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I have a Montague Boston full-sized folding bike which ships as a fixie. I've added a Sturmey-Archer 8-Speed hub and now am trying to determine the optimal chainring size. The stock chainring has 42 teeth. As a consequence I use third and fourth gear mostly on the flat and find even low is a bit lacking on modest hills. Fifth through eighth are mostly wasted.

I will be downsizing the chainring but am wondering if there is some way, other than trial and error, to determine the optimal chainring size. Any ideas?

The stock crank is a Suntour single with an 5-arm spider. Any idea if I'll be able to just switch out the chainring with another significantly smaller one or will I likely have to replace the entire crank set?

Incidentally, I'm an old fart with bad knees and I mostly commute or leisure ride at a dawdling pace.

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    Welcome to Bicycles @Brian. We recommend that new users take our tour; it can make life here a lot easier :-) The only information I think you could / should add would be the BCD measurement. (BTW enough of the old fart stuff! Many of us are 60+, and the oldest regular guy I ride with is 80+ :-) – andy256 Aug 11 '16 at 3:26
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    BCD 131 is non standard, sure its not 130?. Min tooth on a 130mm BCD is 38 - about 10% smaller. The Sturmy Archer has a 14% step between gears, so at best its moving down 1 gear. Changing the crank may let you go smaller but you may have problems with frame clearances. – mattnz Aug 11 '16 at 4:33
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    The day will come when you're riding up a grade or into a headwind when you wish you still had those gears. Likewise, a tailwind someday will see you hooning down the road, feet madly doing 2.5 revs/second, wishing for one more gear. Since you only have the one front chainring, keep your options open. Mine's a 1x6 speed and I want more both at the top and at the bottom end. – Criggie Aug 11 '16 at 5:16
  • Thanks mattnz: Yeah probably 130. That was the result of a raw calculation.When you say "frame clearances" are you referring to the width of the bottom bracket? – Brian Grover Aug 11 '16 at 9:05
  • On some internal hubs, you can substitute a different sized cog, also. – Kaz Aug 11 '16 at 16:14
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Sheldon Brown has a gear calculator available so you can input all your info there as-is, note the gear inches for gear 4, and then change the ring size until your modified 8th gear is near that number. In addition to the info you've already provided, you'll need the number of teeth on your hub's cog to place in the spot of the first cassette cog.

FWIW, I recently had a dawdling bike with an internal 3sp and the smallest ring my cranks would accept and I sometimes wished for a lower gear. Also if your setup looks like this then you may not be able to downsize very far without swapping cranks.

Edit incorporating comment thread

How small can I get and what tools do I need to replace cranks? Sheldon Brown's BCD crib sheet can let you double-check your measurements so far and it also lists the smallest ring any particular configuration will accept. The interface between your bottom bracket and cranks is almost certainly square taper, so you'll need a new crankset with that same interface (which is a combination of quite old and quite common so that's good news for you). The only tool that you should need is a crank puller to get the existing cranks off.

Can I use mountain cranks? Assuming it's got a square taper bottom bracket interface, you can go with any cranks you want. Especially with a multi-gear crankset you should mind your relative chain line. My hunch is that the smallest ring on a triple in your scenario could be sufficiently inboard of the cog for the chain to work its way off the outside of the ring, seeking the plane of your cog. In that case you could swap your bottom bracket for one with with a longer spindle. More tools and expense, but possible.

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    Thanks id est laborum: It took me a while to figure that all out [very clever approach indeed!] and if I did it right I should look for a 23-tooth chainring [smaller even than the 25-tooth rear sprocket. Is that even possible? Yes my set up is identical to the one pictured in your link to Montague's website. Doesn't seem hopeful does it? Ballpark: what's the smallest front chainring I'm going to be able to find without the drastic step of replacing the crank set? – Brian Grover Aug 11 '16 at 5:05
  • Sounds like you're stuck with 38 tooth minimum chainring size unless you change the whole crank. Which is not scarey but it could start getting expensive. – Criggie Aug 11 '16 at 5:17
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    Oh, and a chain tool to shorten your chain. – Fing Lixon Aug 11 '16 at 12:39
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    Could I switch to MTB cranks, many of which sport the size of chainring I'm after? – Brian Grover Aug 11 '16 at 22:56
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    Assuming it's got a square taper bottom bracket interface, you can go with any cranks you want. Especially with a multi-gear crankset you should mind your relative chain line. My hunch is that the smallest ring on a triple in your scenario could be sufficiently inboard of the cog for the chain to work its way off the outside of the ring, seeking the plane of your cog. In that case you could swap your bottom bracket for one with with a longer spindle. More tools. Hence @Criggie 's "expensive" wisdom above. – Fing Lixon Aug 12 '16 at 5:22
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After more searching about I discovered that Sturmey Archer makes a couple cranksets that address this specific issue with their hubs, one with a 33-tooth chainring and a 30-tooth version. I've ordered the 33T Crankset as I found one in Taiwan at half the price of a 30T one from the US, saving both money and delivery time. This should allow me to cruise in 5th and 6th, adding a couple lower gears for hill-climbing, leaving me with 2 higher gears for wheeeee!

Thanks again for all your assistance in understanding the complexities involved and the tools required for the job.

  • Nicely done--I never realized S-A made cranksets. Still mind your chain line. And wow! They make fifty-SIX tooth rings for all the hammerhead time trialists out there using planetary hubs! – Fing Lixon Aug 13 '16 at 4:16
  • @idestlaborum maybe for small-wheel bikes like most folders. If your wheel diameter is say 2/3 of that if a full size bike, a 56 tooth chainring would be like a 37, getting you back to a normal range. And SA years are quite common on folders. – Chris H Aug 13 '16 at 6:47
  • Oh yeah @ChrisH of course! But wait. Why would a time trialist ride a folder? ;) – Fing Lixon Aug 13 '16 at 14:53

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