Coil springs are the most common way of providing suspension on modern vehicles, and with good reason - they're very good at their job. Funny thing is, they're really torsion bars in disguise. Sneaky.

What Are Coil Springs?

  If you took a torsion bar (see previous section), and twisted it into a spiral, you would have a coil spring - which kind of resembles a mutant industrial-size slinky. Like a torsion bar, the spring works by twisting along it's length - but because it's length has been wound into a spiral shape, you can induce this twist by compressing it down. A lot of people think that the energy is stored in a coil spring through bending the material, like a leaf spring - but it's actually through twisting, like a torsion bar. Remember how a torsion bar of a given diameter twists more under load the longer it is? Well the same principle applies with coil springs. The more coils a spring has in a given length, or the larger the diameter it's wound in is (i.e. the longer the torsion bar that makes it up is...), the softer it will be. Similarly, the thicker the material used to make the spring is, the stiffer it will be.

  The advantage that a coil spring has over a torsion bar is that it can be made to be rising rate: The tighter you wind it (the more turns it has for a given length), the softer it is, as each turn has less work to do for the same overall loading. This means that winding a spring tighter at one end than the other gives it variable stiffness along it's length - i.e. a rising-rate spring. As we've mentioned before, a rising rate spring allows for soft springing under light loads, and firmer springing to prevent all the suspension travel being used up under heavier loading. A spring like this is often described as being progressively wound.

  Like a torsion bar, a coil spring doesn't have the friction losses that a leaf spring does - ideal for building a smooth, predictable suspension system. It is this, combined with the ability to make them rising-rate that makes coils so suitable for suspension systems. They are also generally lighter than either leaf springs or torsion bars, which is a plus.

  A coil's stiffness is also related to it's overall length. For example, a ten-inch-long spring with ten coils will be stiffer than a twenty-inch-long spring with twenty coils, assuming the diameter of the material, and the diameter of the coil it's wound into, remain equal. This is because you are in effect dealing with one torsion bar that's twice as long as another - and so easier to twist by a given amount. This increased twisting shows up as greater compression of the spring, but the compression of both as a fraction of their original length (relative deflection) remains the same no matter how long you make the spring (logical really: A 10% deflection would be one inch on a ten-inch-spring, two inches on a twenty-inch-spring and so on). So although two springs of different length may have the same spring rate, in terms of overall compression for a given load, the actual stiffness of an equal chunk from both springs is different.

Back to Suspension Basics 4 - Torsion Bar Springs

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