True, but combine that backbone with a monocoque, and you get an incredibly rigid chassis.
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Maybe, maybe not - depends on how substantial the body was (open topped - only the sills could contribute) and how well fixed to the chassis it was. And torsional rigidity only matters if transferring roll from one end of the vehicle to the other and I still believe Chapman arranged the geometry to avoid that. Not something he would say out loud - Europe frowned at the American practice of body on frame. But if body on frame (bodies rubber mounted to frames ) is so bad, how did it endure for so long? Was it because American engineers (and Chapman) actually knew what they were doing and were immune to the fashion trends that would sweep through Europe?
NB. Chapman never got the backbone chassis to work in a mid engined (where transferring roll from one end of the car to the other is essential) open topped car. The road going mid engined cars were roofed and the open top racers best forgotten.