The major components of the suspension system must be firmly located, to withstand the forces that occur in normal operation.
Control arms fasten a component like a rear axle assembly to the vehicle body, while allowing it to move as it needs to.
Similarly, with a steering knuckle.
The arms must be strong enough to withstand the forces due to normal operation, but light enough to minimize the vehicle’s unsprung mass.
The front suspension can have one, or two, control arms. Parallel-link front suspensions have two control arms.
Vehicles with strut-type front suspension have only one arm. It can be a wish-bone shape with two fulcrum mounting points, or straight with single fulcrum.
The inner end of the control arm is bushed to the vehicle body, and the outer end uses a ball joint to allow changes in the steering angle for turning.
This rear-wheel-drive vehicle has coil springs in the rear suspension; therefore special provision must be made to locate the axle, laterally and longitudinally.
Trailing arms, or control rods, are used to position the axle longitudinally. They have flexible rubber mountings at each end where they locate on the axle housing, and on the chassis frame.
They would allow the axle to move laterally if it were not restrained. Any such movement has to be controlled to keep correct alignment with the front wheels, and the vehicle frame.
This suspension system uses angled, upper-control rods, to limit lateral movement of the rear axle, during cornering.
They also absorb the torque reaction forces, which limits rear axle wind-up during accelerating, and braking.
A panhard rod may be used to restrict lateral movement of the rear axle during cornering. It has bushes or mountings at each end, where it locates on the axle and frame. This allows for variations in load, while maintaining correct alignment.
The axle may also be located by a Watts linkage. A lever, mounted on a pivot near the center of the axle housing, is connected by rods to the frame, on each side of the vehicle.
This maintains the axle in alignment with the frame, while still allowing the suspension to move vertically.