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I just bought myself an old vintage Koga Miyata and just realized today that there's a weird dent on the frame near bottom bracket.

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Is this something I should be worried about? Or is it normal with these kind of frames, and what's it for?

EDIT judging from other pics of the same frameset it seems like it's normal? https://www.velosaloon.com/products/koga-miyata-gents-touring-frame-in-58-5-cm-c-t-57-cm-c-c-with-high-tensile-1024-tubing

Any idea what they are for and why aren't they symmetrical?

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    That's normal for a frame of that vintage. Jul 13, 2021 at 18:12
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    Theres some large rubber in there - do you have sufficient frame clearance for that wide rear tyre? I'd pull it and check for witness marks on the paint. Yes, rubber can wear off steel.
    – Criggie
    Jul 13, 2021 at 22:56
  • @Criggie I second that. Once I had to ride home (~5km) with a buckled rear wheel. The tire sawed through about a mm of aluminum. Basically no friction loss was felt.
    – Vorac
    Jul 19, 2021 at 18:22

1 Answer 1

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The chainstay has been crimped (at the time of manufacture) to provide extra clearance for the chain rings. On some bikes, I’ve seen this clearance provided by having the bottom-bracket end of the chainstay be a solid plate (thinner than the chainstay tube).

The bike almost certainly came with a multi-gear crankset when new.

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    The crank on the photos is multi-gear. Jul 13, 2021 at 17:22
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    Ah yes, it’s missing the small chainring. To make it clear: The frame is manufactured this way intentionally. When done properly crimping can actually improve strength by work-hardening the steel.
    – Michael
    Jul 13, 2021 at 17:42
  • There probably should be an emphasis that this is intentional and deliberately manufactured. There is no evidence that it was damage or an after-market modification.
    – Nelson
    Jul 15, 2021 at 1:00

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