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I bought a Trek FX Series 7.5 in April 2017 and ride every day for about an hour. Two weeks ago the crank arms suddenly gave around the bottom bracket and no longer stayed 180 degrees to eachother. I "fixed" this at first by simply tightening the screws on the crankarms. Last week the problem recurred and I took it to a bike store.

The repairman showed there was actually a spacer in the bottom bracket which broke off, which caused the BB and crankarms to wiggle side to side slightly. He put on a new spacer, gave me the bike, but today the problem recurred.

I don't have any experience repairing more serious problems like this, so I have a few questions. Given my explanation and the photos provided, can anyone guess as to what the cause of crankarms not staying 180 to eachother is? Is it indeed a broken BB?

If so, is this the type of thing that is fixed or should the component be replaced? Should more components than just the BB be replaced?

Any help is greatly appreciated. Thanks.

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Edit: The picture below shows the problem in greater detail. The "spacer" I was referring to isn't part of the bottom bracket, but the crank arm. Also the chain ring bolt had broken/fallen off. The technician put a new chainring bolt and just screwed the crankarms on tightly. As a solution I have bought new crank arms, same model, both right and left. Thanks for the help!

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    Any chance you can point out this mysterious "spacer" the repair tech replaced for you? I think the answers you have are correct, the splines on the left crank arm are damaged and/or the pinch bolts are loose. I'm not sure how a tech would have missed that, or would have thought a spacer was involved. I'm wondering if you have two problems here. (Or if the tech didn't know what he was doing and needlessly replaced something unrelated).
    – dwizum
    Commented Aug 5, 2019 at 20:54
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    Your name looks Germanic, could this be a mere confusion of translated terminology. 'Tretlager' in German is commonly used for crank and for the bearings installed in the frame. In this case your crank is the damaged item.
    – Carel
    Commented Aug 10, 2019 at 12:00
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    The spacer belongs to the crankarm, and I think there was confusion regarding German vocabulary. But also the chainring bolt was gone, so that was also part of the problem.
    – Karl Haebler
    Commented Aug 10, 2019 at 16:48
  • The spacer is referred to as a 'plate pin' by Shimano.
    – Swifty
    Commented Aug 10, 2019 at 17:41

2 Answers 2

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The bottom bracket is not damaged, the left hand crank arm is.

You have a 'two piece' crank where the right side crank is permanently attached to a steel axle, and the left hand crank clamps onto the axle with two pinch bolts. The axle has small splines cut into it where the crank clamps on and the crank has corresponding splines cut into it.

for the crank to come loose and rotate on the axle the pinch bolts must have been loose. You can re-align the crank and re-tighten the pinch bolts, but the crank is likely to move again because the splines in the crank will have been damaged when the crank rotated on the axle. The axle is made of steel but the crank is softer aluminum alloy.

To fix this you need to replace the left hand crank, which means effectively replacing the whole crankset as left hand crank arms are not generally available individually, although you might be a able to find a matching left hand crank arm on Ebay.

I believe the Trek FX 7s used a Shimano R460 crank, which is a lower-end, relatively inexpensive model. Any Shimano Hollowtech II road crank with matching chainring sizes and crank arm length will be a drop-in replacement.

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That's a Shimano Hollowtech II crankset and bottom bracket. Your bottom bracket isn't causing that problem - that's something seriously wrong with your crankset.

The only thing I can conceive of causing that problem is the two pinch bolts on the non-drive side are loose. (your first photograph - the side without the chainrings)

There are two bolts that hold the non-drive side crank arm in place. They need to be tight.

See this Park Tool video:

I also suspect your crankset may be damaged to the point it needs to be replaced. The splines that hold the crank arm aligned are almost certainly badly distorted.

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