The bakfiets was built from a steel donor bike with steel tubing extension on the front. Steel is chosen because of its price and availability, and also it's easier to work with.
Coming from a design background I figured some essential aspects regarding geometry. They are: 1. Fork trail.
Long story short, the trail length should be short to keep the steering stable yet agile enough. I didn't have the privilege of fabricating my own fork, so I have to modify a BMX fork (see "Ingredients"). Having good experience with touring bikes with 50-60mm of trails, I chose to run 51mm of trail on my design. Factoring this and the fork offset on the BMX fork, I got the headtube angle set at about 71,5 degrees. A bit slack. 2. Bottom bracket height.
To keep it stable on high and low speed, I make the bottom bracket height as low as possible. I ended up with about 289mm. You might want to get lower, but keep in mind that if you're using large rear wheel on your donor bike, a bottom bracket that's set too low will disrupt the riding position (the seattube angle will be too steep). 3. Wheelbase
Imagine this bike as a bridge that suspended between the two wheels. You can go bananas by making the bike as long as possible, but you might ended up using bigger or thicker tubings to prevent the whole bike from sagging under heavy load.
Here's some of the ingredients I use for my bakfiets:
1. Steel mountain bike frame. A disc brake compatible model was chosen for extra stopping power. Derailer mount is a plus.
2. Steel BMX fork with 1 1/8" threadless steerer.
3. 16mm x 37mm square-section steel tubing, 2mm thick. I ended up using 9 meters of it.
4. I used 42mm steel pipes for the headtubes (front and rear) because they have internal diameter of 34mm, roughly the same as the outer diameter of 1 1/8" threadless headset cups. You might want to use thinner pipes for this purpose, as long as the internal diameter still be 34mm.
5. Some scrap 5mm steel plates or strips to make steering rod mounts and front brake caliper mounts.
6. I found an adjustable rod with heim joints from our local scrapyard, probably taken from a car or something. The heim joints are used for the steering rod.
7. 15mm steel rod, for the steering rod.