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1.0 Dependant and Independent Suspension Systems

This section of the report discusses the common suspension systems found on motor vehicles. The section is concluded with a table listing advantages and disadvantages of each system.

1.1 Spring

The suspension spring is in place to absorb the shocks caused by bumps in the road surface, the spring stops the shocks from being carried though the tyre and wheel into the vehicle chassis. The suspension springs support the vehicles body weight. When load is applied to the spring, the spring compresses, absorbing the shock from the road surface. The spring is actually a shaft, which, when a load is applied, twists.

1.2 Damper

The damper is in place to reduce the number of oscillations in the spring created by suspension bound. The damper helps to ensure that the tyre remains in contact with the road surface. The damper itself is a variable component, the greater the velocity of the force applied to the damper, the greater the resistance produced within the damper.

1.3 Double Wishbone

The double wishbone suspension system comprises of two arms (the “wishbones”), a coilover, and the wheel hub. The steering linkage will also be attached to the hub assembly, although the position varies between manufacturers.

Figure 2.2 – Double Wishbone Suspension (Hillier 1966: 403)

The wishbones are mounted to the chassis, one above the other, as can be seen in figure 2.2, it can also be seen that the lower wishbone is longer than the upper wishbone, this is a common arrangement and is done to reduce tyre wear due to track change. “Track variation can be reduced by using wishbones of unequal length, the longer length wishbone being placed at the bottom.” Hillier (1966: 404). Hillier (1966: 404) goes on to say “The camber then becomes negative on the bump stroke which improves handling during cornering, although there is a small increase in tyre wear”. This is illustrated in figure 2.2 (a) and (b).

The coilover is mounted to the chassis at one end, and to the lower wishbone at the other, allowing it operate (see section 2.1) The upper and lower ball joints allow the hub assembly to rotate about its axis according to steering input. The Swivel Axis Inclination (SAI) is also determined by the position of the upper and lower ball joints in relation to the centre line of the wheel. The offset is also determined by the positioning of the upper of lower ball joints. Assessing figure 2.2 it is possible to see that shortening or lengthening either of the arms will change the camber angle of the wheel. From this it is easy to see the wide adjustment available for this suspension system.

Another design feature of this suspension system is the mounting of the lower wishbone, note how the rear pivot point for the lower wishbone is mounted above the front pivot point. This type of geometry set up helps to reduce vehicle ‘dive’ under braking, therefore reducing vehicle pitch and load transfer during braking in comparison to other suspension systems.

This suspension system is commonly found on performance vehicle due to its excellent road holding. The space the system takes up means it is more commonly found on vehicles with longitudinally mounted engines where space allows for a larger suspension unit.

1.4 McPherson Strut

The McPherson strut suspension is widely used today mostly due to its simplicity and compactness. The system comprises of a coilover, a lower control arm (LCA), and the hub assembly. The steering linkage will also be attached to the hub assembly, although the position varies between manufacturers.

Figure 2.4 - McPherson Strut (Hilliers 1966: 404)

As can be seen in figure 2.3, the LCA is pivoted at the vehicle chassis at one end, and to the hub assembly via a ball joint at the other, the LCA controls the track during bound and rebound of the suspension.

The coilover is mounted to the chassis at the top via a needle roller bearing, and attached to the hub assembly at the other end, when steering input is applied, the entire coilover rotates about its axis, excluding the spring, which is mounted on a fixed plate. The strut is angled to allow for wheel clearance, as well as allowing enough clearance for a negative offset to be achieved.

It is possible to see that this suspension system is more compact than others, and is therefore often found in vehicles with transversely mounted engines such as hatchbacks and people carriers where interior space is a premium.

1.5 Trailing Arm

The trailing arm suspension system consists of a longitudinally mounted arm (some systems use more than one arm), pivoted at the chassis at one end, and mounted to the hub assembly at the other. The coilover is then pivoted on the arm and mounted to the chassis.

Figure 2.5 - Rear Trailing Arm (Car Bible, 2010)

As the suspension bounds and rebounds, the arm(s) and hub assembly move up and down, this movement, as a result of the transverse mounting the vehicle track and vehicle camber do not change as the suspension bounds, but the wheel base does change due to the arc created by the travel. This type of suspension can also be mounted backwards, created a ‘leading arm’ suspension system.

This system is found on the rear suspension of many hatchbacks and coupes from the 1990s. More modern hatchbacks and coupes have now began using beam axles due to their cheapness and low level of complexity. Some performance cars are fitted with this system as it is an independent suspension system, which therefore offers great road holding.

1.6 Beam Axle

Figure 2.5 - Beam Axle Suspension (Car Bible, 2010)

A rear beam axle consists of a beam, which connects one wheel hub to the other wheel hub, a spring and damper is attached to the beam near each wheel, and fixed rigidly to the body. The beam pivots on the chassis to prove the suspension movement. The ‘panhard’ rod is place to reduce transverse movement in the axle. This system is not independent because any movement at one end of the beam will be transferred to the other end.

Beam axles are often found on modern hatchback vehicles due to their low cost. Many small commercial vehicles also use beam axles for this reason. Independent suspension is not a priority on these types of vehicles.

1.7 Torsion Bar

Torsion bar suspension systems consist of a bar, which is fixed to the chassis at one end, and to the suspension arm at the other. When suspension movement is input, the bar twists. This movement is exactly the same as the movement produced in a coil spring. A torsion bar is a type of suspension spring.

This system is often found on hatchback vehicles, in particular Peugeot vehicles. More manufacturers are beginning to use torsion bar suspension on rear of their vehicles for cost purposes. Honda has recently begun using torsion beams in replacement of trailing arms in its latest generation of the Honda Civic.

1.8 Leaf Springs

Leaf spring suspension systems consist of much suspension ‘leafs’. These rolled sections of metal, which are stacked one upon another act as the vehicles springs. Each leaf is smaller in length than the preceding leaf, which helps ensure constant stress between the leaves. The suspension leaves are fixed to the vehicles chassis at either end, pivoted via a shackle at one end to allow the change in length that occurs under load. The axle is then secured to the leaf springs. The suspension dampers are attached to the axle at one end, and to the chassis at the other.

Figure 2.6 - Leaf Spring Suspension (Car Bible, 2010)

Leaf springs are most commonly found on medium sized commercial vehicles. 1980’s American muscle cars are also well known for being fitted with this type of suspension system.

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