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1

There are several different 18" sizes, and several different 16" sizes. See Sheldon Brown's tire sizing page If the 18" is the 355mm used by Birdy, and the 16" is the 349mm used by Brompton, that's close enough that it might be possible. People have converted Birdys from 18" to narrower 20" (406mm) by moving the brake posts (see "brakeboss relocator"). I ...


1

Unless the bike was specifically designed for a wheel size conversion, like the 18/20 Prodigy from Hoffman in 2003 or so, changing the wheel size is generally not a safe and practical option. Brake mount positions and tire clearance to the frame prevent it.


3

Alternatives to adding a (possibly very low) rear derailleur: add a hub gear instead I'm not sure you can do this in-line with the rear axle (it would have to be joined to the hub shell), so it might need to precede the rear axle and drive a second chain. Actually, I see Sturmey Archer make specific tricycle hub gears, but I have no idea how they're ...


6

Normally I would say "you wouldn't start from there", but since you have... Since you've managed to add a cassette, I assume you're reasonably confident with tools. You're going to have to make a derailleur hanger that sits in the right place. If you can get a sealed bearing over the axle it'll be easier, but if not you will need to make a plain bearing ...


1

I found this image in an untrusted link, on a site called https://www.factorydirectbikes.com/. It seems to show a trike with gears. Perhaps either the people at that site, or Pashley themselves might help you. At the very least, you'll need to add a bracket to the centre frame member to hold the derailleur. See ...



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