Most racecars are fitted with at least one anti roll bar. Anti roll bars are a simple and effective way to fine tune the handling characteristics of a racecar, and yet they are often poorly understood and sometimes used incorrectly. This article will attempt to explain the three things an anti roll bar does, and why you might want to do these three things. Finally, it will attempt to provide an explanation of what change to make to your anti roll bar to achieve the result you want.
There are some technical explanations missing from this article. This is deliberate. We want to keep things as simple as possible, focussing mostly on what anti roll bar change to make to achieve the right result. For more in depth coverage of this topic, you might like to refer to Allan Staniforth's excellent book: Competition Car Suspension.
What does an Anti Roll Bar do?
Resists Roll
The anti roll bar (as is implicit in the name), resists roll. As a racecar drives through a corner, the anti roll bar works to resist compression in the outside suspension, and also works to resist extension in the inside suspension.
Increases Spring Rate
The anti roll bar increases the effective spring rate in the outside suspension when the car is going through a corner.
Transfers Weight
In a cornering situation, the anti roll bar works to transfer weight from the inside tyre to the outside tyre. If anti roll bars are fitted front and rear, the relative strength of the anti roll bars at each end will also work to transfer weight from front to rear or vice versa.
What does all this mean?
Resisting roll is generally a good thing. It maintains the racecar in a relatively stable platform. For an aero sensitive car, it will also keep the wings working at or near their optimum. Most cars also experience some camber change as the suspension moves through it's travel. Any change in camber from optimum means less of the tyre is in contact with the track - this means less grip. The roll resistance provided by an anti roll bar can help reduce this by keeping the tyre closer to optimum camber.
Increasing spring rate leads us into how we use the anti roll bar to tune the suspension. The static spring rate at each end of the car is normally selected to keep the tyre in contact with the track surface as much as possible. Too soft, and the suspension will deflect to the point where the bodywork/floor contacts the ground, or the suspension reaches the end of its travel. Too hard, and the tyre will spend most of its time skipping across the surface. If we go to the extreme, to solid suspension, the tyre will quickly become overloaded and skate through every corner. So, if we assume that the spring rate is already at optimum, an increase will result in some reduction of grip.
Weight transfer means that more of the cars total weight is carried by one wheel than the other. If we imagine a car that, when parked, has equal weight on every corner (say 250kg), all tyres will provide the same amount of grip. As this imaginary car enters a left hand corner, some of the weight is transferred to the outside tyres (the exact amount will depend on roll centre heights and other things beyond the scope of this article). If the anti roll bars front and rear are the same, the weight transfer at the front and rear will be the same. We now have 300kg on the right hand tyres, and 200kg on the left hand tyres. If we were to simplify things, we would assume that this means we now have 20% of our grip coming from each inside tyre, and 30% from each outside tyre - this is not accurate, the relationship between tyre load and resultant grip is not linear. The actual figures may be something more like: 15% from each inside tyre, and 35% from each outside tyre. If we stiffen the front anti roll bar (but leave the rear the same, the result may be something like: LF 180kg, RF 320kg, LR 200kg, RR 300kg. Because the load/grip curve is not linear, the amount of grip at each tyre will be something like: LF 10%, RF 38%, LR 15%, RR 37%. If we then compare total front grip to total rear grip, we see there is 48% at the front, and 52% at the rear. Stiffening the front anti roll bar has given us a higher proportion of grip at the rear. The car is now more likely to understeer.
Putting it into practice
There are a number of ways to remember what effect an anti roll bar change will have.
You may like to think of it as putting a stiffer anti roll bar at one end of the car, will 'tie-down' the opposite end, ie; a stiffer bar in the front will yield more rear grip.
Or you may consider that the anti roll bar reduces compliance at the end it is used, and reduces grip at that end of the car.
Both of these explanations overly simplify what is actually happening, but they do help to remember which way to make changes when adjusting setup at the circuit.
Essentially, to reduce understeer, you may try using a stiffer rear anti roll bar (trade offs will be reduced rear grip, worse power down for RWD cars), or a softer front anti roll bar may be more appropriate.
To reduce oversteer, go for a stiffer front anti roll bar, or soften the rear anti roll bar.
Further Reading
Disclaimer:
Remember, you are responsible for the consequences of any changes you make, and any adverse outcomes. This article is provided for reference only. Neither iRace or the author offer any warranty or any liability for any damage or injury caused by tweaking any racecar.