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As tires improve, the increased rolling resistance of smaller tires becomes less of a concern than it formerly was, but one problem that has persisted is the difficulty of getting decent high gears with small drive wheels.
The gearing of a normal sprocket driven wheel is proportional to the wheel diameter, so if you use smaller wheels with the same sized sprockets you would use for a bike with full-sized wheels, all of your gears will be lower.
Lower lows are rarely a problem, and by using a tight "racing style" cluster in back, you can raise the low gears easily enough if you don't want super low gearing.
The top end, however, has always been a problem with small wheels. There are only three ways to get a decent high gear with small wheels:
If you have 349 mm (16") wheels, you would need to install an 86 tooth chainring to be equivalent to a 52 tooth with common 622 mm wheels!
Even if you could find such big chainrings at an affordable price, you could never find a front derailer that would do a decent job of shifting it.
Special, very expensive freewheels with 9-tooth sprockets have been available for Moulton bikes, but Moulton has been reluctant to market them to owners of other brands of bikes.
Enter the Shimano Capreo system! This is Shimano's first group intended specifically for small wheel bikes. The Capreo system includes:
Special 9-speed Freehub, 135 mm spacing, 24, 32 or 36 holes.
9-10-11-13-15-17-20-23-26 tooth sprockets.
The Capreo cassette won't fit any other hub. The Capreo hub won't accept any other cassette.
The 4 smallest sprockets, 9, 10, 11 and 13 teeth are all special.
Instead of using the male threaded lockring common to other Hyperglide cassettes, the Capreo uses a much smaller female threaded lockring, using the same 22 x 1 mm thread as common crank extractors.
It comes set up for 135 mm spacing, but the left side of the axle has two removable spacing washers, one of which is 4 mm and the other 8 mm thick. By removing spacers and trimming the axle, this hub could easily be adapted to other spacings, though this would result in a wheel with more dish.
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