Many start their frame building the rear forks. This is not a wise thing to do. If the front turns out different from what you had planned, there's no going back on a welded frame. It's my advice to wait with the rear until the front wheels are on in order to have the right geometry.
You can change it on a bolted trike of course. If you find the trike needs a bit more caster, or less, in the worst case you only have to replace the vertical piece of the frame. Most of the times modifying the dropouts or drill the threaded rod higher or lower will be enough to compensate.
The size of the rear wheel is not really important. Smaller rear wheels will result in smaller gears and faster sprints. A larger wheel will be a tiny bit faster for touring. Take your pick. Bare in mind a smaller wheel is more compact and the frame will have less flex.
The construction is easy. Lining out the rear wheel can take some time. With a file the dropouts are adjusted once the rest is done. You leave them a bit higher ( actually lower) on purpose just so you can play around with achieving the right track.It's very difficult to cut and drill the whole rear frame without needing to tune the alignment at the very end.
The size of the tubing can be 30, 40, 50mm. Or maybe some solid material. Important is the stuff is pretty rigid. There's enough flex in the threaded rods holding the forks together. So thin material is not the best choice. You can reinforce the rear assembly by extra rods or L-profile to increase overall stiffness if you want. But I like to keep it simple. Every trike has some flex which in fact is not a bad thing.
Calculate the height of the rear wheel axle from the top of the horizontal rear part of the frame. Add half of the dropout and the size tubing for the rear forks. That's how long the vertical piece of the main frame should be. Don't forget the tire.
If you're off by one cm, no worry. If it's more it will influence steering and ground clearance. So please check it twice and a third time.
The dropouts are made of either L-profile or plate material. 5 to 8mm thick. If you only have thin metal do a double layer. You don't want flex here.
L-profile is shaped, the axle hole drilled out and cut in at an angle and attached at the bottom of the rear forks. .
If the dropouts are out of plate material they go on the inside of the
rear forks. Pay attention the bolts leave room for the smallest cog and
chain on the cassette or freewheel. Adjust the rear spacing to 135mm.
The rear wheel doesn't necessarily have to be dished on a trike. It even can be far out the middle of the trike like on Windcheetah trikes. But you can simply adjust the spacing left and right till it is in the exact center.
I have now changed it to this. Lighter, stiffer, and pretty easy to align. This particular wheel is not dished. So the forks are out of center. BTW the wheel doesn't have to be right in the middle! It will handle just as good. For example Mike Burrows Windcheetah has the wheel way off center. But in our case for the chain to clear the frame the wheel can't be to the left of the frame.
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