True, I am making a generalization and generalizations are bad (sometimes ). It also (I assume) comes down to body configuration ie: fwd, rwd, midengined ,length of wheelbase, width of track and how much overhang the front and rear the vehicle in question has. Also,depending on where the engine/trans/heater/dash/occupant is in relation to the suspension points it may help or hinder what the factory can add for body bracing. I'm sure the engineers that do that math stuff have a list of maximums that they expect a car to see routinely and have a design up to those limits. Throw some stickier tires and bigger brakes and more power so on and so on,the car will twist more just because of the added forces.

Both unit body and body on frame cars can be made stiff but like all things, it's a compromise.

I still think the original reason for the unit body was just lighter weight not increased rigidity. Body on Frame would (generally) have superior beaming stiffness. Torsional...it's a tossup.

 

I stitched my drag car, and it will also get a full cage. Anything you can do to help keep your car stiff will be worth it. Flexing of any sort can break factory felds and stress crack your sheetmetal. Not to mention that the power being used to twist your car isn't making it to your tires!

 

isnt it easyer just to add spot welds closer to each other?

 

Easier, but not as effective. Also, don't think more welding is better. Don't weld more than 1.5-2" in a row, then jump 1.5-2" and weld some more. You don't want the entire seam welded or it will create a stress point and will crack under loads. Just a FYI

 

The spot (arc) welder you talk about doesn't really clamp that hard. FYI, it's a sub-$200 tool, nothing speshul.

One of VW Richard's (local guy, I used to turn wrenches with him at an old job) friends has an old 1G Scirroco. He braced all the suspension points with RHS bars, then drilled holes between the factory spotwelds and filled them up with a MIG. Close enough for rocket science.