Chassis flex, does it only effect the front of the car

[BlackGMC]

After attempting to do some research on subframes and chassis flex on unibody cars, I ended up with a question. Does chassis flex only effect the front of a car. I mean all the stuff I have seen subframe connectors, export/firewall bracing, g-braces, etc are on the front of the car, so does that mean the rear section of the car does not move or flex?

[David Pozzi]

I'm sure there is flex all over the chassis, but the front is probably the weakest because any bracing has to go around the engine, and most cars have very little bracing from the factory. Early cars didn't need much torsional rigidity because they came with soft suspensions and skinny tires.

This is one of the best discussions we've had on the subject: https://www.pro-touring.com/forum/showthread.php?t=23278
David

[Norm Peterson]

Everything flexes when you load it. How much depends on the types of load involved (axial force, bending, shear, torsion) and their magnitudes - and the sizes, shapes, and arrangement of the structural elements.

As Dave has said, the front subframe comes in for the majority of stiffness upgrades because it (and I'll include its attachment details here) is the least rigid portion of the chassis. The biggest bang for the buck comes when you stiffen up the thing(s) that most closely resemble a hinge. I'll add that bolt-on subframes in general tie in to the rest of the chassis at very few points. If I'm not mistaken, the OE subframe mounts all attach in pretty much a single horizontal plane, which isn't the best solution stiffness-wise given the locations and directions of the front suspension forces. In the earlier cars, it comes that way for reasons of manufacture, maybe for servicing, and because it's either "good enough" or intentionally a bit softer-riding to suit a wider range of buyers. Later cars with more stingent crash standards may force a somewhat different solution in order to provide energy absorption without deforming the passenger cell.

Flex happens around rear suspension attachments, too. Even the suspension pickup brackets themselves flex/twist slightly (I have it on pretty good authority that some of the more flexible brackets aren't a whole lot stiffer than the bushings in the link ends that connect to them). It's fairly common to see strut tower braces and lower tie bars on modified imports with strut rear suspensions, particularly in those cars driven by the more serious autocross competitors in classes that permit such modifications - currently there's a huge "discussion" going on within SCCA circles that's more or less centered around how effective these things should be allowed to become (and how to write the rule). Closer to home, perhaps, is the diagonal PHB brace on 3rd/4th gen F-bodies that is there for similar reasons - it's not just there to keep the cantilevered chassis pickup from bending over or being broken off.

Norm