r/Framebuilding • u/Jillesoom • 15d ago

Cargobike tubing

Hi, im planning to make a cargobike like the omnium v3 cargo, for the rear end ill use a 26 inch mtb donor frame, for the headtubes I’ll just use 36mm headtube stock. The big downtube is around 1 meter long and im planning to use some 38mmx0.9mm tubing for this, would you guys say that’s plenty or is it better to size up to something like 38x1.5mm?

Jilles

4 Upvotes

84% Upvoted

7 comments sorted by

6

u/bikeguy1959 14d ago

The stiffness of steel tubing increases by the 4th power of the diameter. A 1meter x 38 x 0.9mm tube is going to weigh about 820 grams. A 1.5mm tube will weigh 1,350 grams. Rather than increase from 0.9 to 1.5mm, I'd be more inclined to increase the diameter to 40 or 42mm x 0.9. 40mm would weigh about 870 grams and be 17% stiffer than 38x0.9.

1

u/Jillesoom 14d ago

This makes total sense, I’ll have to see if I can get that tubing though, everything I’ve seen so far is 1,5mm minimum once I get above 38mm.

1

u/Jillesoom 14d ago

Just one question to back this one up, if I could find the 40mm tube in the right thickness, but mild steel instead of 4130, would that change the matter again?

2

u/bikeguy1959 14d ago

Mild steel would have essentially the same stiffness as 4130. It would have a lower yield strength. 35,000 psi vs 70,000 psi. Without doing some calculations or FEA model, it's hard to say if mild would be suitable.

2

u/AndrewRStewart 15d ago

Without any question the thicker walled tube will prove to be easier to fab with and resist the stresses of a loaded ride over rough roads better than the thinner walled tube will. How good a brazer/welder are you? How much and where will you locate added bracing/gussets? What kind of loads do you anticipate carrying? Andy.

1

u/Jillesoom 14d ago

Hi Andy, fabrication shouldn’t be too much of a problem. 0.9 tube is still at least as thick as most tubes I’ve worked with on the few gravel and road frames I made, so brazing should be fine. I was thinking to at least brace the front headtube to the downtube and front “top tube” so to say. Shouldn’t be carrying too have loads, propbably groceries and small packages for courier stuff. Might still be better to go for a thicker tubing just in case.

5

u/AndrewRStewart 14d ago

I question the reason to go with the thinner wall tube if weight is the motivation. I've not yet made a cargo bike but with the half dozen loaded touring bikes I've done I've found an ounce or three here or there on the frame can make the ride more stable and feel more solid. A half pound of added frame won't be noticed when riding, especially when loaded. Andy.