How Do Anti-Roll Bars Actually Work?

Most of us know the function of an anti-roll bar, but how do they actually work, and is bigger always better?

Remind me later
How Do Anti-Roll Bars Actually Work? - Tuning

The chances are you’ve heard of anti-roll or sway bars. And you probably know what they do - the clue is after all in the name: they combat body roll, duh. But how exactly?

The part itself is quite simple. It’s a U-shaped cylindrical length of metal (typically steel) that joins together the suspension either side of an axle generally via the control arms. It will usually be attached to the chassis in the middle at two points with rubber bushings. As for why you want one, we have to look at the joys of body roll.

Image via Evan Mason/Wikimedia, showing a front suspension setup with the anti-roll bar marked in red
Image via Evan Mason/Wikimedia, showing a front suspension setup with the anti-roll bar marked in red

During cornering, the sprung mass of the body will naturally be shifted towards the outer side of the car, otherwise known as body roll. This isn’t something you want - as the car leans the wheels start to tilt, reducing the contact patch of the tyres. Excessive roll also results in a car that isn’t as responsive, as it’ll take longer to react to your commands while trying to settle.

If you have an anti-roll bar fitted, the energy from the more heavily loaded side of the suspension will be transferred through the bar via a twisting force, effectively ‘pulling’ the wheel at the opposite side of the axle up towards the body. This won’t get rid of the roll entirely, but it will drastically reduce it by evening out the forces across an axle to an extent.

How Do Anti-Roll Bars Actually Work? - Tuning

There is a downside, however: having one side of the suspension affected by what’s happening at the opposite end isn’t always ideal when a car is dealing with imperfections in the road. This is, after all, why independent suspension is normally considered to be the best setup. Adding an anti-roll bar into the mix is a compromise (albeit a necessary one most of the time), and it can introduce a jolting motion sent through the cabin that can be uncomfortable and make the car feel nervous. So, you don’t want the anti-roll bar to be too stiff - it’s a balancing act between comfort and performance.

Some manufacturers get around this problem - while also improving anti-roll bar effectiveness - by fitting an active system. This replaces the single part with two roll bars protruding from a central motor. Sensors monitor factors like steering angle and yaw, and when necessary, that twisting force we talked about earlier is applied electronically, lifting the less heavily-loaded wheel up and counteracting the roll.

In terms of modifying your car, aftermarket anti-roll bars can improve the cornering ability while also changing the balance of the car. For instance, stick a stiffer sway bar on the back of a front-wheel drive car, and you’ll reduce understeer, while a studier piece on a rear-wheel drive car will cut down on oversteer.

It’s important to note that a larger diameter anti-roll bar isn’t necessarily going to be more rigid, as some are hollow. The length and position of the lever arms can affect stiffness too. More commonly seen in the world of motorsport, adjustable anti-roll bars allow for these factors to be tweaked, giving a quick and easy way to adjust the balance of the car to the preferences of the driver.

In the realm of off-roading meanwhile, some choose to remove the sway bars entirely, allowing for much greater axle articulation.

The torsion beam of the FK2 Honda Civic Type R is stiff enough to negate the need for a rear anti-roll bar
The torsion beam of the FK2 Honda Civic Type R is stiff enough to negate the need for a rear anti-roll bar

Have you ever changed the anti-roll bars on your car? What did you go for, and why? Let us know in the comments.