coming from the crank/BB area that I haven’t been able to fix.

Equipment: Cervelo S3 (rim), Sram BB30, Rotor 3D+ cranks, Rotor 54/42 oval rings, Sram eTap, Speedplay Zero stainless steal, NOT ceramic BB

Each time either shoe passes the top of the crank circle I get a good 'thunk' sound. I can hear it and I can also feel it resonate in the carbon frame, no matter speed, RPM or watts. My cleats are on tight, pedals on tight, I removed the cranks and reinstalled them to spec, bolt rings are tight. I put my finger and turned both bearings using my finger tip. Both bearings are still ultra smooth.


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    Have you previously been riding the same setup with no issues, or is it new? – Andy P Mar 14 '19 at 16:14
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    Could this be from somewhere else on the bike? Does the noise occur when you are out of the saddle, or if you are not holding the bars? If your feet are on the ground and you turn the crank can you reproduce the noise that way? – Ed Neville Mar 14 '19 at 17:07
  • I see you took advice you got on FB :-) – Argenti Apparatus Mar 14 '19 at 18:57
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    Unfortunately (from a diagnostic point of view) could be anything. Could be bad bearings in the pedals, could be bad bottom bracket bearings, could be that the BB cartridge is loose, could be a loose chainring, could be loose cranks, could be the chain jumping on the chainring. And a few others. – Daniel R Hicks Mar 14 '19 at 20:29
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    Get an assistant to hold the bike and pedal with your hands on the crank to try and identify a source. Having your head right down by the suspected problem area may result in a different direction from the sound. – Criggie Mar 14 '19 at 20:32

My bet is that this is caused by the bike rocking side-side as you shift body mass and pedalling forces. The noise is caused by the trainer shifting or perhaps the bike frame shifting on the trainer. The vibration or noise is coming through the frame and feels like it is emanating from the bottom bracket area.

Its sounds like you have ruled out the noise coming from the bottom bracket / crank area. I would spin the crank by hand with the bike in the trainer to make sure of this. Also grab the crank arms and push/pull in all directions to check for any play or movement in the bottom bracket.

If you have a smart trainer check that its axle does not have any play.

Try sitting on the bike and shifting it side-side, as if you were sprinting, but without rotating the pedals. Can you replicate the noise this way?

Can you ride the bike off the trainer to see if the noise occurs then also, or goes away?

Can you recruit a friend to listen and try to locate the source of the sound while you ride the bike? (Being careful to not get kicked in the head or tangled in the drivetrain of course.)

  • "try riding your bike on the road" sounds like an excellent diagnostic idea. If not a road then a cycle path or some other not-road facility. I'm slightly flabbergast that someone chooses a cervelo for an indoor bike. – Criggie Mar 14 '19 at 20:35
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    @criggie, perhaps OP is posted away from home and only has the indoor option – Argenti Apparatus Mar 14 '19 at 20:41

My suspicion at this point is around a BB that has some problem internally.

Since the noise is at a fixed crank angle, its not related to gearing but I bet it happens in both small and big chain ring.

You could help isolate this by removing the chain and pedal in the trainer - even with no-load you should perceive the noise or sensation still.

Cranks are relatively solid items, and if they were moving relative to the BB then it would be at 3 oclock where pressures are higher. Maybe test this by setting the cranks level, and push/pull both sides at the same time. See if the noise now exhibits.

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