An anti roll bar (ARB) is that part of a modern vehicle that is designed to reduce body roll when a car negotiates a sharp corner or other road irregularities. It is also referred to as an anti-sway bar, roll bar, stabiliser bar or ARB. It is fitted on the opposite side of the front and rear wheels of a car using short lever arms that are connected through a torsion spring. The bar stiffens the suspension to help the vehicle avoid the body roll. Anti roll bars were not common in pre-war cars as they had a stiffer suspension and could prevent body rolls. The 1950 car models, however, came fitted with the bars especially vehicles with a softer coil spring suspension. These bars have ends that bend at angles that vary from 60-90 based on the type of vehicle. As the inner suspension arms move downwards and the outer arms rotate upwards, the bar is fitted below torsion. Its thickness determines the roll’s resistance. Unless a vehicle gets involved in an accident, the stabiliser bar is unlikely to be used throughout the life of the car.
Types of Anti Roll Bars
There are two types of anti roll bars the front suspension bars and the rear suspension types. Modern vehicles are fitted with front suspension anti roll bars using two mounting methods. In the first method, the bar is mounted on the sub-frame, chassis or any other part of the vehicle’s structure. The upper end passes through the horizontal opening of the bar and the lower end attached to the suspension arm in the same way.
The second mounting method uses the bar as a suspension link. This mounting method is widely used on vehicles that have MacPherson strut suspension systems like the Fiat 128, Ford Escort, Capri Mk I, and Japanese and Ritmo models. In this case, the bar is mounted on the sub-frame or the chassis just like the first method, but the ends pass through a rubber-insulated eye on the lower suspension arm.
Rear stabiliser bars are common in older vehicles, and the attachment methods are similar to those used when fitting front suspension anti-sway bars. Alternative designs may have the bar mounted on the axle casing with a bend joined to the bar to prevent the final drive housing. This kind of mounting allows the bar ends to project forward.
Another type of anti roll bar is the adjustable type, which is designed for auto racing vehicles. These bars can be adjusted externally when the car is in the pit, and other systems are modified in real time by the car owner from the inside of the vehicle. The premise is to allow the driver to alter stiffness by reducing or increasing the length of the lever arms. A perfect example is the Super GT, which features adjustable anti-sway bars.
How ARB's improves a Car’s Performance
Manufacturers of car models produced in the 60’ and 70’s only emphasised on horsepower and not the handling capabilities of the vehicle. While the suspension components were adequate, handling and cornering capabilities of most cars produced during this time were only a secondary consideration when it came to performance. As mentioned earlier, stabiliser bars increase the stiffness of the suspension system by increasing resistance to a body roll.
Normally, suspension springs ensure vertical resistance to body roll while the stabiliser bar offers horizontal resistance. When driving on a flat road where both wheels are going in the same direction, the bar may not be necessary. However, if one wheel is moving upwards and the other downwards, the anti-sway bar generates an opposing force to even out the body-roll during fast cornering. The bar adds to the spring rate of the outer wheel and pulls down the spring on the inside of the wheel to flatten the car’s position when cornering.
As a result, the bar reduces the vehicle’s cornering force and the amount of grip that the tires exert. While thicker anti-sway bars increase the vehicle’s resistance, they can affect the amount of under-steer on the suspension system. Additionally, thick ARBs can cause bumps to be more severe on rough roads, which is a condition known as over-barring. It occurs when the suspension system becomes so stiff that the vehicle loses standard suspension components between both sides of the car. Consider a car that has an ARB with a diameter of ¾; increasing its width increases the stiffness of handling to the power of four, i.e., if the bar is upgraded from ¾ inches to an inch, the percentage of stiffness is increased by more than 300 per cent. Over-barring can also lead to loss of traction in the tyres and the loss of their gripping capabilities.
Alternatively, car owners can stiffen the anti-sway bar by adding improved bushings bolted to the frame and link ends. Most aftermarket suppliers sell urethane or poly-graphite bushings, which are stiffer and longer-lasting than their traditional rubber alternatives. They increase the effectiveness of the bar by 20-30 per cent by adding support and reducing the amount of camber change in the alignment of the wheels.