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A car or truck body would react sharply to road surface irregularities without some means to suspend the vehicle above the wheels. Imagine a shopping cart crossing a speed bump at 20 miles per hour; surely no calm ride for the contents. You might feel some shock duplicating this stunt in your car or truck, but you would stay seated, and maintain control. Torsion bars and coil springs are two different ways to support a vehicle above its wheels. While the result is the same by either method, singular advantages are present in both designs.
Independent suspension is achieved in the absence of a straight axle. Instead of a straight axle between the front wheels, arms pivot on attaching points at either side of the frame. The outboard end of the arm mounts to the wheel at a joint that allows the wheel to swivel when steered. The arms help restrict wheel movements fore and aft and are known as "control arms." Their unique actions prevent up and down motions of one front wheel from affecting the other. Therefore, right and left control arms require independent and uniform elastic support. Torsion bars or coil springs provide the muscle for this monumental task.
Coil springs support the weight of a vehicle whether they are part of a strut assembly, or standalone suspension components. Spring pressure is applied between the lower control and the vehicle by either design. Coil springs are made from round bars of steel. The bars are heated until pliable and wound around a die of a specific diameter multiple times. The bar thickness and the diameter and spacing of the coils help establish the deflection strength, or spring rate. The rate and overall height of a finished spring are varied to satisfy particular vehicle designs.
Torsion bars are straight sections of round spring steel used on front suspensions instead of coil springs. Both bars run lengthwise along the inboard sides of the frame rails. The ends of each arm are formed into shapes that mate with recesses in the undercarriage, typically hexagons. The forward ends of the torsion bars are inserted through the lower control arm pivot points. The rearward ends are inserted into anchors on a frame crossmember or bracket. Vertical movements of the control arm occur when the wheel encounters a bump. The pivoting action causes the torsion bar in that arm to twist. The twisting action is resisted by the bar which deflects the load, and keeps the vehicle suspended.
Both types of springs are effective ways to support a vehicle above its control arms and either style can weaken over time. A vehicle might settle into an ungainly stance or lose some handling qualities. You can adjust torsion bars to alter the ride height or restore tension. You might tailor this adjustment to meet a personal preference, even if no faults exist. However, you have to remove coil springs from the vehicle to make effective adjustments. You can replace coil springs to recover original performance, or modify them to customize the ride. You can sometimes shorten coil springs to allow a lower profile, or you can use heavier or taller replacement springs to stiffen the ride or raise the vehicle.
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