... is to find donor bikes. Two are needed. One with 26" wheels for the rear frame, and one with 20" wheels for the fork and front wheel. Almost all cargo bikes available and self-made use these sizes. On the trike I had a 700 wheel at the back, and two 26" at the front, and it was a bit to high, thus less stable.

I've tried too find an electric bike for the rear frame, that would have been nice to go with a motor from the beginning to help, but every bike I found used was made with aluminium frame, that most people (including me) cannot solder. So the next choice is to use a mountain bike, which is made to be strong. On this bike we keep the rear wheel, the crank, the frame, the direction pivot, the saddle.

The second bike is for the front pivot, the fork and wheel. With a 20" wheel, the obvious choice is a BMX, which are strong bike too.

Both bikes where bought for 30€ each.

The first step, prior to dismantle bikes, is too measure them. Lot of people go with a global schematic and adjust while doing, I prefer to have a globally set design before to start. so, measurements. The only tool used here is a measure tape. It's fun to see that every measure is round, except from the tubing sections.

From here I start by drawing a sketch in my favorite CAD software. In this sketch are just included the points of the geomerty, viewed from side. From these points is drawn a wireframe, and then each section is drawn to build the volumes. It can be quite messy with all the references planes needed, and all the sketches for sections !

There are several things that are made easy by using a cad software :
Every tube junction can be easily exported as a developed shape.
The global weight can be accurately estimated.
The needs for tubing can be precisely calculated.
And, last but not least, the deformations and constrains of the frame can be tested under load ! I'll come back to this later.