Torsion bar

A torsion bar suspension, also known as a torsion spring suspension (but not to be confused with torsion beam rear suspension), is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, the torsion key, mounted perpendicular to the bar, that is attached to a suspension arm, a spindle, or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar's torsion resistance. The effective spring rate of the bar is determined by its length, cross section, shape, material, and manufacturing process.

Torsion bar suspensions are used on combat vehicles or tanks like the T-72, Leopard 1, Leopard 2, M18 Hellcat, and the M1 Abrams (many tanks from World War II used this suspension), and on modern trucks and SUVs from Ford, Chrysler, GM, Mitsubishi, Mazda, Nissan, Isuzu and Toyota. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for heavier or lighter engines. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock key too far can bend the adjusting bolt and (more importantly) place the shock piston outside its standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump-stop prematurely, causing a harsh ride. Aftermarket forged-metal torsion key kits use relocked adjuster keys to prevent over-rotation, and shock brackets to keep the piston travel in the stock range.

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Wikipedia