with foam, as 'structural frequencies' aren't much use to anyone but engineers. For a car there should be a before and after test. Since track and skidpad's varying conditions(along ith driver error/ confidence) pretty much rule this out I was thinking of a simple 'jack' test where the car is placed up on four jacks-one on each wheel. Then two, opposing(example FrontR, RearL) are removed. Deflection and amount of drop could be measured at these corners with both an initial drop, and then after a few seconds(after all who cares how much a car flexes after a few seconds). Then repeat the process with the previously unsupported corners. With a stiffer frame torsional flex(measured amount of drop) should be much less than with stock/previous components.
