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I'd like to build a "bakfiets"; a cargo-trike with the box in front.
I've found several resources, but none of them talk about the framebuilding material.
What is are my material options; steel? what thickness is best to choose? do I choose a square or U-profile. One of the things I'm very concerned about is weight: the stronger the frame, the heavyer it will be. I can't work with aluminium.

Any advice on this?

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How much do you anticipate the you, the box and cargo will weigh? –  mikes Feb 17 at 16:37
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Steel is probably the only material most people can build things with. Have you talked with a professional frame builder to assess the needs of your frame? A good one can help you pick the type of steel and geometry necessary for the trike. –  Batman Feb 17 at 16:42
    
@mikes I have not anticipated anything about weight. I realise it should be as little as possible. The whole thing is more a hobby than a serious project. –  Brtrnd Feb 17 at 20:20
    
Have you considered computer modelling? Free CAD tools are available and once you have an accurate model you can test it using Finite Element Analysis programs. Although FEA could be very difficult to master, learning 3D design tools would help you optimise the design to use less of your chosen material. –  Emyr Feb 26 at 15:15
    
FEA requires quite a lot of instrumentation of the prototype if your model is going to be accurate. It's very hard to model even a diamond frame bike because the usage is so dynamic. –  Mσᶎ Mar 2 at 5:42

1 Answer 1

up vote 3 down vote accepted

I suggest starting with a prototype made out of square tube and old bikes, especially if this is your first framebuilding project. Square tubing is a bit easier to work with (you spend less time mitring tubes) and the goal for the prototype is that it should be easy to build. Take shortcuts, in other words. Get something you can ride, then ride it. When things break, fix them.

As far as wall thickness, I'd buy whatever is cheapest. Weight for the prototype is not critical. For a final build in chrome-moly steel the thickness will depend on tubing size and how much stress it's under. You can buy plain gauge cold drawn 4130 steel tube in a range of sizes and wall thicknesses, usually by the metre. Which ones you want will depend on the detailed design.

Groups like Atomic Zombie and a lot of HPV and velomobile enthusiasts spend a lot of time just working this way, because it's cheap so you can afford to build things just for fun.

This site shows a steel prototype long john bike built and ridden, then eventually CroMo final bike built. There's some description of the design process and you can see the idea progress from welding random bits onto a steel chassis rail through to riding the finished bike.

One note of caution: you will need to tilt the pivot axle back from vertical to get more stability, but that design is inherently unstable.

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thank you, you're right it's more for the fun than for having a dependable bike. I'll see how far I get. –  Brtrnd Mar 3 at 16:23

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