[Eric]

If a vehicle suspends the chassis/body system on springs, the chassis must simply be stiff enough to resist bending in torsion AND beam in order for the suspension to function as intended. More precision from the suspension requires more chassis stiffness. Simple as that. the degree of which the chassis must resist flex is a function of the purpose of the car.

The reason it is not possible to compare modern downforce loaded racing cars to road cars is simply that with high downforce loads, the suspension springs are ridiculously high rate, and therefore the chassis must match. It was getting harder and harder to make this work and stay lightweight enough with metal chassis, hence the change to ultra-stiff carbon chassis.

The SAE does not specify anything for chasis stiffness. Every manufacturer has its own bechmarks for this. lets also be aware that production cars are broken down into several structures, and each requires different propereties, also understand that a primary drive for modern cars is how theey fold up in impact rather than torsion stiffness.

The higher the performance required from the suspension, the stiffer the chassis must be to allow it to function properly in geometric changes. This is a given, however as has been said before, if your car is on rubber bushes, it is most likely that they deflect far more and long before the chassis does. If you are on solid suspension mounts (rose joints) - any flex in components will become quickly apparent.

I will also say that in chassis design, it is not so simple as designing for torsional or beam rigidity....they are simply related measurements to the more important design factor...LOAD PATHS. How load is distributed by the chassis from the 4 input points is critical. It is much more important to be able to see a chassis and know how it is distributing loads than to just have some simplified torsional number that is not truly comparable.

Okay, enough, its starting to look like a book

cheers
Eric