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I've got a 10-year old bike I'm trying to revive. It was a single speed that I converted to a fixie. I'm anticipating just casual riding on flat ground.

It has an old-school looking Shimano SGX 46t chainring that I reused plus a Avenir fixed gear cog 1/2 x 3/32 16t in the back.

Because I'm not sure what type of chain the Shimano part takes, I'm inclined to do a 1/8 chain. The one thing that's holding me back is it looks like the offset between the chainring and the rear cog is around 8-9mm, which seems kind of high. I haven't figured out if I can include some rear cog spacers to reduce that, but I've read 3/32 chains are more flexible and forgiving.

I just don't know if my front chainring can take 3/32 and I can't for the life of me find any information on that chainring.

  1. Is the offset enough to be unsafe and I should figure out some spacers?
  2. If the Shimano SGX accepts 3/32, perhaps the increased flexibility can account for the offset?
  3. Or just get a 1/8 chain and be done?

Happy to add photos or additional information if it helps others to help me.

enter image description here

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    Added a photo, hopefully that's adequate - I have the chain draped over the chainring and cog, tried to make it look as tight as possible.
    – fronk
    Dec 7, 2023 at 20:06

3 Answers 3

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The single front chainring won't care about chain width, because it never sees the outside only the inside of the chain. Same goes for a single rear cog.

You do need the two to be aligned as well as possible - 1/8" chain does not kink sideways as much as 3/32" chain can. When looking straight down on the chain from above it should be a straight line as much as possible.

You might need to move the chainring inboard, or move the single rear cog outward. There may be a need for washers or spacers and perhaps even longer chainring bolts.

If there remains significant misalignment, your chain/chainring/cogs will wear faster, and there's a greater risk of dropping the chain. Braking with the pedals will also stress the chain more unevenly, risking a chain snapping.

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    Thanks Criggie. I have a Shimano UN55 BB Square Taper Bottom Bracket (68x122.5mm) installed currently, but no idea if I have spacers currently in or perhaps I have to find a shorter bottom bracket. Luckily, I do have the tools to take a look later this week. Misplaced my lockring wrench, so need to find one to figure out the rear cog situation.
    – fronk
    Dec 6, 2023 at 23:05
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    Although, looking at it now, I'm not sure if my frame supports a shorter bracket? I might just have to bring this into a shop, I'm a little out of my depth. (This is what happens when you google your way into building your own bike 😂).
    – fronk
    Dec 6, 2023 at 23:15
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    My experience was different, running a wide chain with narrow singlespeed freewheel led to a lot of chain dropping. Replacing the chain and front ring with narrow fixed the problem.
    – ojs
    Dec 7, 2023 at 12:33
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    Solution: there's more than 10mm of space between my chainring and frame, so getting a UN55 BB square taper bottom bracket 68 x 107mm, which reduces the offset by 7.5ish mm seems to be ok.
    – fronk
    Dec 8, 2023 at 0:47
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Update: finished the bike build with the help of folks here.

  1. Had to get a shop's help to get the cranks off to access the bottom bracket.
  2. Installed a 68x107mm bracket instead of the original 122.5mm I had.
  3. Got a new chain.
  4. Chain alignment looks rad, doesn't have any noise like before when tightened.

enter image description here

enter image description here

Thanks everyone!

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The photo looks like you have plenty of chainring to chainstay clearance to move the chainring inboard. If I'm doing the arithmetic correctly, a 68 x 107 UN55 would bring it in around 7.5 mm. There are other length options if double-checking measurements suggests moving inwards a bit less would be better.

For the low-ish price of a UN55 and it being a fairly simple job (assuming you have a crank extractor and remember which one of the BB threads is left handed) that's what I'd be tempted to do. As a bonus, you'll have a new BB!

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  • Math checks out! Just have to get my cranks off, they seem to be stuck on pretty handily.
    – fronk
    Dec 8, 2023 at 0:48
  • @fronk you need a crank puller and a big spanner. Don't half-arse it with hammers and crowbar, that only works if crank was already loose.
    – Criggie
    Dec 8, 2023 at 1:53
  • yeah I have the properly sized crank puller from Park Tool and two large spanners. It's stuck pretty good in there though.
    – fronk
    Dec 8, 2023 at 21:07

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