Flug,
No worries mate. I appreciate your concern and input and have zero issues with you working through the numbers to make a point. Let me address the issues raised, if I may.
The way I drew the frame tubes it would be easy to insert a cross tube to take the upper engine mount.
I'm not trying to design his frame - just to throw some ideas out for him (and you) to think about.
Good to see you digging out the Strength of Materials. The calculations are probably OK and I'm not planning on checking them.
The 1/2" comment applied to the strut in compression! But in reality it would come pretty close for the forces you calculated for the downtubes. The frame wouldn't be very strong in terms of the suspension and cornering loads though.
At 90,000 PSi Tensile, 4130 would get pretty close to holding up the seat...LOL
BTW, offhand remarks never need to be called out because they are just that. But I digress. No one would seriously suggest using 1/2 tube" for anything much other than a fairing or footrest mount. I'd use tube either the same size, or one size larger than the lower cradle and in a much thinner wall thickness. But maybe that's just me. I like frames that are light and rigid in the right directions.
Stout is not a term I use, but the frame needs to deflect a certain amount in certain directions. Too much is bad and so is too little. Yes, triangles are the strongest design and the trick is not in the amount of metal but in the shape of the design as I think you were pointing out.
Large diameter thin wall tubes beat thick wall small diameter tubes every day of the week.
Rob North made some nice frames in the early seventies for BSA/Triumph triples that had a huge curve at the rear - as did the Mk1 and Mk2 Seeley frames. Both were carefully boxed in/braced in critical areas to render the curve almost irrelevant to the shape.
Going back to that small tube again, how much force do you calculate would be required to bend a 1/2" tube to failure? Do you have access to stress analysis software by any chance? It would be very useful to see the stresses in living color. Dr Rob Tuluie did that with the Tularis. That could be why Renault recruited him as their Head of F1 R&D.
If you have access, try twisting the frame in different directions and check out the massive deflection with some frame shapes and small diameter tubes and then replace some of the tubes with larger diameter thin wall and see the differences.
This is all good mental exercise.
