bogus wrote:
Anytime you weld metal, you weaken it as much as bond it.

I stand by my prior theory on how to reinforce the C4 frame - fill the damned thing with autobody expandable foam. Make that thing dense as MDF.

Dick Guldstrand did his GS90 custom C4 in 1990 and reinforced the frame. He removed the hatch floor and welded in reinforcment rods, then welded in a b-pillar roll cage reinforcment. It still doesn't solve the lack of a binding reinforcement from the a- to b-pillars.

Which is why the entire thing shimmies! Dave McClellan wanted to have a t-bar. Marketing didn't. He protested, and asked for $1m to reengineer it, they threatened to have him fired. I hate old GM; more concerned with the marketing than a safe and stout vehicle. However, he had to scramble without the added cash. He then came up with the high sills. The problem? The firewall STILL cantelevers off the sills! ugh.

That's where the tension rods and the xbrace come into play. By tying those points to gether, they simply can't flex. The other obvious solution is the targa truss.

This is also why we shouldn't drive these cars with the roofs out for extended periods of time. The hood and firewall will feel the effects over time...