We take a closer look at what anti-roll bars are, how they work, and how they can improve the handling performance of your car.
Guide from Fast Ford magazine.
The idea of an anti-roll bar sounds simple, doesn’t it? The name says it all: it’s a bar that resists body roll… But what should be a straightforward subject is surprisingly complicated, and by altering the front and rear anti-roll bars we can have a massive influence on how a car behaves.
And then we need to consider other factors such as spring rates, what the car is used for, what kind of handling balance we’re trying to achieve – and even a bit of good old-fashioned physics – and then try to string together the perfect setup for our specific application.
Some cars need stiff anti-roll bars, some need softer bars, and some don’t even need any roll bars at all. And there’s no hard-and-fast rule that says ‘cars like this need roll bars like that’, because each car and set of circumstances is different. There are too many variables to cover here, but we’ve covered the main areas to bring you a beginner’s guide and quick taste of what anti-roll bars are all about.
Centre of Gravity
Before we start looking at the anti-roll bars themselves, we first need a quick recap on some basic physics. Centre of gravity (CoG) is an entire topic worth discussing all on its own, but we can’t really talk about anti-roll bars without first understanding what the CoG of our car is, and how it affects the handling. In short, the CoG is the centre point of all the car’s mass, and the point at which all cornering forces act – which is why we need to look at it in relation to anti-roll bars.
A lower CoG is beneficial when it comes to handling performance, as it results in less lean or body roll when cornering, and there are a few things we can do to lower the CoG on our cars. The most obvious is to lower the ride height, as this lowers all the car’s mass closer to the ground – but that can have knock-on effects with other geometry settings such as the roll centres (we’ll come onto that in a minute). Other modifications to lower CoG include removing weight from higher up – such as replacing heavy glass sunroofs with lightweight carbon fibre roof skins and so on.
Hollow vs Solid Anti-Roll Bars
There are two main types of anti-roll bar construction: solid bars or hollow tubes. If you had a solid bar and a tube of the same diameter, the bar would have a greater torsional stiffness. But only marginally. This is because the inner part of the bar does very little to increase torsional stiffness. What it is very good at, though, is increasing weight.
Therefore, many performance ARB upgrades are hollow to reduce weight, but have a greater diameter to increase torsional stiffness. American suspension specialist Hotchkis posted some of its test results in a video online: the 35mm hollow ARB was 19 per cent stiffer yet 27 per cent lighter than a popular solid 32mm bar.
Other considerations we need to be aware of before looking at anti-roll bars are the roll centres and roll axis of our car. Again, this is a complex subject and something that race teams spend many hours calculating and adjusting. On our road and track cars there’s not too much we can do to alter the roll centres, but it is something we need to be aware of when choosing the right characteristics of any anti-roll bar we want to fit.
The roll centre is the point about which the car rolls during a corner; it should be on the centreline of the car from left to right, but the height will depend on the rest of the suspension geometry. The roll axis is the line drawn between the front and rear roll centres; on front-wheel-drive cars the front roll centre will be lower than the rear and so the roll axis will incline towards the rear of the car, or on rear-wheel-drive cars the rear roll centre tends to be lower and so the roll axis will decline towards the rear of the car.
Whereas the roll centres look at individual axles, the roll axis gives us a much better picture of how the car behaves overall; a car with an inclining roll axis (like most front-wheel-drive cars) will have the tendency to lift the inside rear wheel during cornering, as the weight of the car is ‘rolling’ around an axis that lifts at the rear and drops at the front – although this does depend on the roll stiffness of any anti-roll bars fitted and the spring rates used, which we will cover later.
When in a corner, the tyres are effectively pulling the car body in towards the inside of the corner. And if we revisit our school physics lessons for a second, you might remember Einstein’s third law: every action has an equal and opposite reaction. Because of this, there’s an equal but opposite force pushing the body outwards (away from the corner), acting on the CoG. In turn, as the CoG is higher than the roll axis, you get a rotational force causing the body to roll.
What this means for your car, and importantly your tyres, is that the CoG moves more over the outside tyre. This increases the vertical load on the outside tyre and reduces the same on the inside tyre, otherwise known as weight transfer.
As you might expect, more vertical load pushing the tyre into the ground allows the tyre to provide more lateral force, or grip. But – and this is a big but – if you double the vertical force on a tyre you do not double the grip available. This means that if you have more weight transfer, the total grip available across the axle is lower; more grip is lost from the inside tyre than is gained by the outside tyre. Ultimately, the sum of the total grip across both tyres is less.
This might lead you to think that body roll is a good thing, minimising weight transfer. But it does have its downsides: agility is reduced as it takes longer for the car to change direction, and allowing the suspension to move to the extremes of its travel (as it would do in a big roll motion) can have serious negative effects on the camber and toe, and other geometry settings of the wheel alignment too.
How Anti-Roll Bars Work
So, how do we control body roll? It’s easiest to imagine anti-roll bars as three component parts all joined together: the main ‘bar’ is fixed to the body/chassis of the car with special mounts that allow it to rotate freely but hold it steady, and on each end there is a cantilever that attaches – often via drop-links – to the rest of the suspension system.
This means the anti-roll bar is connected to both left- and right-side wheels, and its torsional properties act as a spring to resist body roll; when a car corners, weight is transferred to the outside wheel, which tries to rise up into the wheelarch. At the same time, the inside wheel wants to move out of the wheelarch. By connecting both sides together, an ARB resists this twisting motion and stops this from happening – transferring more load to the outside tyre to push the body back upright again, which in turn increases weight transfer. The stiffer the ARB, the more weight is transferred to the outside.
An anti-roll bar does very little when in a straight line (unless we’re talking about drag cars launching at the start line, which we’ll cover separately later); both left and right wheels are subject to the same bumps and undulations, and the ARB simply rotates in its mounts as the suspension is compressed and rebounded. But during cornering, the ARB is subjected to a twisting force, at which point it works together with the springs and dampers to control the car.
Anti-Roll Bars – Different Applications
Different cars doing different jobs will require very different ARB settings to perform at their best. A stiffly-sprung circuit racer with big, fat, slick tyres and lots of lateral cornering forces might want a very stiff ARB setting to keep everything nice and flat through the corners.
In contrast, a rally car that needs to have much softer suspension with a much larger travel to accommodate all the bumps of rough terrain would need to address the body roll issue completely differently; a stiff anti-roll bar would prevent the suspension from doing its job by restricting the ability of each corner to move up and down independently.
In these applications a softer ARB setting is required, and the body roll/weight transfer is managed via the spring rates – using things like dual-rate coilovers with a softer initial rate to absorb the rough terrain and then a stiffer second rate to help control the body roll and weight transfer through corners.
We said earlier that anti-roll bars don’t work in a straight line, but that’s not strictly true. An ARB doesn’t work when there isn’t any twisting action applied to it, but when you dump the clutch at the start of a drag strip there are a lot of torsional forces applied, without even turning the steering wheel, and an ARB will help resist this twisting motion as you accelerate.
The amount anti-roll bars resist the twisting force it’s subjected to during cornering is often referred to as roll stiffness; a stiffer bar will resist the twisting action more than a softer one. Generally, this stiffness comes from the cross-sectional area (or thickness) of the bar. Going back to school again very briefly, you’ll recall your maths teacher telling you that the area of a circle is ‘pi times the radius squared’. This means that even a small diameter increase – typically between 3 and 6mm larger – can have a big effect on the stiffness of an ARB.
Many aftermarket anti-roll bars available and will offer a level of adjustment, usually by way of having multiple connecting points for the drop links. These series of holes effectively alter the length of the cantilever (remember our three component parts?) and therefore alter the stiffness of the ARB; the longer the cantilever, the less effort required to move it, or the softer the setting. The shorter the cantilever, the stiffer the setting.
Some motorsport applications make use of a different design, known as bladed roll bars, which offer much more adjustment and levels of fine tuning.
Bladed Anti-Roll Bars
On many race cars, a bladed-style ARB is used. Going back to our three main components as we stated earlier, the main bar still is fixed to the car in the same way as ever, but now the cantilevers on each end that connect that ‘bar’ to the wheels have a bladed shape. This allows for a much greater window of adjustment than simply having various mounting holes to alter the effective length of the cantilever, and it allows for a much more compact and neater installation in many applications too.
The torsional strength of the main bar can be easily swapped for a stiffer or softer bar, but key to the design is the bladed shape of the cantilevers, which allows for significantly different levels of stiffness simply by rotating the blades through 90 degrees. When the blade is horizontal it is in its softest setting, allowing some deflection in the blade itself before acting upon the torsion bar. Rotate that through 90 degrees and now the (vertical) blade is in its stiffest setting, allowing much less deflection in the blade before it starts acting upon the torsion bar.
These blades are mounted within spherical bearings that allow adjustment through the full 90 degrees, and are often attached to a cable system that allows the driver to adjust them from the cockpit while driving – ideal for changing the handling setup during a race, when it starts to rain, for example.
Making Adjustments To Your Anti-Roll Bars
Unless you have some quite clever simulation models going on, adjusting your anti-roll bars will mostly be down to time behind the wheel and feel for changes. Don’t just assume the stiffest ARB you can find, or the stiffest setting, will give the best results, though – the whole package needs to be considered to work together with the springs and dampers for optimal results. In fact, it’s important to note that if you’ve already fitted an adjustable ARB and change from softer road-spec springs to stiffer race-spec coilovers, you’ll probably need to soften the ARB settings too.
If the car has too much oversteer through a corner, you can either stiffen the front ARB or soften the rear ARB to dial out the oversteer characteristic. If the car tends to understeer through a corner, you would either soften the front ARB or stiffen the rear ARB – or a combination of both.
As a rough rule of thumb, front-wheel drive cars will benefit from a stiffer rear ARB (to reduce understeer), while rear-wheel drive cars generally benefit from a stiffer front ARB (to reduce oversteer), and all-wheel drive cars can be either: if it understeers, fit a stiffer rear ARB; if it oversteers, fit a stiffer front ARB.
Anti-Roll Bar Mounts & Bushes
An ARB is mounted to the body or chassis of the car, and a strong, positive mounting location is required in order for the ARB to work at its optimum. Traditionally, most factory mountings use rubber bushes; these are cheap to mass-produce and keep the NVH levels low, but they do perish over time and can allow for excessive movement under heavy loads – neither of which are particularly useful for performance. Replacing rubber bushes with polyurethane is a great upgrade for many road and track cars, as the firmer material reduces unwanted flex, doesn’t wear out, and is still compliant enough for regular road use. Also, stock mounting brackets have been known to fail on some applications – not surprising when you consider the huge forces that go through the roll bar when smashing over kerbs on a racetrack at speed – which is why some roll bars come with, or have the option for, uprated mounting brackets for added peace of mind.
Race cars take any movement out of the system altogether with spherical bearings, meaning no energy is lost through unwanted movements, but these can increase NVH and aren’t as well suited to the dirt and grime of regular road use.
How Stiff Should Anti-Roll Bars Be?
There will be a limit to how stiff you can go with an ARB, though, and that all comes back to weight transfer. The easiest way to visualise this is to imagine a hot hatch or front-wheel-drive race car doing the famous ‘cocking a wheel’ trick. Front-engine, front-wheel-drive cars are usually prone to understeer due to their weight distribution and tyre loading. As such, they run a stiff rear ARB to help balance the car, which means the rear suspension is so stiff that 100 per cent of the weight is transferred – leaving no weight on the inside rear wheel and allowing it to lift off the ground. Obviously, if this is happening, fitting a stiffer roll bar at the rear won’t reduce any more understeer as all of the weight has been transferred. The only option in this situation is to soften the front ARB (or spring rate).
One adjustment will always have a knock-on effect elsewhere, and finding the optimum setup will require some time trying different settings and understanding how the car is behaving, and more importantly, how any changes will affect that behaviour.