Torsion Bar vs. Spring Suspensionby Richard Rowe
Torsion bar suspensions may seem like outmoded technology, but they're far from extinct. While coil springs long ago came to dominate the automotive market, the simple, cheap and reliable torsion bar has carved out a nice niche for itself where the pavement ends. At the end of the day, it comes down to a choice between ride comfort, suspension articulation, adjustability and cost.
There are really only two types of springs: those that bend, and those that twist. Leaf springs are long strips of spring steel attached at either end to the chassis. The axle sits in the middle of the strip, which bends upward in order to absorb axle movement. A torsion bar runs longitudinally through the chassis; it attaches at one end to the chassis and runs through the pivot point of a vehicle's control arms the way that the through-bolts normally would.
Coil and Torsion Bar Movement
A coil spring is actually a combination of a leaf and coil spring in that it both bends and twists to absorb movement. This dual-action of bending and twisting lends itself to very precise tuning, more so than the single-action torsion bar. Most automotive coil springs are progressive in design, meaning that they compress more easily for the first inch or two of movement, but get progressively stiffer as they compress. It is possible to make a somewhat progressive torsion bar, but it's more difficult and expensive to engineer.
Coil Springs Pros and Cons
Versatility is the coil spring's primary trump card. The coil spring's dual-action progressive nature means that engineers can build one to absorb fairly large impacts with minimal driver discomfort while aptly controlling body roll during cornering. Coil springs also package easier in modern unibody vehicles, because the gap in the coils leaves room for the shock absorber. But there's a flip side to the packaging equation too, because the open space in the coil spring makes it dimensionally larger than the torsion bar. The coil spring needs its own pocket and support engineered into the chassis, while a torsion bar simply slides underneath it.
Torsion Bar Pros and Cons
Apart from packaging considerations, torsion bars do have one glaring advantage over coil springs. By rotating a torsion bar relative to the frame, a driver can easily raise or lower his vehicle by several inches for either more ground clearance or for a pavement-scraping ride. This adjustability makes torsion bars perfect for off-road vehicles, which is where you'll normally find them. The down side is that raising a vehicle with torsion bars will not increase the suspension's range of motion, and that preloading it in such a way may shorten its lifespan.
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