What is an Anti-roll bar?
An anti-roll bar is a tube of metal that works as a spring between left and right wheels. An anti-roll bar connects the vertical motions of the left and right wheels and works in parallel with the coil springs.
During cornering, the outside wheel (be it front or rear) wants to move up in the wheel arch and the inside wheel wants to move downwards. If the two are disconnected then this would take place and the car would experience roll. With the two connected, via an anti-roll bar, the rising wheel is being held down by the falling wheel and vice versa therefore reducing the roll of the car.
The effects of an Anti-roll bar
The main effect of an anti-roll bar from within the car is what the name suggests; it reduces the amount that the body of the car can roll.
Anti-roll bars alter the distribution of lateral load transfer between the front and rear axles of car. This is due to an anti-roll bar increasing the amount of force upon the outside tyre when cornering. Due to this, the stiffer the anti-roll bar, the more lateral load transfer that occurs across it.
The phenomenon is often very misunderstood within motorsport as it is a confusing principle of science. What it effectively means is that the stiffer end of a car will experience more load transfer across the wheel during cornering which will result in over/understeer at that end.
For example, if a car has a very stiff front anti-roll bar and a soft rear anti-roll bar then the front of the car will tend to understeer during cornering. Alternatively, if a car has stiff rear anti-roll bar and a soft front anti-roll bar then the car will tend to oversteer during cornering.
This key effect of an anti-roll bar is very important to understand when tuning a car on track and adjusting a vehicle based upon driver feedback.
When is an anti-roll bar working?
An anti-roll bar is inactive in a straight line when both wheels on opposite axles are moving up and down the same distance at the same time. This is due to the way in which the roll bar is linked to the suspension system and will just pivot in its mounts in this scenario.
When a car is cornering the roll bar will be forced to twist and becomes active taking effect upon the suspension system and working in parallel with the coil springs to control the car.
An anti-roll bar is most useful during steady state cornering. This is usually mid corner when the car has turned in, body roll has occurred and the car is sat mid corner turning at the point before the corner exit when the car will roll again. At this point, the dampers have stopped working meaning that the only tuneable suspension component working at this point in the corner is the anti-roll bar.
It is very important to remember that mid corner handling is affected most by the anti-roll bar as this can be adjusted based upon driver feedback to solve most mid-corner steering issues.