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Just wanted to show (with some back-of-the-envelope-typed-during-a-coffee-break sums) that you didn't need anything too crazy to sustain that kinda load.
Also worth noting that this is a classic plane-stress situation (ie very thin walls) so you only really have the axial load to consider. Would be interesting to see how much the load of the rider etc cancels with that of the internal pressure...
hamrack-
Just wanted to show (with some back-of-the-envelope-typed-during-a-coffee-break sums) that you didn't need anything too crazy to sustain that kinda load
Fair point. I was just pointing out that the problem is really at the ends, mostly in how you cap your pressure tube in a way which is practical when the tube has to extend past the caps to the frame joints. A 60mm diameter × 300mm long pressure vessel inserted into the downtube gives twice the volume of just the top and down tubes, allowing for lower pressure and simplifying fabrication :)
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@mdcc_tester So for idiots like me, and aside from an independently made, purpose specific vessel that goes in the middle. Is it a good idea or just a vanity piece?
gbj_tester
Chak
Thinner than an actual frame tube, in fact. Generally you need something better than 4130 to get away with <0.6mm walls in the main tubes of a frame. You also need to account for the tensile load from the two ends being forced apart by the contained pressure, and of course all the usual frame loads which don't disappear just because your pipes are full of gas. In practice, that probably means that using the frame tube as the pressure vessel is probably no better than using the frame tube as a frame tube and adding a completely separate pressure vessel alongside it, should you be minded to use your bike to transport pressurised gas. If the issue is getting tubeless tyres to seat, you'd probably be better off carrying a gas which is liquid at a relatively low pressure at ambient temperature, as that makes your pressure vessel much lighter. People who run very low pressures on off road 4×4s and knock their tyres off the bead seat as an occupational hazard have been doing it for decades
https://www.youtube.com/watch?v=1QDLx6HSOAw