When a vehicle is in motion, several forces operate to displace the wheel units - driving thrust, braking torque, and cornering force. These forces must be transferred to the frame of the vehicle, but while they act, the wheel units must stay aligned with each other, and with the frame.
They must be located longitudinally, and laterally, while still having the freedom to move vertically, to allow for suspension travel.
On vehicles with non-independent suspension, leaf springs provide a simple means of performing these functions.
On a rear-wheel-drive vehicle, the axle housing is located on the spring by the spring centre bolt, and clamped to the spring by U-bolts. Since the front eye of the main leaf is located on the frame at the fixed shackle point, the spring then prevents the axle moving longitudinally. A swinging shackle at the rear allows the semi-elliptical spring to flatten out under load, which provides for vertical movement. As well, the driving forces can be transferred through the spring to the frame.
Driving thrust is transferred from the tire contact patch, through the axle housing and front half of the spring, to the fixed shackle point. This pushes the vehicle along the road.
During braking, the torque tries to twist the axle housing around its centre, but it’s held firmly, clamped to the spring, and any twisting effect is resisted by the relatively stiff spring.
During cornering, lateral movement of the axle housing is prevented by its location on the spring centre bolts, and by the spring shackles.
Leaf springs perform these functions well, but the transfer of forces interferes with the suspension function, and ride quality is compromised.
On an axle of this type, the use of coil springs allows the spring to carry out the function of suspension only.
The axle housing is usually located longitudinally, by upper and lower control arms, and laterally, either by the angle of the control arms, or by a separate Panhard rod, or Watts linkage.
The control arms have flexible bushes at each connecting point, to allow for suspension movement. The driving thrust and braking torque are transferred through the control arms and bushes to the frame.
