Although simple, torsion bars give good performance in vehicle suspension, and have been used in a variety of vehicles for decades.

What Are Torsion Bar Springs?

  A torsion bar is a a very simple device: A metal rod that can be loaded by twisting it, and then twists back when released. By securely anchoring one end, and fitting a lever of some kind to the other, it can be used in a suspension system to handle the vertical movements of a wheel. It's a similar energy-storage principle to the wound-up elastic band found on toy planes and the like. The stiffness of the spring is governed by both the resistance of the torsion bar to twisting, and the length (and therefore leverage exerted) of the suspension arm it's attached to. The important thing to remember is that a torsion bar's resistance to twisting is governed by it's thickness and length. If two torsion bars are the same diameter, whichever is longer will have the least resistance to twisting - and if they're the same length, whichever has the lower diameter will be easier to twist.

  Most commonly made from steel, a torsion bar has splines at either end which are used to attach it. There are two common ways of using a torsion bar: Either anchored to the chassis at one end and the suspension at the other, or mounted to the chassis in the centre, with a suspension unit at either end.

  The latter is almost exclusive to having the bar running across the width of the vehicle, supplying the springing for both left and right wheels at either the front or rear. It is almost never used to spring both left or both right wheels. This transverse (across the width of the car) configuration is best demonstrated on the front suspension of VW Beetles (old ones; later models used a different design). Most Peugeots use one torsion bar for each side on their rear suspension, but the principle is exactly the same - as shown in the picture on the right (I know it looks like there are three torsion bars, but only one is the suspension spring - the others are to prevent body-roll when cornering, which we'll cover in a later section anyway).

  The difference comes in the way torsion bars are used for the front suspension on many modern pick-ups and SUVs: Here, the torsion bar is mounted longitudinally (along the length of the vehicle), with the suspension arm levering off it at right-angles to the chassis. This has the advantage that a far longer torsion bar can be used, so it can be twisted through a greater angle overall for a given amount of twist per unit length - allowing for long-travel suspension without undue stressing of components, hence the suitability for off-roaders. Of course, the longer bar has to be larger in diameter to give the same resistance to twisting as a shorter equivalent. The picture on the left shows the torsion-bar front suspension on the Hummer H2 (The poncy suburban bling-wagons, not the US Army's pet monster trucks. They use coil springs, which is the next section).

  Advantages of a torsion bar include the fact that it is very simple and reliable, as well as the fact that it means more of the mass of the suspension system can be mounted low-down, reducing the effective centre-of-gravity of the vehicle. Many systems also incorporate adjusters to add or remove a small amount of twist at the chassis mounting point, which can be used to take up sag as the springs get older, or to balance the two sides of the vehicle if the springs do not match perfectly on stiffness.

  A torsion bar system is not perfect, however, it's main failing being that the spring is purely linear, and cannot provide the kind of rising-rate performace that is often desirable for automotive suspension. Several designs compensate for this by having additional springs etc that come into play as more of the suspension travel is used up - but adding hardware like this kinda defeats the object of using a torsion bar system in the first place. Also, you're pretty much stuck with the stiffness of the bar as it is: If you want to stiffen it up, you have to buy a new one, though pulling the splines out of one mount and refitting the bar one spline further round can sometimes help with things like lowering or raising suspension.

Back to Suspension Basics 3 - Leaf Springs

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