oneclick 

Building a small bike, 24" wheel, might use a crankset that's on hand with a 45T big front ring, would need a very small top gear in the back to make it work.
120mm spacing,

pastorbobnlnh 

Not that I'm aware of. 13T is the smallest.

juvela

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and even when one gets down to 13T, on the blocks where it is possible it is often necessary to remove the small cog in order to mount the gear block remover

this is the case with the CycloPans block for example


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Jeff Wills

SunTour 5-speed and Ultra-6 freewheels are relatively easy to remove without removing any cogs. The usual technique of retaining the removal tool with the QR skewer is necessary.

I’m glad the OP defined his need for a 120mm spacing, regular-5 or Ultra-6 freewheel. I recall that SunTour made an 11-tooth cog for the New Winner Pro freewheel (“Z” position??) which threaded onto a 13-tooth second position. This was only available for regular-6 speed spacing.

pastorbobnlnh 

Jeff Wills specifically, I was also thinking about the Winner family of Suntour freewheels when I posted above (I was posting from my phone and only had time for a quick reply). The Suntour Perfect/ProCompe only had a 14T as their smallest sprocket.

IIRC, those tiny 11T and 12T sprockets for the Winner bodies, are for seven-speed configurations and won't work on an Ultra spaced six-speed. But I could be mistaken.

oneclick your best bet on a small-wheeled bike with 120mm spacing would be to mount a larger front chainring. This is why we see vintage Moultons, Dahons, etc., with large chainrings. Fortunately, ebay and Amazon sell these 50T-58T ones (usually in 130BCD) as imports from China inexpensively. I have no idea if these chainrings are durable.

oneclick 

I knew about the 7-speed winner small cogs, they are *incredibly* difficult to separate from their larger brothers.
Thy won't work because the larger mounts in the last space on the fw body, and the smaller mounts on it - so the fw is wider regardless.
Somewhere in the bins are the remains of an Everest with a 13.
I know also about large gears (gratuitous pic below) but would like to use a Maxy 3-pin crank because it's shorter and comes out of the bins; it's possible one of the 52/3/4t in the bins could be modified.

Jeff Wills

IIRC, they were regular-space 6-speed only, possibly because the contour of the cog couldn’t work with 7-speed. Ultra spacing wouldn’t work because the outside cog threads on to the body and it would be limited to 13 teeth.

Ah… vintage SunTour freewheel trivia. So much fun.

bulgie 

Correct, 11 was for regular-width 6-speed only. I used them as sort of "proto-Microdrive" in the early days of mountain biking, to allow smaller chainrings and short-cage derailers. I also made an adaptor to use a freewheel cog as the granny gear on my triple crank. I settled on 18t for the "chainring" but in retrospect, that was too small. I broke chains twice in races, and then went back to 24t as the smallest ring and didn't break any more chains. Chain force goes up inversely proportional to chainring size. But I digress!

Sounds like you're looking for a low-cost solution, so the Grand Bois may not interest you. But it's a Shimano-compatible cassette freehub that comes in 120 mm OLD for 5-sp and narrow-6. Haven't tried it, but it looks like it'll work for 11t.

There's also the Dura-Ace EX that came in 5-sp 120 mm and had the option of an 11t cog. Those I have actually owned and used, but they're super-rare now. The 11t cog was threaded and acted as the lockring, like other Shimano pre-Hyperglide freehubs, but a smaller thread for the small cog. I ground one down thinner (built-in spacer) to use it on a narrow-6, after the thinner cassette spacers became available, not too difficult if you have a belt-sander

Mark B

Piff 

Not to derail the thread, but those Grand Bois hubs are interesting. However, I can only imagine that replacement freehub bodies will either be impossible to find or heinously expensive a decade or two down the line? Or will they take regular 7 speed shimano freehub bodies?

bulgie 

I know a guy who has one, and he's a tinkerer, might well have taken his apart to look — I could ask. But I'll bet you a dollar that it's proprietary. That's the safe assumption anyway, don't buy one assuming you'll be able to get replacements from anyone else.

Don't good freehubs last a long time though? Do you often need replacements? If I actually rode it for two decades and it wore out, then I'd probably feel OK about the return on investment, myself. But yeah if you want parts availability decades later, stick with Shimano. Shimano won't still sell the replacements, but ebay will. Assuming society doesn't crumble into pre-technological dark ages before then.

Piff 

Fair points on all accounts.