I've done a trial version of converting my wife's Schwinn Meridian trike to multi-speed, using leftover parts. I've got more leftover bike parts than most--For normal people a donor bike would be the easiest way to get the needed parts. I'm planning to look for one to solve some of the problems I've found
The donor I'm looking for is a department-store 21 speed with a 3 piece crank. Ideally it would have a frame-mounted derailleur and a chain guard, but that's not critical. Better mountain bikes will have a cassette, which won't work in this application. I've seen bikes that would work for around $20. All we need are the drive train parts--if the rest of the bike is trashed, it won't affect the build. Since it is hard to fit a bike in a Cavalier, I'm taking a tubing cutter so I can chop the frame in pieces to get it home.
Like many trikes, the Meridian uses an intermediate gear system. The main chain goes between the crankset to an intermediate hub. The intermediate hub has a freewheel, a brake and a fixed sprocket. A chain runs from the fixed sprocket to another fixed sprocket on the rear axle.
The conversion is relatively straightforward. I removed the old freewheel on the intermediate hub and replaced it with a 5 speed freewheel. I also installed a low-end Shimano derailleur, the type that is held on by the back wheel.
The first step is to disassemble the trike and remove the old freewheel. This is a fairly standard BMX unit, however they neglected to add the notches usually used by a removal tool. This makes non-destructive removal difficult. I opted for destructive removal. (Even with the slots I may have gone with destructive removal rather than either buying the tool or paying to have the old freewheel removed)
The two holes are meant for a pin spanner. I don't have the right size so I used a pair of needle nose pliers with the tips ground to fit. Threads are opposite, so clockwise unscrews. On other freewheels, I've used a punch and hammer.
If you might re-use this, put something under the hub to catch the thousands of little ball bearings that will fall out. (I suspect once you see how many there are, you'll give up on putting it back together...)
Once you unscrew the ring, you will be able to lift the sprocket off the body. The aforementioned thousands of balls will fall out. This exposes the part of the body that unscrews from the hub. I used a vise, locking pliers would probably work. This is standard threads, removes by turning counterclockwise.
Ideally, I should have used a longer axle in the intermediate hub--The derailleur side needed some extra space to fit. I was able to shift it enough so there is just (barely) enough axle on each side. I'd probably want more thread engagement on a bicycle, but this doesn't hold the wheel on so stress should be less.
I need to do something here--I need just a little more space than I have, and I am at the limit of the original axle. If the donor bike's axle is longer I will use it, otherwise I will fabricate a different derailleur mount that doesn't rely on the axle nuts.
To shift the axle--Remove the nut holding the band brake in place--That will let you remove the brake itself, giving access to the cones. You will need a cone wrench (Harbor Freight's bike tool kit is a really good value, and includes the specialized bike tools needed for most bike repair) Loosen both cones from their jam nuts, move the axle over, shortening the part that sticks out on the brake side. Re-adjust the cones and tighten. (Hint: Get the adjustment close, but a bit loose and tighten both cones. Use 2 big wrenches on the jam nuts and tighten until you've removed the play)
You will need some washers or spacers to add some space to the freewheel side, so the nut is past the freewheel. You also need some washers to go between the main frame and the rear axle carrier to get a bit more space. Since the main frame is aluminum, you need to be careful not to spread it too far--Aluminum is not as forgiving as steel to being spread.
I need about 5 mm more space...I gained about 3mm. This is just enough to let the freewheel fit when the intermediate hub was positioned right, but not enough to let the chain ride on the smallest cog without rubbing and probably jamming up on the frame. Not a major problem in this application--The goal was better hill climbing, not speed, and the 4 remaining gears give one gear higher and two gears lower than the original--the ones that would be most used. I adjusted the travel screws to lock out the smallest cog. I can get the space in the frame, but unless I either get a different axle or a better way to hold the derailleur, I don't have enough space in the hub.
The next problem was the derailleur. The standard mount for this cheap type uses a special D-shaped nut that sits in the axle slot to keep the derailleur aligned. Since I had virtually no extra space, this nut was enough to rub on the freewheel. Instead, I moved the derailleur and hub back a bit, and drilled and tapped a hole in the bike frame to replace the D nut.
5 speed index shifters are rare, but 6 speed use the same spacing and will work with most Shimano 5 speed freewheels. The spare shifters I have were 7 speed. Luckily, they have a friction mode, letting them be used with the 5 speed spacing. The final version will most likely use 7 speed spacing, so I'll leave them for now.
Future plans are to change the spacing of the freewheel. If I use 5 gears with the spacing of a 7 speed cluster, I'll save another 2mm or so, which should be just enough to use all 5. There are a few different possibilities--The most likely is to salvage cogs and spacers from a 7 speed and mount them to the 5 speed body. I could also shave the existing spacers down.
Another reason to get a donor bike is the chain and front chainrings--Single speed bikes almost all use a wider chain, with wider sprockets. These will not work with derailleur parts.
I took the largest chainring of the replacement off in hopes of making it work with the old chain guard. Unfortunately the guard bracket rubbed the inner chainring, so I still had to remove the guard. I will try to find a donor bike with the proper chainrings and a guard. (I passed one by at the thrift store that would have been perfect, days before I started this project) Trikes aren't likely to go as fast as bikes, so higher gears aren't really needed--The middle and small rings from a mountain bike set are plenty--In fact, for this use, I am only using the smallest chainring, since there is no easy way to mount a front derailleur.
Update: The summary of what I did is at:
I don't know of any kit for this, I just used parts off of a couple of cheap used bikes. It worked out OK, but Wife eventually wanted a recumbent. We found one and gave the Schwinn away.