Originally Posted by
Hoopdriver
Some manufacturers use Shimano's Capreo hub and cassette for 20" wheels. This system uses 9-10-11-13 tooth cogs on the small side, which effectively gears the bike/trike in a way that compensates for the smaller wheels. It also means that you should not consider gearing to be a big factor in what wheel size you use.
Not quite true. Cogs smaller than 13 teeth suffer progressively more efficiency loss and higher wear. Also, you're talking about getting a one-of-a-kind hub and cassette. So they're harder to drive, more expensive to replace, and you replace them more often. Again, that only matters if you expect to go fast enough that you need gears over 80" or so. Slower than that and you don't need to normalize the gearing.