The Exhaust System:
Torsion Bar Suspension
A torsion bar suspension is also known as a torsion spring. The torsion bar consists of a long spring-steel bar. One end of the bar is secured to a nonmovable mounting called an anchor. The other end is fastened to a suspension arm or lever.
Vertical motion of the wheel causes the suspension arm to raise, causing the bar to twist around its center, which places it under a torsional load. The bar resists the twisting and gives a spring action, always returning to its original position unless it is overloaded. Spring rate depends on the length of the bar, and its diameter. The shorter and thicker the bar, the stiffer its spring rate.
Torsion bars are usually made to take stress in one direction only and often are marked by an arrow stamped into the metal to indicate the direction of stress.
Torsion bars can be used across the chassis frame on the same principle, in a trailing arm suspension, or as part of the connecting link between two axle assemblies, on a semi-rigid axle beam.
After a lot of use, a torsion bar can sag. On many vehicles, it can be adjusted to allow for this.
When the vehicle is turning, centrifugal force acts on the body, and tends to make it lean outwards. The anti-roll bar, or stabilizer, tries to use its connections to each side of the suspension, to resist this roll tendency.
Torsion Bar Suspensions are currently used on trucks and SUV's from Ford, GM and Dodge. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for heavier or lighter engine packages. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock keys too far can bend the adjusting bolt and (more importantly) place the shock piston outside the standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump stop prematurely, causing a harsh ride. Aftermarket forged torsion key kits use re-clocked adjuster keys to prevent over-rotation, as well as shock brackets that keep the piston travel in the stock position.
The main advantages of torsion bar suspension are durability, easy adjustability of ride height, and small profile along the width of the vehicle. It provides a longer travel than leaf spring systems, and takes up less of the vehicle's interior volume compared to coil springs. A major disadvantage is that torsion bars, unlike coil springs, usually cannot provide a progressive spring rate, forcing designers to compromise between ride quality and handling ability—progressive torsion bars are available, but at the expense of durability since they have a tendency to crack where the diameter of the bar changes. In most torsion bar systems, especially Chrysler's, ride height (and therefore many handling features) may be adjusted by bolts which connect the torsion bars to the steering knuckles and require nothing more than crawling under the car with a wrench in hand. In most cars which use this type of suspension, swapping torsion bars for those with a different spring rate is usually an extremely easy task.
Some vehicles use torsion bars to provide automatic leveling, using a motor to tighten the bars to provide greater resistance to load and, in some cases (depending on the speed with which the motors can act), to respond to changes in road conditions. Height adjustable suspension has been used to implement a wheel-change mode where the vehicle is raised on three wheels and the remaining wheel is lifted off the ground without the aid of a jack.
General Motors has used torsion bars since 1966, starting with the E-platform vehicles (Oldsmobile Toronado, Cadillac Eldorado), 4 wheel drive S-10 pickups, and since 1988, full size trucks (GMT400, GMT800, and GMT900 series).
Some front-wheel drive automobiles use a related type of torsion beam suspension, usually called a twist-beam rear suspension, in which the rear wheels are carried on trailing arms connected by a laterally mounted torsion beam. The torsion beam functions both as wheel-locating arm and as an anti-roll bar to resist lateral motion of the wheels as the body leans in turns. Its advantages are that it is inexpensive to manufacture and install, and engages a minimum amount of interior volume, leaving more space for the carriage of passengers, cargo, and other components. Because the torsion beam acts in the lateral plane, not vertically, the twist-beam axle cannot provide ride-height adjustment, and it suffers, to some extent, similar car handling limitations as other beam axle suspensions. However these limitations may not be apparent on the road, because of the trend towards firmer, more sporty suspension setups with more limited wheel travel. Torsion-beam rear suspensions were pioneered on the Volkswagen Golf in the early 1970s, and remain common on compact cars and minivans. |