Anti-roll bar

An anti-roll bar is suspension part that resists vehicle body roll, most often through the twisting of a bar. On a traditional anti-roll bar, laterally opposite wheels each have a drop links connected to the anti-roll bar. A blade style anti-roll bar resists relatively opposite suspension movement through the twisting of a blade.

What's the Point

A vehicle's body "rolls" during cornering due to the center of mass not being in the same location as the roll center. This creates a moment known as a roll couple. So why do we not just let the car body roll? In order for the aero package to work consistently well. A rolling body creates unequal sized gaps between the undertray and the ground creating adverse wakes that decrease downforce while increasing drag. Additionally, while piloting a car with significant body roll the driver will feel disconnected from the tires.

Anti-roll bars change the vehicle's roll stiffness. A change in an axle's roll stiffness further changes the distribution of load front-to-rear during lateral load transfer. Anti-roll bars do not change the load distribution left-to-right. Using anti-roll bars to change front-to-rear load distribution allows us to change the balance of the car during lateral load transfer (aka "mid"-corner).

your wish	your genie
to reduce understeer	stiffen rear anti-roll bar or soften front anti-roll bar
to reduce oversteer	soften rear anti-roll bar or stiffen front anti-roll bar

How it works

The beauty of anti-roll bars is that they only affect the dynamics of the suspension when there is a relative difference between the left and right suspension positions. Therefore, with the use of an anti-roll bar the dynamics of roll can be isolated.

An anti-roll bar does nothing during pitch and heave because when both sides of the suspension are compressed, the anti-roll bar does not twist and therefore provides no suspension resistance. When anti-roll bars twist they are essentially acting as springs.

Traditional anti-roll bar

parts of a traditional anti-roll bar

A traditional anti-roll bar connected to either side of the suspension by drop, or end, links. The anti-roll bar is also fixed the chassis at 2 points. When the outside suspension compresses and the inside extends, the anti-roll bar twists, therefore reducing the extent which the inside suspension extends and the extent which the outside compresses. This keeps the car's body more horizontal thus body roll has been reduced.

Note that on two identical vehicles aside from an anti-roll bar, when experiencing an equal lateral acceleration, the outside suspension of the vehicle with the anti-roll bar will compress less than the vehicle without. Essentially, the anti-roll bar forces the left and right suspension to "share", meaning the outside suspension is compressed less and the inside is extended less.

The stiffness of the anti-roll bar is inversely proportional to drop link length. To calculate an estimated torsion stiffness of a traditional anti-roll bar use the following:

$k_a{}_r{}_b=\frac{\pi D^{4}G}{32l}$

• k_arb is the torsional stiffness of the anti-roll bar
• D is the diameter of the anti-roll bar
• G is the anti-roll bar's material's shear modulus
• l is the anti-roll bar's effective length

There are other equations that are probably better lowkey. The torsion stiffness can be used to calculate roll stiffness.

Blade Style anti-roll bar

Rear blade style ARB on an IndyCar. The orange part is the blade that gets twisted under roll.

A blade style anti-roll bar is also connected to either side of the suspension, but unlike traditional anti-roll bars they are often connected to rocker arms. When both springs compress, the blade is not twisted. However, when there is relative movement between the springs, the blade is twisted therefore reducing the extent which the inside suspension extends and the extent which the outside compresses.

Front blade style ARB on a 1999 IndyCar. The actual blade that gets twisted is under the carbon and cannot be seen

The advantage of the blade style anti-roll bar is packaging, weight, and adjustability. Because the blade is often connected to the rocker arms, it is often easy to transfer the twisting force to a "far away" blade allowing for better packaging and an opportunity to lower the center of gravity. The weight advantage is largely due to taking advantage in the coilovers proximity to each other allowing for a much smaller anti-roll bar that does not have to span almost the entire tract width.

Because a non symmetrical blade is being twisted, depending on its orientation its effective spring constant changes. Thus the anti-roll bar's effective stiffness can be changed quickly, sometimes even by the driver.

Honestly just watch this short IndyCar explanation: INDYCAR's Anti-Roll Bar technology is next level