Back purging the insides of tubes with Argon is good practice even for the welding of the mild steel tubes we use. That's a reason we use mild steel and the wall thicknesses we do. Argon is a lot heavier than plain air and will sit in a well. So I think titanium is weldable with good standard practices. I see professional welders using diffusers under their work and other tricks to do good shielding. Frames are a worst case situation because of their size and shape.
Quote:
I have no doubt Car 9 By Marcus here beats the torsional stiffness of the Caterham because Car 9 isn't bound by those traditions.
Thanks Cheapracer...
Quote:
Sticking to the traditional 'Seven' shape, though, about the best you can do with the cockpit bay is beef up the backbone (which is limited by low second moment area) and/or introduce fixed transverse torsion boxes, as per Frank Costin's patented arrangement.
If you do a good job with the driveshaft tunnel it can be a strong backbone. There is at least a small issue with the Locost chassis because of how the backbone connects into the front of the frame, the FEA we did show it yielding with a reasonable load.
The tunnel though always bothered me a bit, I resented the investment in material there when it seemed it could be used other places which would have an additional benefit of also providing more protection for occupants.
For Caterham the Seven shape is pretty specific and I think there was some legal finding there to differentiate Westfield for example. I found when doing the FEA that just raising the upper frame rail a few inches could increase the stiffness %25. A frame tube %25 larger is twice as stiff and will also contribute to overall stiffness and safety. One of our organizations specifies 1 3/8" x .083" tube for roll cages on light cars so that seemed a reasonable choice. Using that spec for tubes that corresponded with a roll cage produced a chassis with something like 7k-8k lbs./deg stiffness.
As Cheapracer also mentioned having a truss under the scuttle, connecting that to engine bay braces was a really big win for stiffness. It turns out this can also connect to a removable bar over the passenger seat and that little bar increases stiffness by another %50 for just 3 pounds of weight.
I don't think my frame is unique in this way, these approaches would apply to any frames of this style. Using the driveshaft tunnel material to invest in a larger diameter upper rail and raising the upper rail can mostly be hidden under the bodywork, except for the part under the driver's elbow. Improving bracing under scuttle and engine bay are pretty much hidden.