An anti-roll bar (also known as an anti-roll bar, stabilizer bar, stabilizer bar, torsion bar) is a part of many Automotive Suspension System that helps to reduce the vehicle’s roll force during fast cornering or unusual road crossings. It connects opposite (left/right) wheels to each other via short swingarms with torsion elastic linkage function. Anti-roll bars increase the anti-roll stiffness of the suspension.
The first anti-roll bar patent was awarded to Canadian inventor Stephen Coleman of Fredericton, New Brunswick on April 22, 1919.
The anti-roll bar is a U-shaped metal bar that connects both wheels on the same axle to the chassis. Basically, the ends of the rods are connected to the suspension on either side of the corresponding two wheels while the middle part of the rod is connected to the body of the vehicle.
In order for the body to roll over, the suspension on the outer edge of the vehicle must compress while the suspension on the inner edge is simultaneously stretched. However, since the anti-roll bar is attached to both wheels, such a movement is only possible if the metal rod is allowed to twist. (One side of the bar must twist upwards while the other side twists downward.) So the torsional stiffness of the rod – or resistance to torsion, determines its ability to reduce body roll. Less torsion bars result in less counter-movement on either side of the suspension (dragging, compression) – this results in less body roll.
Purpose and function of anti-roll bar
The anti-roll bar is intended to force each side of the vehicle to lower or raise to similar heights, to reduce the vehicle’s side-to-side tilt on curves, sharp corners or large bumps. When this bar is removed, the vehicle is tilted markedly, as shown in the image below.
There are many variations on anti-roll bar designs, but all have a common function of forcing the shock absorber, spring, or torsion bar of the opposite wheel to lower or raise it to the level of the other wheel.
In a quick turn, the car tends to tip out of the corner, leaning closer to the outside wheel, and the anti-roll bar will force the opposite wheel to come closer to the vehicle as well. As a result, the car tends to “hug” the road closer when making quick turns, where all the wheels are closer to the body of the vehicle. After a quick turn, downward pressure is reduced and the paired wheels can return to normal height above the vehicle, held at the same level by the connecting swingarm.
The anti-roll bar provides two main functions. The first function is to reduce the body tilt. The reduction in body tilt depends on the total anti-roll stiffness of the vehicle. Increasing the vehicle’s total anti-roll stiffness does not change the total load (weight) at steady state transferred from the inner wheel to the outer wheel, it only reduces the body tilt. The total lateral load is determined by the height of the center of gravity and the width of the track.
Another function of the anti-roll bar is to adjust the vehicle’s handling balance. Under- or over-turning behavior can be corrected by varying the ratio of total anti-roll stiffness from the front and rear axles. Increasing the anti-roll stiffness ratio at the front increases the ratio of total transmission to which the front axle responds – and decreases the ratio to which the rear axle responds. In general, this causes the outer front wheel to run at a relatively higher slip angle and the outer rear wheel to run at a relatively lower slip angle, which is a lack of revving effect. Increasing the roll stiffness ratio on the rear axle has the opposite effect and reduces under-rotation.
How does the anti-roll bar work?
The anti-roll bar is usually an elastic torsion bar to counteract the rolling motion of the vehicle body. It is usually composed of a cylindrical steel bar, formed into a “U” shape, in the middle connected to the body, connected to the suspension on the left and right sides respectively to the two wheels.
If the left and right wheels move together, the anti-roll bar simply rotates around its mounting points. If the wheels are moving relative to each other, the rod is subjected to torsion force and forced torsional motion. Each end of the rod is connected to an end link via a flexible joint. The end link of the anti-roll bar in turn connects to a point near the wheel or axle, transferring power from the heavily loaded axle to the opposite side.
When turning around a corner, the inflated mass of the body creates a lateral force at the center of gravity (CG), which is proportional to the lateral acceleration – the inertia force. Since the center of gravity is not normally on the roll axle, the lateral force that creates a torque about the roll axis tends to overturn the body. (The flip shaft is the line connecting the front and rear roll centers). Called flip pair.
The flip pair is countered by the anti-roll stiffness of the suspension. The use of anti-roll bars helped the designers to reduce spring/suspension stiffness purely in the vertical direction, allowing for improved controllability and handling without sacrificing ride quality.
One effect of overturning the body (frame), for a typical suspension geometry, is the positive camber of the wheels on the outside of the turn and negative on the inside, reducing their cornering grip (especially is with criss-cross tyres).