What is an anti-roll bar?
A roll bar is essentially a spring, just like the ones over your shocks, but without the helical shape. A straight spring like this is known as a torsion spring. You can appreciate its effects by holding a rule firmly at one end and twisting the other end (gently). When twisted it bends, when you let go it returns to its original position - just like a helical spring.
So what does it do?
To see what the effect of adding a roll bar is, let's consider what happens when a car enters a corner with a rear roll bar attached; Normally (without roll bars) the car begins to roll, i.e. the outer suspension begins to compress and the inner suspension begins to lift. However, because we've linked the two sides with a roll bar, when the outer suspension compresses, (some of) the force is transferred to the inner suspension which prevents it lifting, even compressing it slightly (depending on the strength of the roll bar). In this way the roll bar reduces lateral roll, making the car less likely to roll-over. This is the first effect.
Secondly, due to the effects described above, as you enter the corner the suspension compresses more than normal, because the inside is being compressed as well as the outside. This makes the car sit lower, generating more grip.
Thirdly, when you reach the middle of the corner and the suspension has equalised, it is compressed less than normal, as some force has been absorbed by the roll bar itself, hence you have less grip. However, the weight is distributed more evenly over the wheels, so the car is better balanced, giving a smoother feel.
So in summary, you have less body roll, less rear grip (more steering), but a more consistant motion through the corner.
Putting a roll bar on the front has a similar, but opposite effect i.e. less steering, but more consistant cornering.
You can choose roll bars of different thicknesses, whereby a thicker roll bar has a more pronounced effect.
