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I have enountered a strange problem with a fixed gear build: when the wheel/crankset spin, the crankset's rotation creates a periodically pulsating sideways wobble on whole the bike. It can be felt when riding, and upsets the balance (riding is possible, but not pleasant). See image below for details.

At first, I assumed the wheel's rotation was somehow creating that force (angular momentum of a wheel spinning at a crooked angle, or untrue wheel). However, the wheel looked true. So I took the chain off to decouple the crank and wheel rotations. Holding the bike up by the saddle, front wheel on the ground, I spun the wheel and cranks separately. Spinning the wheel did not produce fluctuation, but spinning the cranks produced a lot of wobble.

The strength of the wobble got higher the faster the cranks spin. I assume it might be caused by one of these:

  • Bent crank makes one side much heavier than the other? These are 90's Shimano Dura Ace cranks, so such a bend appeaning unnoticed is unlikely
  • Issues with bottom bracket axis or bearings, which makes the angle of the crank spin not parallel to rest of bike? The angle seems good enough from looking (chainring distance from frame stays the same when rotating), so also unlikely
  • Some other mystery reason?

Has anyone encoutered this? Any advice on finding the issue and how to fix it?

Graphic of the rotation issue

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    Try free-spinning the crank with the chain disconnected. The bike will do the same "wobble" which means its the nature of the crank, and its distance from the tyre/ground patch. Also try hanging the bike from a single rope/cord clear of the ground, and repeat the crank-spin
    – Criggie
    Dec 11, 2022 at 2:04
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    Thank you for the comment! There is definitely a weight/angle imbalance when the cranks spin. I have disconnected the chain, and just spinning the cranks wobbles the bike. Weirdly, I cannot see an angle issue on the drive side at least: the horizontal distance between the chainring and frame, and vertical one to the ground, are visibly consistent when rotating. So at least axis of rotation is perpendicular to the frame. Could it be bad bearings on the BB?
    – goose_lake
    Dec 11, 2022 at 10:23
  • Bad BB bearings are likely to feel either wobbly or notchy. It's often easiest to detect with the ctanks removed, just turning the spindle. Do you have compatible cranks to swap on for a test? Does the wobble seem matched to the rotation (e.g. the frame wobbles right as the right crank goes down)? What about spinning the cranks backwards? Does it feel any different if you take the cranks off, rotate them with respect to the spindle, and put them back on?
    – Chris H
    Dec 11, 2022 at 12:30
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    Maybe also weigh the cranks while they're off, just in case of a sneaky mismatch despite labelling
    – Chris H
    Dec 11, 2022 at 12:31
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    @ChrisH Good advice, thanks. Cranks weigh the same. The BB isn’t wobbly, feels solid and well oiled. Wobble is matched to rotation, appears regardless of spin direction. When doing this test I noticed a big issue: one of the cranks is actually not set on a straight axis: when looking at the circular hole in the crank (dust cap removed), the hole changes position when rotating, meaning it’s not perpendicular to the axis. So either bb is bad or crank. Will try a different set of cranks tomorrow to find out.
    – goose_lake
    Dec 12, 2022 at 18:30

1 Answer 1

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The mystery is solved: this issue was caused by a defective non-driveside crank. Since I bought the frame without wheels, but with the BB and cranks installed, I trusted the seller that they were all good. Turns out I shoud not have.

Previous investigation (decoupling the crank chainrings from the wheel sprocket) isolated the wobble origin to the cranks spinning. Since the wobble amount was synchronized to the crank rotation, it was clear there is an imbalance of the weight on one (or both) side(s). Therefore, there could be three potential culprits: the cranks (bent or off-center), the bottom bracket (faulty bearings or axis), and the frame itself (crooked angle of BB tube).

I got a different set of cranks with the same fittings, installed them, and gave it a spin. The wobble was gone. To be extra sure, I installed them on the bike I took the other cranks off from, and, as expected, the other bike started to wobble when they were spinning.

Now I just need to replace the cranks.

My thanks to the people who left comments and advice.

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  • Nice find! Can you add a photo of the bad cranks to this answer? I'm curious to see if the cause is visible.
    – Criggie
    Dec 14, 2022 at 19:58
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    Ah, didnt see your comment on time! Already donated the cranks to local bike workshop, I hope the drive side crank can still come in handy. From visual inspection there was nothing obviously wrong with it. I think it’s because the misalignment came from the axis spindle hole (these were square taper cranks, Shimano FC7400), and was therefore quite hard to note. Quite confused where this defect could have originated from. Maybe sustained pressure from the side? Only the original owner would know…
    – goose_lake
    Dec 15, 2022 at 17:57
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    could have been crash damage in the past too.
    – Criggie
    Dec 15, 2022 at 22:16

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