Hard to see the floor drawing, but in the next few pictures, I'm laying frame pieces on the floor drawing to check for alignment of things. Will it work in steel?
The donor frame is cut, removing most of the chain stays, all of the seat stays, and most of the top bar. No battery can fit in the last few inches by the headset anyway, so I left a stub, that I will use to weld the seat stays to. At this point I'm looking at things like will the headset angle work, and will the crank land too high off the ground, or too low. Also looking at where the seat stays will land. There has to be at least enough seat tube to attach them there. I wasn't entirely sure how the geometry would really play out once the donor frame is placed in position.
Now for the chain stays. I was going to use some beefy square tube I have, but it's just too thick. It's crazy heavy. But I noticed that some 3/4 conduit was exactly the same diameter of the cut down chain stay. Hmmm. Hate using conduit. But it's there, and it's the right thickness to be light. It bends so nice with my conduit bender. And,,, I'll regret this one later.
But meanwhile it was easy to fabricate the joint. A couble slight bends, and then a piece of 1/2 inch conduit inside for reinforcement. This will be a pretty critical weld, so the inner tube is just there as a backup, if my welds break. If the weld fails, the inner tube will help keep me from going splat at 20 mph.
Before welding, the main tubes had some small holes drilled in them. Then when I welded, I filled the holes so the inner tube is spot welded to the outer tube.
From various angles. Another floor drawing helped me get the tubes attached straight, so one tube is not straight and the other at some kooky angle. Once straightened up, I used some sandbags to hold them in place for the first tack welds. A jig would be nice next time.