Rear Suspension Systems are as critical to ride control as front end systems. An automobile must be in optimum pitch and balance to ride right. There are two types of conventional rear suspension systems: coil spring and leaf spring.

Dependent (Linked) Systems

Solid-axle, leaf-spring

The drive axle is clamped to the leaf springs and the shock absorbers normally bolt directly to the axle. The ends of the leaf springs are attached directly to the chassis, as are the tops of the shock absorbers. The main drawback with this arrangement is the lack of lateral location for the axle.

Solid-axle, coil-spring

This is a variation and update on the system described above. The basic idea is the same, but the leaf springs have been removed in favor of either 'coil-over-oil' spring and shock combos or separate coil springs and shock absorbers. Because the leaf springs have been removed, the axle now needs to have lateral support from a pair control arms. The front ends of these are attached to the chassis, the rear ends to the axle.

Beam Axle

This system is used in front wheel drive cars, where the rear axle isn't driven. Again, it is a relatively simple system. The beam runs across under the car with the wheels attached to either end of it. Spring / shock units or struts are bolted to either end and seat up into suspension wells in the car body or chassis. The beam has two integral trailing arms built in instead of the separate control arms required by the solid-axle coil-spring system. Variations on this system can have either separate springs and shocks, or the combined 'coil-over-oil' variety. One notable feature of this system is the track bar. This is a diagonal bar which runs from one end the beam to a point either just in front of the opposite control arm or sometimes diagonally up to the top of the opposite spring mount. This is to prevent side-to-side movement in the beam which would cause all manner of nasty handling problems. A variation on this them is the twist axle which is identical with the exception of the panhard rod. In a twist axle, the axle is designed to twist slightly. This gives, in effect, a semi-independent system whereby a bump on one wheel is partially soaked up by the twisting action of the beam. Yet another variation on this system does away with the springs and replaces them with torsion bars running across the chassis, and attached to the leading edge of the control arms.

4-Bar

4-bar suspension can be used on the front and rear of vehicles. The parallel design operates on the principal of a "constant motion parallelogram". The design of the 4-bar is such that the rear end housing is always perpendicular to the ground, and the pinion angle never changes. This, combined with the lateral stability of the Panhard Bar, does an excellent job of locating the rear end and keeping it in proper alignment. The triangulated design operates on the same principle, but the top two bars are skewed inwards and joined to the rear end housing much closer to the centre. This eliminates the need for the separate panhard bar, which in turn means the whole setup is even more compact.

Derivatives of the 4-Bar system

There are many variations on the 4-bar systems. For example, if the four angled bars go from the axle outboard to the chassis near the centerline, this is called a "Satchell link". Both of the these angled linkages can be reversed to have the angled links below the axle and the parallel links above. The roll center will be lowered with the angled bars under the axle, a function which is difficult to accomplish without this design. The other variation on the "four bars" not shown are the Watts and Jacobs bar linkages to replace the Panhard rod for lateral positioning. Another linkage is the two parallel bars above the axle and a triangulated link underneath - a design you will find on the Lotus 7 - where the lower link has its base on the chassis and the apex under the differential. Then there is the Mallock Woblink, which could be described as half way between a Jacobs ladder and a Watts link, and makes it possible to place the rear roll center quite low without sacrificing ground clearance.

Independent Systems

Simplified versions of all the front suspension independent systems described above can be found on the rear axles of cars. The multi-link system (4-wheel independent suspension) means all the wheels are independently mounted and sprung. The drive towards 4-wheel independent suspension is primarily to improve ride quality without degrading handling.

Ford Control Blade Suspension

Control blade is an evolution of trailing-arm suspension. The primary purpose of Control Blade suspension is to increase the interior space available in the vehicle. Most suspension systems used in daily drivers have strut towers front and rear. In the front it's not really a problem, but in the rear it impedes on trunk space. Ford wanted to give more space in the back and needed to find a good way to remove or reduce the size of the strut towers. The result is their Control Blade system which in essence separates the shock absorber from the springs. The control blades themselves are basically the trailing arms which give lateral support and provide the vertical pivot point for the entire unit.

Hydro-lastic Suspension

The principle is simple. The front and rear suspension units have Hydro-lastic displacers, one per side. These are interconnected by a small bore pipe. Each displacer incorporates a rubber spring (as in the Moulton rubber suspension system), and damping of the system is achieved by rubber valves. So when a front wheel is deflected, fluid is displaced to the corresponding suspension unit. That pressurizes the interconnecting pipe which in turn stiffens the rear wheel damping and lowers it. The rubber springs are only slightly brought into play and the car is effectively kept level and freed from any tendency to pitch.

Hydra-gas Suspension

Hydra-gas is an evolution of Hydro-lastic, and essentially, the design and installation of the system is the same. The difference is in the displacer unit itself. In the older systems, fluid was used in the displacer units with a rubber spring cushion built-in. With Hydra-gas, the rubber spring is removed completely. The fluid still exists but above the fluid there is now a separating membrane or diaphragm, and above that is a cylinder or sphere which is charged with nitrogen gas. The nitrogen section is what has become the spring and damping unit whilst the fluid is still free to run from the front to the rear units and back.



	REAR COIL SPRING SUSPENSION

On a Coil spring suspension, the spring is mounted between the axle housing and frame. A lower control arm connects the axle housing to the frame. Some vehicles use an upper control arm for added stability.

Control arms are not required on leaf spring suspensions. The leaf spring is connected to the axle housing with the U-bolts. The shackle assembly allows spring movement.

In both applications, shock absorbers connect between the axle housing and the frame and absorb excess energy from the system. If the vehicle is being used to carry heavy loads on trailers, load-assist shock absorbers are recommended.

Routinely inspect rear shocks whenever under-car service is being performed.



	LEAF SPRING REAR SUSPENSION

IAUTOINFO.COM: AUTOMOTIVE CENTER

Automotive Information for the consumer