Independent and Live Axle Suspension

Tuesday, January 01, 2013

This article was found surfing the net and we felt that it was a good reference document.

The suspension of a vehicle can be described as the system of links that couples the wheels to the rest of the vehicle. However, in the automotive world, the suspension often refers to the springs and dampers as well as the linkages, i.e. to the complete assembly of parts that control the movement of the wheels. The configuration of the suspension arms controls the path that the wheels follow, while the properties of the springs and dampers determine the speed and acceleration of the wheels as they move along this path.

A good suspension system should be able to:

l. Isolate the framework from the road surface, to improve comfort and control. It does this by using springs to convert the kinetic energy of the up or down wheel movement into an oscillation, and employing dampers to convert this cyclic movement into heat energy. One of the important elements that come into play is the ratio between the sprung and the un-sprung mass. The sprung mass is that part of the vehicle above the springs, in other words the part that benefits from the fact that the vehicle has a suspension, while the un-sprung mass is mainly the wheels and tyres, plus a proportion of the suspension linkage. If this ratio is high, for example, if the sprung mass is large or the un-sprung mass is low, then there will be less body movement for a particular wheel movement, so that comfort will be improved. In addition, the lighter the wheel, the more easily the body can keep it down, i.e. in contact with the road, so that road holding is improved. This follows from the fact that there is a spring between the sprung and un-sprung mass, and this spring controls the movement of both in proportion to the masses involved. A low sprung/un-sprung ratio, resulting from heavy wheels and tyres, combined with a lightweight vehicle, will have the opposite effect - comfort and road holding will be diminished.

2. Keep the wheels mainly upright while they follow the path that the links allow. This implies that there should be no camber changes. For many years this was regarded as the ideal, but it is now known that a small amount of negative camber (wheels tilted inwards at the top) increases grip on the road. However, many modern systems deviate from the ideal without any apparent harmful effects. For example, many cars display large positive camber angles at the front wheels when turning sharply.

3. Have enough strength and be mounted in such a way that the forces produced by the tyre/road interface are fed into the framework at locations that are strong enough to resist deformation.

4. Help to control roll when a vehicle corners. This function depends on the suspension geometry, and the important criterion in this case is the distance between the centre of mass, and the roll centre. The centre of mass is a point somewhere above the ground where the forces acting on a car can be assumed to act. The roll centre is a point about which the sprung mass rolls during cornering. Front and rear suspensions will each have a separate roll centre, and the line connecting the two is the roll axis about which the car will roll during cornering.

5. Prevent excessive changes in attitude when braking or accelerating. The above are all functional qualities, but other criteria may have an over-riding influence in the mind of the designer. Of prime importance would be the cost of the components, their mass, the space occupied by the components in relation to the engine, as well as in relation to the driveline, boot space, fuel tank, and exhaust. Ease of assembly and servicing should also be considered, but the latter is often neglected. Ask anybody who has tried to replace a MacPherson strut at home.
Suspension systems are normally divided into two major groups, depending on how much freedom the wheels have. An independent suspension system is one where the wheels are able to move up or down independently of each other, while a non-independent system usually employs an axle to link the wheels.

Non-independent front suspension systems. Historically, these systems predominated until the late ‘40s, because the suspensions of most early cars were based on leaf springs as used by carriages for hundreds of years. Virtually all non-independent front suspension systems employed a beam axle suspended on leaf springs. Advantages included simplicity, ease of manufacture and robustness. It was also an easy way to keep the wheels upright and parallel, except when steering, when the outside wheel had to turn less than the inside wheel. Furthermore, the friction between the leaves supplied a certain degree of damping, and the leaves located the axle, not only longitudinally but also transversely. However, the last two qualities delayed the introduction of independent suspension because they blinded most suspension engineers to the need for separate controllable damping, as well as the need to locate the axle positively.

This layout had some important disadvantages, such as a large un-sprung mass due to the heavy axle, but it served many early designs well, provided the designer kept the mass of the axle and wheels as low as possible. In fact, on a smooth surface, not many modern designs will out-corner a beam axled semi-elliptic sprung Bugatti or Alfa Romeo from the late ‘20s and early ‘30s. On the other hand, a transverse leaf spring mounted on top of a beam axle gained a reputation for indifferent road holding for at least 30 million Fords, from the Model T, through the A and B, small British and German models, and even the V8 right up to 1948.

By the mid-’30s the days of the beam axle were numbered, at least at the front of the car, because it was responsible for three phenomena, all aggravated by the growing trend towards fatter tyres, bigger wheels and faster cruising speeds. The culprits were called shimmy, tramp and patter. Shimmy is a rapid movement of the front wheels from side to side, felt at the steering wheel as a brisk to-and-fro movement. It usually occurs just after one wheel has hit a bump, and is caused by the fact that the rising wheel tilts the axle, causing the other wheel to tilt as well. Both wheels are gyroscopes, and tilting a gyroscope causes it to precess i.e. tilt at 90 degrees to the initiating tilt, so that the wheels start to flutter around an axis at 90 degrees to the tilt of the axle.

Tramp, a rhythmic vertical rise and fall of the front wheels, is also a gyroscopic effect, caused when braking on an irregular surface that causes one wheel to have more grip than the other, so that the axle gets out of line, as seen from the top. Patter is a persistent rocking motion of the front wheels and axle about the longitudinal centreline of the car such as occurs just after a series of potholes have been traversed.A great deal of research was conducted on the causes of these phenomena, and the results showed that going to an independent set-up could eliminate all three, because the wheels would no longer be linked. This changeover started in the early ‘30s and was virtually complete by 1950. Another non-independent disadvantage was that the situation of the beam axle meant the engine could not be moved forward to obtain more passenger space. The narrow distance between the front springs that enabled a reasonable steering lock implied a low front anti-roll resistance, so a substantial anti-roll arm was needed to eliminate an over-steering tendency. It was also impossible to make the front springs soft enough to prevent a pitch-free ride, because this would encourage some of the gyroscopic effects just described.

Non-independent rear suspension systems.

Beam axle semi-elliptic spring rear suspension is still very much with us on millions of commercial vehicles, including all the one-ton single- and double-cab pick-ups that South Africans are buying by the thousands. What makes it so attractive? It is familiar, robust, and can be reasonably comfortable, and the fact that the wheels move parallel to each other promotes a long tyre life. Furthermore, the wheels do not have to steer, so the gyroscopic disturbances that plague the front wheels are not present.

Major drawbacks are the so-called torque reaction, which tends to make a powerful car spin its right-hand rear wheel when accelerating hard, and the massive un-sprung mass due to the heavy differential and right angle drive. This promotes axle tramp and a harsh ride. The two main rear axle constructions are the Hotchkiss drive, which has two universal joints and relies on springs and control arms to resist the drive torque, and the torque tube drive, favoured for many years by Ford and Peugeot. This employed an enclosed driveshaft that swivelled just behind the gearbox, but was bolted rigidly to the crown wheel and pinion casing.

The axle is sometimes located sideways by a Panhard rod, which links one end of an axle to the chassis, or a Watts linkage, which forces the axle to move up or down in a straight line. A beam axle can also be suspended on coil springs, but when this is done the axle has to be located positively, usually by means of trailing links. This reduces the un-sprung mass, and was very popular in the US and Europe at one stage, being fitted to makes as diverse as Oldsmobile and Peugeot. It has been superseded by front-wheel drive and/or totally independent rear systems.

Suspension: Forever Shackled Together, Jake Venter - 02 May 07