Vehicle body roll occurs when lateral acceleration and transverse weight transfer cause a car to roll toward the outside of a turn while cornering. The amount of roll is referred to as the 'roll angle'.

Roll resistance engineered into a suspension can help reduce body roll and is achieved by suspension springs, anti-roll bars, or a combination of the two. By building roll resistance into the front and rear suspension a balance between under-steer and over-steer can be achieved.

Under-steer occurs when the front end of car wants to go straight ahead while cornering, typically experienced as the front suspension 'ploughing' through the corner rather than going around it.

Over-steer occurs when the rear end of a car wants to continue straight-ahead while cornering, typically experienced as the rear of the car 'fish tailing' or swinging to the outside of the corner.

Spring rate can be used to reduce roll angle, however stiffer springs that reduce body roll will also affect ride quality and handling. For example, to halve a roll angle of two degrees in a vehicle with 500 lbs-in. front spring rate would require 1000 lb-in. front springs. This would make the car ride very stiff, reduce tire/road contact due to tire hop over bumps, and cause under-steer.

Spring rates, anti-roll bars & roll resistance...

Development of the anti-roll bar enabled control of body roll while allowing softer springs to be retained. Anti-roll bars achieve this by transferring suspension load from one side of the car to the other while cornering.

In principle if a vehicle suffers from under-steer, softer front springs, stiffer rear springs, a less stiff front anti-roll bar, or the addition of a rear anti-roll bar can help achieve balance between the front and rear suspensions. This changes the balance or 'roll force distribution' on the vehicle.

Conversely, if a vehicle over-steers this can be corrected by adding an anti-roll bar to the front (or a stiffer bar if it already has one) rather than stiffer springs on the front or softer springs on the rear.

Anti-roll bar diameter versus arm length...

The effective stiffness of an anti-roll bar can be achieved in two ways. As bar diameter increases, the stiffness increases dramatically. For example, a 1.25-inch diameter anti-roll bar that's 25% bigger than a 1-inch bar will be almost 2.5 times stiffer. A 1.5-inch diameter anti-roll bar that's 50% larger than a 1-inch bar will be over five times stiffer.

The second factor that affects the effective stiffness of an anti-roll bar is the length of the arms that feed load into it. Assuming the diameter and steel characteristics remain constant, a short arm will make the anti-roll bar effectively stiffer, a long arm making it less so. Relatively speaking, halving the length of the arm will double the effective stiffness, and doubling the arm length will halve the stiffness.

   • Click here to see different anti-roll bar arm function


This way it's possible to use a small diameter (lighter weight) anti-roll bar with short arms, yet achieve high roll resistance. In this example however, the short length of the actuating arms would require it to be located close to the control arms of an independent suspension, or live axle.

Whether on a vehicle with independent or live axle suspension, an anti-roll bar must be securely anchored to the chassis to ensure its torsional stiffness applies load to the chassis to help decrease roll angle.

Camber angle, tire contact and cornering power...

With independent suspensions, as body roll occurs the camber angle of the tire relative to the road surface changes. Tires provide maximum traction when perpendicular to the road with the contact patch is at its maximum, however as roll angle induces positive camber tire contact is reduced. This leads to less cornering power, which in turn can affect acceleration out of a corner as power is applied.

Static negative camber angle can be engineered into a suspension design, however too much can cause accelerated wear on the inside edge of the tire. Negative camber 'gain' can be designed into an independent suspension through the use of unequal length A-arms (swing arms), so that as the outside suspension compresses under load while cornering, the amount of negative camber increases. This is a common feature of modern independent suspension design.

   • Click here to see camber change on independent suspension


However, camber change is affected by the length and position of swing arms, the roll center height, center of gravity, and other factors that limit the practical amount of camber gain possible. At best, negative camber gain will be less than the positive camber caused by body roll, so the net result is still marginal positive camber with reduced tire contact and less cornering power.

Live axle or independent, all suspension can benefit from the addition of anti-roll bars...

On independent suspensions, a lighter front spring rate will help stop under-steer, but at the expense of increased roll angle due to lower roll resistance, and less tire contact due to increased positive camber. For this reason, a better solution is usually the addition of a rear anti-roll bar.

Generally speaking the same applies to vehicles with independent rear suspension where a lighter rear spring rate can help stop over-steer, but at the expense of decreased roll resistance, increase roll angle and potentially reduced tire contact. A better solution is the addition of a front anti-roll bar or a stiffer bar if one is already in place

Hot rods that feature live axles front and rear get the theoretical benefit of maximum tire contact patch due to camber angle being fixed at zero degrees or close to it. However, this benefit is offset because the end of each axle is affected by what's happening on its opposite end. For example, while the outside tire is under load from lateral weight transfer while cornering, the inside tire is less loaded, which can lead to less traction, and less cornering power.

   • Click here to see fixed camber on live axle


For all vehicles, the ideal means to limit roll angle is to use a combination of spring rates tailored for ride quality and handling, with anti-roll bars mounted front and rear to tailor the balance between under-steer and over-steer..........read more

 

 

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