TR4/4A Reducing Chassis Flex on TR4A IRS

A few years ago I saw a TR6 race car without a cage or any obvious additional strengthening that handled like it was on a rail. The only observation was a series of large pop-rivits spaced pretty closely thru the floor to the frame (the rivits followed the shape of the frame) TR6 frame and 4A frames are essentially the same. I asked the owner if that was his method for strengthening chassis flex and he just smiled. A few years later I again saw the car at Watkins Glen and asked him again, this time he was a bit more forthcoming and nodded his head in the affirmative. Sort of created a unibody out of a chassis on frame construction. He again was very competitive until his engine went up in smoke.

There are three aspects to the rigidity of the frame.
Flexing
Twisting
and Racking

Racking is the easiest to reinforce with some diagonal braces or "X" braces in the frame.

Flexing is a little harder and usually requires additional depth added to the frame. Ive seen 2" x 2" x 1/8" square tube welded to the bottom of the frame or plate steel welded to the sides of the frame members.

Twisting is the toughest to cure. The idea of using the body to resist twisting is good as it gives you lots of depth, except at the door. A combination of riveting the body to the frame and reinforcing the frame in the area of the door might work best.

I'm a structural engineer but not an expert on auto frames.

Steve