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

  • 1
    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


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.

  • 1
    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
  • 1
    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
  • 1
    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
  • 1
    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

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!


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!

  • 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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