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I replaced my back tire. After putting it back on, I noticed this under the toolbox. Only one. It is either from the circular disc brake thing on the right, or the gears on the left.

I thought it could be a washer, but it doesn't seem to slide through the threads as the inner hole is too small. Also, for some reason the inner hole has teeth.

I tried finding a service manual online, but nothing came up.

EDIT

After some research I found it must have come from the dynamo hub on the front tire. The anti-rotation answer makes sense, since there is a part that rotates (the part where the wire goes into the wheel) that shouldn't rotate assuming everything is tightened. I ended up putting it back, having it touching the dynamo hub.

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Its called a crinkle washer and is normally found between the underside of a bolt head, and the face into which the bolt passes through before threading into another part. The inner teeth will give an idea of the shaft diameter for the bolt it fell off.

It is an anti-rotation washer intended to add friction and resist the bolt's desire to back itself out under vibration.

It is not from the gears on the right-hand side of a bike. It may be from a disk brake caliper, but I've never seen them on a disk rotor.

It also may not be from your bike at all.

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  • 1
    I know them as "lock washers," a term which also encompasses a couple additional washer types. Apr 3 at 15:56
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    @Michaelcomelately fair call - I'd understand it as crinkle washers are a specific kind of lock washer, which are all specific kinds of washer.
    – Criggie
    Apr 3 at 18:38

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