i'm not arguing your theory/opinion.....but i got a different view for a car chassis...this is what was i'm thinking...maybe i'm wrong....but at least this is what i think for now...huhuhuhu
the gokart we all know already that it doesn't have any suspension...so the chassis had to be designed in order to allow some degree of flexing in order to maintain tyre contact with the tarmac surface especially on uneven surface....and also since kart doesn't have differential (straight lock axle) the chassis flexing allow the inner side of rear tyre to lift a bit from tarmac surface and induce better turning radius....
as per car chassis, opposed to that,too much chassis flexing would makes the car wobble on rough surface, therefore resulting in poor steering feel and a major headache to tune the suspension.....with stiff chassis,all the bumps/rough surface change can be made only on the suspension setup and the setup change would work more consistently and predictable between changes...thus drivers can get direct feedback on any change/setup he needs easily without wasting valuable time (especially on race day) to test various suspension setup changes......another good example was the F1 car...because of the carbon fiber chassis tub was so stiff,they only need suspension travel of a total less than 5cm (50mm) to get a good mechanical grip...
and this explain why when normal road cars using stiff spring it goes too bumpy on undulated surface...think as the road rough surface was a length of consistent wave graph....and the suspension travel along it with inverted graph..when taking the spring rate and damping rate of the suspension doesn't match the road surface graph, the car simply bouncing over the graph length....it became worse when the car chassis was flexed during the run makes the suspension graph even more inconsistent with the road graph....in this case a pair of fender bar would make a different where it helps hold the chassis stay rigid and not causing any further distraction to the suspension graph versus road surface graph...and this is also what i think where most ppl say their car becomes better in term of comfort after installing chassis reinforcement bar....the suspension do all their busy job without the chassis flexing causing the suspension behave differently from what they intend to do....
p/s: sorry for the long post....feel free to correct me if there's mistake in my post....me also still learning