Roll steer is the geometric steering angle the rear axle develops relative to the chassis centerline as the car rolls in a corner, driven by trailing-arm or 4-link angles.
When an oval car dives and rolls onto the right-rear corner, uphill or downhill 4-link bars, leaf springs or truck arms move the right-rear wheelbase backward and the left-rear wheelbase forward (or vice versa). If the rear axle points slightly toward the outside wall in roll, the rear end steps out and helps rotate the car through the center of the corner (roll oversteer, common on dirt late models and modifieds). Too much roll oversteer makes the car snap-loose on corner entry; dialing out bar angle reduces roll steer so the car drives straighter off the exit on slick tracks.
Roll steer controls mid-corner rotation and forward drive off the corner, making 4-link and trailing-arm angles critical setup-sheet entries.
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Trackside setup apps vs chassis simulation (Performance Trends & SusProg3D)