Sub-frame connectors... Please build them correctly!

I agree 100% with you 72 - any frame connector is better than none.

Thanks for the video, longarm, that test was very informative and was similar to the test I saw some time ago.

I like to think of a car body as a box, a box with a longer length dimension than width and height dimensions. Of course, due to many considerations, car bodies are not built as true boxes, but rather as much modified boxes.

Consider a flat plate of sheet metal with a length four times its width, pretty flexible along the longitudinal axis. Now, weld together some rectangular cross section tube steel together with the same dimensions (WxL) and weld it to the sheet metal. That will substantially stiffen the sheet metal along the same axis, but it will still flex somewhat with adequate force acting on a corner. Welding an additional identical piece of sheet metal to the top of the tube steel with further stiffen the structure. Other than internal cross bracing, we are now out of options.

Now, consider taking the original single piece of sheet metal with the one tube steel frame attached and making another identical construction. Take some more tube steel, say with length of about 1/8 of the length of the construction, and weld them vertically to the perimeter tube steel. Then invert your second tube steel-sheet metal construct and weld it to the new vertical tubes. And, lastly, weld new sheet metal around the perimeter and onto the vertical tubes to make a completely enclosed box. A typical car has at least four vertical planes of stiffening bracing - the radiator support, the firewall, cross-bracing over the rear axle area, and the rear panel behind the trunk. These vertical planes of stiffening greatly reduce longitudinal flexion by transferring loads. In structural engineering, this is called a bridge truss in three dimensions.

This is essentially a car body and we have dramatically improved the strength and torsional resistance of the structure. Along the same lines, it can be seen that increasing the height dimension of the structure (or reducing the length/width dimensions) will stiffen the structure. Therefore, a cube shape would eventually be the strongest such box structure for torsional stability (length, width, and height being identical). Of course, this is impractical for a car, unless you have one of those South Korean shoeboxes on wheels.

My bottom line is that, with cars, we must work with the hand that was dealt. An around-town grocery-getter will really not need additional body stiffening. A drag racer will need more stiffening. A road racer will need even more (they do much better with all four tires on the ground at any one time). The more stiffening, the better.