A torsion bar is a heat-treated steel spring rod splined at both ends that twists along its length to support the chassis instead of compressing like a coil spring.
Sprint cars, midgets, micro sprints and classic torsion-bar modifieds mount splined torsion bars across the frame rails, linking one end to a torsion stop (adjuster bolt) on the chassis and the other end to a torsion arm connected to the axle. Bar stiffness is rated by shaft diameter in thousandths of an inch, such as a .975, 1.000 or 1.050 bar: thicker bars twist less and raise wheel rate rapidly (stiffness scales with the fourth power of diameter). Between hot laps and the feature, crews swap a torsion bar in under two minutes or turn the stop bolt to adjust ride height and cross-weight.
A 0.025-inch jump in torsion bar diameter produces a noticeable rate change, and torsion arm angle plus stop height must be logged together to repeat ride heights.