Multi-link suspensions have largely replaced conventional wishbone suspensions, increasing driving dynamics and comfort. The modern design, explains TRW, features one or both wishbones broken down into multiple components, so that significantly more moving parts are installed.
The distance between wheel centre and steering axle – the so-called disturbing force lever arm – is relatively short. This allows disturbing forces (propulsive, braking and lateral guiding forces) to be kept away from the steering as much as possible. However, the joints move closer to the brake, which automatically exposes them to higher temperatures, resulting in significantly higher demands on the installed rubber sleeves and plastic bearing shells, as well as for the grease used.
More moving parts also means more joints connecting the parts, this results in a higher sum of ‘moments’, which are divided into breakaway torque and running torque. Breakaway torque results from the force required for setting the ball pin into motion from its resting position. The running moment results from the force required for rotating the ball pin or moving it back and forth in the joint.
Development engineers try to keep these moments as small as possible as this ensures, for example, that after cornering the front axle returns to the centre position independently and smoothly and that the suspension responds more sensitively. Today, so-called low-friction ball joints are used that have a substantially lower friction moment than previous types.
While a smoothly moving ball pin used to indicate increased wear of the ball joint, this is no longer the case. Modern ball joints have distinctly lower breakaway torques and running torques.
DEVELOPMENT AND DESIGN OF A MULTI-LINK SUSPENSION
The product specifications defined for a multi-link suspension are determined during extensive test procedures on a test bench as well as in the vehicle. The associated moments (running torque and breakaway torque) as well as the damping properties of a joint always have to be considered within the overall system.
The influencing factors for steering systems, suspension, springs, brakes and tyres are extremely important for the functioning of the overall vehicle and for vehicle safety. Altering individual components and specifications is only permitted with consideration of the overall concept and always requires approval from the vehicle manufacturer.
Mechanics should observe several points when replacing individual components.
When replacing a ball joint, the eye and the rubber contact surface in which the joint is installed have to be cleaned and any rust removed. The contact surface must be free from rust or the rubber sleeve will rub against the rough surface and leak. Dirt and moisture can enter the joint, which leads to premature joint failure.
During installation, it has to be ensured that the corrosion protection layer of the circlips is not damaged, as rust causes them to lose their spring force, allowing moisture ingress into the joint, significantly affecting service life.
Never tighten the ball joint with an impact wrench. There is a risk the ball pin could start to rotate so fast that the plastic bearing shell becomes deformed by the frictional heat, leading to play in the system. Furthermore, the tightening torque can be exceeded, causing the ball pin to move too high into the eye of the stub axle. This means the rubber sleeve can no longer fulfil its sealing function, allowing dirt and moisture to enter into the ball joint.
The rubber bearing on a control arm may only be tightened when compressed and without load to avoid twisting and therefore applying pre-tension to the bearing.
The installation errors described here can lead to premature wear or even failure of the replaced part. Garages should always carry out a wheel alignment after replacing suspension parts, even if only axle components were released. If these simple rules are followed, all suspension repair work can be carried out successfully.