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4WD Suspension.

There are two major aspects to a suspension system: The geometry is defined by the collection of rods, linkages and bushes that locate the wheels or attach them to the vehicle while allowing them to move up and down over bumps. The springing consists of the springs and dampers that control the suspension movements.

Leaf Springs:

The oldest suspension systems are based on leaf-springs which usually serve the double purpose of locating the axle and providing the springing. They are often called "cart springs" in honour of their heritage. The most common form of leaf spring is the semi-elliptical spring. The spring is attached at one end to the chassis with a bush and 4wd.sofcom.com --> a through-bolt. The spring passes under (sometimes over) the axle and is attached to it with U-bolts. As the spring flexes, its length changes so it is attached to the chassis at the other end with a shackle which takes up this change. The leaf spring is usually made up of several leaves. These slide against each other as the spring flexes. Only the top or main leaf can have a firm attachment to the end bushes. Consequently, if the main leaf breaks, not only is its springing function lost but its function in locating the axle is impaired. Sturdy four wheel drive leaf springs therefore often wrap the second leaf around the spring bushes (loosely) so that there is some backup if the main leaf 4wd.sofcom.com --> fails. One-piece composite (fibreglass) leaf springs have been used on commercial vehicles.

Leaf springs are crude and heavy. The friction of the leaves sliding against each other provides a degree of damping although it is not exactly designed-in or well controlled.

The quarter-elliptical leaf spring is literally half of a semi-elliptical leaf spring. The "thick part" is bolted to the chassis and the other end is attached to the axle. This arrangement is not common but the Russian GAZ69 has 4wd.sofcom.com --> double quarter elliptical springs on each side.

Another system, now little used, is the transverse leaf spring in which the centre of an inverted semi-elliptical spring is attached to the chassis, and each end provides springing for the corresponding end of the axle. Some extra linkages are needed to resist fore and aft forces on the axle.

Live Axles:

The wheel is attached to an axle in some way. The simplest way uses a live or beam axle. This is just a strong tube which carries the differential near its centre. Half shafts run down the tube to drive the wheel hubs and thus the wheels. The springs are attached to the tube. The tube has to be strong and together with the differential is a substantial weight. This weight is all unspring weight - it is between the spring and the road. Designers, especially designers of racing cars, go to great lengths to reduce unsprung weight because it has a serious effect on handling.

When a wheel strikes a bump it is driven upwards, compressing the spring. After the bump the spring pushes the wheel down. Relative to the car body, the wheel can vibrate up and down rapidly, particularly on corrugated roads (wash-boards). A vibrating mass has a natural frequency and vibrations can build up wildly near this frequency if there is a source of energy (the car's motion). Naturally this could cause the wheel to loose contact with the ground for most of the time and lead to a loss of control. Shock absorbers, more correctly dampers, effectively control such vibration. Their job is made easier by reducing the amount of unsprung weight. Ideally, but impossibly, unsprung weight would be zero.

Independent Suspension:

Unsprung weight can be reduced dramatically by attaching the heavy differential to the (sprung) chassis and by discarding the heavy tube of the live axle. Each wheel hub must now be supported by a hub carrier which is attached to the chassis by links. Such systems are called independent suspension systems because the left and right wheels are no longer connected to a single axle tube.

Current Formula One cars use double wish-bone suspension. A wishbone is a A-shaped frame, hinged to the chassis at the lower ends and to the hub carrier at the apex. There is one wishbone for the top and one for the bottom of the hub carrier. The track-rods control the "pointing" of the front wheels and a radius arm keeps the rear wheels pointing straight ahead. Angles and distances from the differential to a driven wheel change with suspension movements so each half-shaft requires two universal or constant-velocity joints and a slip joint.

[M151A2 susp, jpg]

Double wishbone front suspension is quite common on four wheel drives such as the Mitsubishi Pajero (to 1999) or the GM Holden Jackaroo. Pictured is the military M151A2. This suspension is less common at the rear for various reasons: it is rather complex and expensive, it intrudes into the load space, and it is less robust than a live axle. But there is a trend to all independent suspension lead by the Mercedes M-class, Mitsubishi Pajero 2000 and BMW X5.

Macpherson-strut suspension uses a single lower wishbone. A telescopic strut continues upwards from the hub carrier to an attachment point under the wing. The strut invariably (?) contains an integral hydraulic damper and generally has a coil spring around it.

Brian Barrett-Park writes: "A variation often called a 'modified McPherson strut' applies the spring force to the control arm, just like most double A-arm designs mount the spring on their corresponding lower control arm. Examples include the Ford 'Fox' chassis (Mustang, and earlier Fairmont), an earlier version of the GM 'F-body' cars (Camaro/Firebird), and the Honda Civic of 1984-1987. The Honda is particularly interesting (but then, I own one...) since it uses torsion bars - the others use coils. Other designs (such as the Innocenti Mini, and probably others) use a transverse leaf spring, solely for springing (not location) linked to the control arm: this is exceptionally compact.
This 'modified' version has advantages of less 'stiction', and a more compact strut assembly. Since the strut housing carries no spring load, the upper mount is a simple rubber bushing, rather than a tapered roller bearing to allow steering movement."

Double wishbone suspension, and to a lesser extent Macpherson-struts, keep the wheel at a near perfect angle to the road as it moves up and down over bumps - which is good for on-road handling. They do have some disadvantages off-road: they may not be as robust as a live axle, the lower wishbone hangs low and reduces ground clearance, and the differential (attached to the chassis) dips under braking which reduces ground clearance at a critical time.

Various swing axle arrangements are also used: The hub carrier as attached to a frame which pivots about an axis on the chassis. This axis is the central chassis backbone in the case of the Pinzgauer four and six-wheel drives by Steyr Daimler Puch. The angle of the wheel and tyre to the road changes as the suspension moves which is undesirable in a sports car and can give "surprising" handling. This is less important in an off-road vehicle. The system is strong and simple and avoids the use of any universal or constant-velocity joints (except in the steering swivels) in these vehicles. Portal axles with hub-reduction gearing improve the ground clearance. Tatra trucks also use this system.

The hub carrier and frame can also swing about an axis across the vehicle as in the case of the Austin Gipsy. The distance between the hub and the differential changes in this case so each half shaft incorporates two universal joints and a slip joint.

Live Axles Not Dead:

Despite all the ingenuity shown in independent systems, there is a deal of life left in the live axle as applied to four wheel drives, certainly at the rear but also at the front: it is robust and simple, ground clearance does not change under loading or braking and it is in many ways easier to provide large suspension movements with a live axle.

[Landrover Discovery suspension at work]

The live axle can be rejuvenated by replacing the leaf springs with various links to control its movements more precisely. It needs control for thrust (left and right sides), torque, and lateral movement. Radius arms are often used to control thrust. A Panhard rod control sideways movement. Torque can be controlled by an extra arm to the top of the axle or by attaching radius arm(s) twice to the axle with bushes that allow a certain degree of flexing. If both arms are so attached, a degree of roll-stiffening is added. The Range Rover (classic), Discovery (left) and Defender use an A-frame to the top of the rear differential to control both torque and lateral movement; the Boge self-levelling unit was also attached to the A-frame on coil-sprung models.

Anti-Roll Bars:

A comfortable ride can be had with soft, long-travel springs, but this leads to a lot of body-roll under cornering. The usual on-road fix is an anti-roll bar, sometimes inaccurately known as a sway-bar. The anti-roll bar is a torsion-bar spring (see below) that operates between the two ends of an axle. Its job is to resist, to a certain extent, large differences in movement between the left and right wheel and to make the car corner flatter. Sometimes the bar is chassis-mounted with short links to the axle or to its locating arms. Sometimes the bar is mounted on the axle or its locating arms with short links to the chassis.

Body roll can be unnerving on-road but does not necessarily detract from road holding and a succession of agile French cars stands witness to this fact. However anti-roll bars are common fitments on many cars. The anti-roll bar may improve on-road handling but it certainly detracts from axle articulation off road. By resisting large differences in wheel movements it tends to unload the lower wheel which can lead to wheel spin. For this reason, the Nissan Patrol has an anti-roll bar that can be disconnected for off-road use.

In some ways the perfect off-road arrangement would be anti-roll bars between diagonally opposite wheels which would resist dive, squat and roll but not axle articulation - although it would be difficult to build. The Bucher Duro all wheel drive truck manages to get a similar effect with clever links and rockers on its de-Dion suspension.

A degree of roll-stiffness can also be built into a suspension system using means other than anti-roll bars. Coil-sprung live front axles commonly achieve it by attaching both locating arms to the axle tube with two bushes each; the bushes are tortured under roll. The Range Rover II achieves it at the rear axle by torturing the composite (fibreglass?) trailing arms which are firmly attached to the axle.

Springing:

Leaf springs are crude and heavy but they do perform the double duty (imperfectly) of springing and location. More sophisticated systems use links for control together with some better form of spring.

A coil spring is just that. It is often mounted between a lower suspension wishbone and either the chassis or a chassis outrigger.

[experimental air bag suspension on Bush Ranger]

Air-bags (left) can be used in place of coil springs. The air pressure is supplied by an engine-driven pump and pressure can be varied to change the ride height and to compensate for loading - as in the Range Rover II. (Air bags are becoming very popular on heavy trucks.)

A torsion-bar is a rod of spring steel that is twisted as the suspension moves. It can thought of as a straight coil spring! It is often used in conjunction with wishbone suspension, attached to the A-frame at one end and to the chassis at the other. The bar is usually installed neatly along the chassis rail. Torsion bars are sometimes adjustable.

A shock absorber, or damper, consists of a piston moving inside an oil-filled tube. Its purpose is to damp out unwanted axle movements. The piston has various holes and spring-loaded valves to control its resistance to movement. The piston arm moves in and out of the cylinder through an oil seal. As it does so the volume inside changes so there must be an air or gas space to allow this. Some dampers contain pressurized gas (behind a seal) to pressurize the oil and reduce cavitation. This puts a force on the piston and provides some load-bearing capacity. Some dampers can even be pumped up to "help" with heavy loads.

The various springs and dampers described above are usually attached fairly directly between axle-tube or hub-carrier and chassis. They can however be mounted remotely with suspension movements carried to them by linkages. (This is done on current Formula One cars to get the units out of the air-stream.)

Suspension movement can also be carried by fluid under pressure: The suspension unit operates a hydraulic ram and the hydraulic fluid compresses gas, possibly remotely. Damping can be built in as an integral part of the plumbing. Units can be interconnected, as on some B.M.C. cars of the 1960s, so that if one wheel hits a bump the other wheels react to reduce pitching. The Citroen DS used an engine driven pump to pressurize the fluid in a more sophisticated system. The ride height was adjustable and the car would settle when turned off and rise when woken. The Citroen C5 uses a later version of the system.

Kinetic of W.A. have developed a hydro-pneumatic suspension for four wheel drives. This allows extreme wheel travel while keeping near equal pressure on all wheels thus reducing the chances of wheel spin. Kinetic also claim significantly improved on-road handling and ride comfort. The hydraulic rams are passively controlled and the system is not active (see below). The system can include a computer which allows the vehicle's attitude and height to be controlled at will - useful for weapons platforms. Kinetic have an agreement in place with a major US manufacturer for new vehicle installation and kits for the aftermarket will also be produced. Kinetic was bought by Tenneco in 1999.

- 4wd.sofcom.com/4WD.html

Active Suspension:

All of the springing systems described above are passive: The wheel hits a bump, it moves up, compresses the spring which generates force to resist the movement, and the damper prevents oscillations building up. Modern electronics have made it possible to build suspensions which are active in controlling wheel movement, as pioneered by Lotus, Koni and Volvo, although they have only been used in experimental vehicles and in Formula One (now banned).

An engine driven pump provides a source of high pressure hydraulic fluid. Computer controlled valves operate hydraulic rams which replace the conventional springs and dampers. This allows any desired suspension characteristics to be programmed. Control has to be very rapid as (just) 90 mph corresponds to 132 feet per second or about 1.5 inches per milli-second.

The most exotic active systems use micro-wave transmitters and receivers to detect bumps and pot-holes before the wheel reaches them so that the computer can plan its behaviour. Needless to say this is very expensive and strictly experimental - so far.

So called semi-active suspensions take computer control of the dampers only, not the springs. This is rather easier to implement, the only new mechanical components being small motors in the damper to control the valves therein.

Potentially, active or semi-active suspension would be very useful in off-road applications. Speeds are lower than in racing, giving more time for a system to operate, although suspension movements are greater. Higher cross-country speeds would be possible with a more comfortable ride. If the military are working on it they are not telling.

See the G-Force (the Auto Edge), Kinetic, Fielder and mechanical pages


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