my chassis is all . 134 wall x 1 3/4 dom, on 2x3 x . 125 wall, so the vast majority of flex is in the bars themselves. At one point years ago I had another driver tell me it actually had one rear tire off the track on the hit. Few ladderbar chassis have the ability to get set below the neutral line. Mine is one of them. When I had the front of the bars in the bottom hole in front, they are just at or under the neutral line.
On the hit the driver's side would rise, and passenger side would want to squat, producing a violent twist at launch. My first anti roll bar was 1 3/ 8 x .125 wall me tube, and it was permanently twisted out of shape on the first pass. Upgraded to the big tube, no more twist. One way to prove or disprove my assumption is to suspend your car under the center of the differential . Then support the rear of the chassis with jackstands to immobilize it. Measure exactly how hi both rear tires are off the ground. Suspend 100 lbs of weight on the outside of the right rear . You will see the flex in how much the right rear is pulled down. If you want to check for chassis flex, suspend the front of the chassis so tires are off the ground suspend the back with a jack at the center of the rear housing. A long beam of some sort attached
crossways firmly to the back of chassis that extends out far enough to apply a lot of leverage. Add weight to the outer end of the beam and measure all four corners of chassis height to gauge flex. It may be very little, but if you calculate the torque values going through the drive line it will make sense. Suspending 100 lbs five feet from the center of the rear is only 500 ft lbs of twisting force.
If you have 750 ft pds engine torque, X 2(converter) X 2.45 (low gear) that works out to about 3700 lbs of rotational torque being applied at the hit, trying to twist the rear axle like a propeller. If your ladder bar mounts are 2.5 feet apart, 3700 divided by 2.5 is 2800 ft lbs divided to each side of the chassis. That means at the hit the driver's side is being pushed apart by 1400 pds of force, and the pass side is being pulled together by 1400 ft lbs. Those short term forces try and force the driver's side ladder bar to rise, and the pass side to go down. When way above the neutral line, this is overcome by higher forces of the housing trying to produce rear rise.