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Background: A couple years back I built a chopper with a 26" front wheel and a 12" rear wheel. It was a lot of fun, but I had to pedal like mad to get anywhere. Unfortunately I could not find a rear cog small enough or a front cog large enough to get me somewhere useable on long rides and ultimately had to increase the wheel size to 16". The size increase worked out great, but I want to try to build a new chopper with a 12" (if not 8").

Are there cogs small/large enough out there to make this work?

My other thought was to weld a bottom bracket shell to my seat post with a multi-speed chain ring and chain one sprocket to the front and the other to the rear. Would this work? If it would and this seems like the right way to go, what sprocket would need to be connected to what sprocket?

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    First thing that comes to my mind is a 9 tooth driver off a BMX hub. Although making it work could be tricky as it is not a standard cog attachment. Sounds like a cool project though! You could always off set the rear end and have a really long raked out fork with the smaller wheel in front. – Nate W Aug 17 '16 at 18:30
  • I think I've heard tell of a 56 tooth chainring as well. If you could pair that with the bmx sprocket you'd have a little over 6:1. That's not far off being equivalent to 2.5:1 on a 26" wheel. That's a low gear but not stupidly so. I wonder what the smallest wheel with a hub gear is (from a folding bike probably) – Chris H Aug 17 '16 at 18:40
  • You could custom build a bigger chainring -- vimeo.com/70921986 uses a 100+ tooth chainring. – Batman Aug 17 '16 at 18:47
  • @Batman, if that's what I think it is, he's a frame builder with machinists to help. That makes custom gears easier – Chris H Aug 17 '16 at 20:47
  • Shorter cranks will also make a contribution to a higher gear. Whatever solution you find to bring about higher gearing, say, 155mm cranks will steepen it quite a bit compared to 170mm. IT's a little hard to find good ones at a reasonable price. – Kaz Aug 17 '16 at 22:28
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Have you considered an intermediate jackshaft with a two-stage gear?

Easiest thing would be to salvage a bottom bracket and a triple chainring, and weld it somewhere between the normal pedals and the rear wheel. This would give you:

46 tooth front chainring = 26 tooth triple inner / 46 tooth outer = rear cassette

So my maths is rusty but that should be the same as a 81.3 tooth chainring for the purposes of gearing calculators.

If you used the 36 and 46 of your middle triple, it would be a 58.7 tooth chainring.

The high revolution rate on the rear chain and cassette will make it wear faster - you might be looking at a new one in as little as 1000 miles.

Later Look at a Schwinn adult trike enter image description here

Side view enter image description here

Another view of a trike enter image description here

If you were really good, you could have the primary and secondary chains, and use a single loop of chain as a tertiary chain between two identical but opposed cassettes, and rig two derailers so that the tertiary chain moved up-one and down-the-other cassette. This could be a 5 speed chopper with a T shifter !

  • I just realised - we're describing a tandem chainline, but with different sized timing chainwheels. Does that help picture it, or would you like a sketch ? – Criggie Aug 18 '16 at 1:39
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    I believe a similar scheme is used with some hand cycles, but I've not been able to find an example. – Daniel R Hicks Aug 18 '16 at 1:44
  • Thank you! This helps a lot. If I go down to an 8" someday, so I can simply add another jack shaft? – user2525 Aug 21 '16 at 16:43
  • @user2525 Could do - or you could increase/decrease the size of the gears involved, if you build them to be removable. Or weld them on for simplicity. – Criggie Aug 22 '16 at 2:00
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Using an 8 or 9 tooth BMX cassette hub will get you into the normal useful range for singlespeed gearing pretty easily with a normal large road ring. 53-9 on a 12" gets you about 70 gear-inches for example, which is a common choice for fixed/singlespeed road bikes. Most present-day freestyle BMX rear hubs would work fine. (Most are 9t, some are 8 - 8s wear out fast and are considered going too far by many.)

Using a 56t tri ring and a normal 12t cassette cog gets you into around 56 gear inches, which would probably be fine for a chopper. 56t rings, while not exactly common, are easy to get. Origin8 makes a 56x130. If this gearing worked for you it would probably be a cheaper route.

Since you mention going on longer rides with it, the good answer may just be to run it with the Shimano Capreo group and a 56t ring, giving you the benefits of a derailer drivetrain. I'm pretty sure the Capreo derailer should still clear the ground on a 12", but you might want to corroborate that somehow before doing it. It may not on an 8".

  • J&B sells a 20" wheel with a 3/8 axle and 9 tooth for about $60 retail at a shop (#640156). They sell the hub by itself as well, branded BlackOps but it's a KT hub. They show no stock though. – Nate W Aug 17 '16 at 21:51

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