Anti Roll Bars
Anti roll bars, or sway bars, are one of the cheapest, easiest and most powerful ways to influence car handling. This article will look at how they work and how to use them to your advantage.
An anti roll bar is a metal tube which connects the suspension assembly on one side of the car to the other side. There is usually one on each axle of the car, although older cars or cars with certain suspension designs may only have a front anti roll bar, or even not have any at all! The purpose of the anti roll bar is to reduce the body roll of the vehicle during cornering without stiffening the suspension spring rates. This allows the vehicle to retain some compliance over bumps, whilst also providing handling benefits.
Reducing body roll improves vehicle handling by increasing response (the less the body moves on the suspension the less time there is for the car to change direction), reducing excessive weight transfer to the outside tyres allowing the inside tyres to do more work, and also by allowing the suspension angles and geometry to remain in check during cornering. After you have carefully set your alignment, the last thing you want to happen is for all the angles to completely change as soon as you turn in.
The roll bar works by limiting the difference between the left and the right side of the car by providing a semi-rigid link. As you turn in, the body wants to compress the outside suspension and roll outward. The suspension tries to compress, but the inside suspension is not compressed - the anti roll bar has to deflect (bend) to allow any more suspension movement. It is important to understand that the anti roll bar is not a rigid component - it is a stiff metal bar that is designed to flex under load, giving the effect of a stiffer spring but only in roll. Changing for a stiffer bar will increase the anti roll effect, and changing for a softer bar will decrease the effect.
As mentioned earlier, this allows roll stiffness (the vehicle’s resistance to body roll) to be increased without increasing the suspension spring rate. This means that the car can have soft springs giving a good ride (on the road) or adequate compliance (on the track) while still having good roll stiffness.
Anti roll bars are a powerful lever in influencing handling because the relative stiffness of the bars front to rear affects the handling balance. Stiffening the rear axle can reduce understeer or induce oversteer, and stiffening the front axle can reduce oversteer or induce understeer.
As most road cars come with a very predictable and safe handling setup that tends to understeer at the limit, stiffening the rear anti roll bar is an extremely cost effective and easy way to transform the handling, without any real disadvantage in terms of practicality or comfort. Kits are available for many cars, some of which are adjustable. Adjustable bars have several different holes they can be attached through, which effectively increases or reduces the bar stiffness.
Anti roll bars are often sold by their diameter in millimeters - the Civic Cup Car for instance has a 19mm rear anti roll bar as standard from the factory. A 22mm bar is available from Honda (as fitted to other models) which we ran in 2015. 24mm and 28mm adjustable rear bars are available from aftermarket suppliers. For 2016 we are switching to a thicker (stiffer) rear anti roll bar, as we hope that this will allow us to run less toe out at the rear (see the article on toe). This is advantageous, as toe out at the rear gives the car an extremely snappy handling characteristic, which is particularly noticeable in stability under braking and through high speed corners. A stiff anti roll bar with less toe out will in theory make the car more stable in high speed bends and under braking while maintaining great turn in and rotation in medium and low speed corners. The new roll bar is also adjustable, which will give us more options to get a setup the driver is happy with.
There are however some slight disadvantages to anti roll bars. Linking the suspension from left to right on the vehicle means the suspension is no longer truly independent, which reduces the ability of the suspension to soak up isolated bumps. All bumps on the left side will to a certain extent be transmitted to the right - this can have negative effects in terms of ride quality, and also in terms of ultimate grip on a particularly bumpy surface. It is therefore usually undesirable to fit extremely stiff anti roll bars on the road. For track use however a stiff roll bar will usually give far more benefit than negative effect.
Another ‘disadvantage’ of anti roll bars are the droplinks used to mount them. These are the links which attach the bar to the suspension assembly at either side, and in many vehicles are notorious for breaking. It is well worth checking these are in good condition - they aren’t expensive to replace, but if they are broken you will have completely lost the effect of the bar and the handling will be severely compromised!
For a road car, it’s well worth checking out if another model in the range has stiffer bars - these are a great option as they won’t be as extreme as an aftermarket solution and are often extremely affordable (from about £90). An aftermarket bar will cost more in the region of £150-£300, and a top end adjustable bar may cost more. It’s worth getting new droplinks if you are changing the bar. They are famous for seizing and you often have to cut them off.
To sum up - an anti roll bar is a fantastic modification to improve the handling of a road car. For most cars, a stiffer rear anti roll bar alone will reduce understeer and make for a more fun and responsive drive, both on road and track. I would recommend this as the first handling modification to do to your car. Some less sporty cars may also benefit from a stiffer front anti roll bar to increase overall roll stiffness, but the reason that this hasn’t been mentioned much is that for many cars the rear anti roll bar is enough - we run a standard front anti roll bar on the Civic Cup Car. Whether setting up a road or race car, go by the general rules below:
To reduce body roll: Stiffen either or both bars
Reduce understeer/increase oversteer: Stiffen rear bar
Reduce oversteer/increase understeer: Stiffen front bar
In the wet: Soften both bars (possibly more off the rear to increase stability)
Do not go too stiff on the road, or on a very bumpy circuit. As usual, on the road less is often more, and on the track try a few setups, testing is king!