Suspension System

The MacPherson strut Suspension

The MacPherson strut is a type of car Suspension system commonly used in many modern motor vehicles. This includes both front and rear suspensions, but usually located at the front of the car. It provides a steering pivot (known as a kingpin) as well as a suspension mountings for the wheel. 

Rear positioned struts are also use but these are less common. In 1957 Colin Chapman of Lotus applied the design to the rear suspension of the Lotus Elite. As a result, MacPherson strut suspension at the rear of an automobile are now commonly called Chapman struts.To be really successful, the MacPherson strut required the introduction of unibody (or monocoque) construction. This is because it needs a substantial vertical space and a strong top mount, which unibodies can provide and also by distributing stresses greatly increases the set up's results in the handling stakes.

The McPherson strut normally also has a steering arm built into the lower inner portion. This assembly is extremely simple and can be pre manufactured into a unit at the assembly line. By removing the upper control arm, it allows for more width in the engine bay, aiding in any maintenance work or engine design requirements.

This is useful for smaller cars, particularly with transverse-mounted engines such as FF drive designed vehicles.  Further simplification is possible by substituting an anti-roll bar (torsion bar) for the radius arm. Also it offers an easy method to set suspension geometry. Ultimately making the production overheads more cost effective and making this a very common design set up in today's marketplace.

Although the MacPherson Strutt is a simple design and has low manufacturing cost, there are always compromises and a few disadvantages. The quality of ride it produces and the handling of the car may suffer or be less effective then other set ups with more Suspension geometry adjustments. 

Geometric analysis has shown that MacPherson Strutt design can not allow vertical movement of the wheel without some degree of either camber angle change, sideways movement (or both). So double wishbone suspension is favoured for Motorsport applications, due to this fact.

With the  MacPherson Strutt set up, the wheel tends to lean with the body, leading to understeer under extreme cornering. In a FF designed car, adding to the already natural tendency for understeer is far from ideal. The ideal situation would be neutral handling, so this might benifit other drivechain set ups.

Another disadvantage of the design is that it tends to transmit noise and vibration from the road directly into the body shell of the car. This results in higher road noise levels and sometimes a harsh feeling to the handling compared to a double wishbones set up. This results in manufacturer's adding extra noise insulation in a bid to reduce the the negative effects, which can lead to some weight gains as expected.

Double wishbone suspension

In Motorsports the application of the double wishbone suspension set up, is the preferred system and this is used in F1 for example.This is partially because it allows the engineers more freedom to choose camber levels and roll centre settings. Which ultimately will affect the car's handling in certain situations and could affect lap times/ handling effectiveness of the vehicle. 

Each wishbone (or A arm) has two mounting points to the car's chassis and one joint at the knuckle. The shock absorbers and coil springs, mount to the wishbones to control vertical movement.

The main advantages of the double wishbone suspension set up is that it is reasonably easy to work out the effects of the moving joints. This allows engineers to easily tune the kinematic of the set up to optimize wheel motion. In Motorsports where a tenth of a second a lap can mean the difference between winning and losing, having a suspension set up with easy adjustments, will yield greater performance for competitiveness.

Double wishbone suspension is more effective in working out the loads that different parts of the suspension are subjected to under loads, which could mean continued development and progression of lightweight parts especially in a racing environment.

This design also provides increasing levels of negative camber, throughout suspension motion uding full jounce travel. Unlike the MacPherson strutt design which provides negative camber gains, only at the beginning of jounce travel and then reverses into positive camber gains at high jounce level amounts. The Double Wishbone design has disadvantage, in that it is slightly more complex than other systems like a MacPherson strut and will be more expensive to manufacture.

Trailing-arm suspension

Trailing-arm suspension is a design in which one or more arms are connected between the axle and the chassis, at the front and basically allows the rear to move up and down. Commonly found in Pre 1990's application, when it was then mostly replaced by Multi-linkage suspension. 

Viewed as an older suspension technology set up, trailing arm design does take up a lot of space in the rear chassis area compared to other modern systems. A good place to look is the original Beetle front suspension set up, which uses a double trailing arm set up.

Semi-trailing arm suspension is a supple independent rear suspension system for cars where each wheel hub is located only by a large, roughly triangular arm that pivots at two points. Viewed from the top, the line formed by the two pivots is somewhere between parallel and perpendicular to the car's longitudinal axis; it is generally parallel to the ground.

Trailing-arm and Multi-link suspension designs are much more commonly used for the rear wheels of a vehicle where they can allow for a flatter floor and more cargo room especially in commercial vehicles. Many small, front-wheel drive vehicles feature a MacPherson strut front suspension and trailing-arm rear axle combination.

Leaf springs suspension 

Leaf springs suspension was common right up to the 1970s in Europe, Japan and up until the late 1970's in America. When the implementation to front wheel drive and more sophisticated suspension designs were implemented, this convinced car manufacturers to use the later designed systems like the coil springs design instead. Leaf springs are still used in heavy commercial vehicles such as vans and trucks, SUVs, and railway carriages. As they are cheap to manufacture and have excellent load carrying capacities.

The main advantages are for heavy vehicles, they spread the load more widely over the vehicle's chassis, whereas coil springs transfer it to a single point. This reduces the pressure and structural loads which result in a more robust design. 

Unlike coil springs, leaf springs also locate the rear axle, eliminating the need for trailing arms and a Panhard rod, thereby saving cost and weight in a simple live axle rear suspension.

Also with more recent technological developments of the parabolic leaf spring, the design is characterised by fewer leaves whose thickness width varies from centre to ends, following a parabolic curve. With this design, inter-leaf friction is unwanted and there is only contact between the springs at the ends and at the centre where the axle is connected.

Spacers prevent contact at other points through out the design. Apart from a weight saving advantages of this design, the other benefits of parabolic springs is their greater flexibility, which means vehicle ride quality that is comparable that of coil springs.

But there is a trade-off in the form of reduced load carrying capabilities, so this will only be used in appropriate applications. Characteristic of the parabolic springs include better ride quality and not a "harsh" as conventional "multi-leaf springs".

It is widely used on buses for better comfort of the passenger on board, where comfort takes a higher priority over load capacity. A further development by the British GKN company and by Chevrolet with the Corvette amongst others, is the move to composite plastic leaf springs.

Another variation and a update on the leaf spring design is the solid axle coil spring suspension design. This concept in theory is very similar, but the main difference is that the leaf springs have been removed in favour of either coil-overs spring and shock combination, or separate coil springs and shock absorbers.

Due to that fact that the leaf springs have been disposed of, the axle will now need lateral support from control arms. One end attached to the chassis and the other to the axle. There can be variations in the different layouts, but fundamentally this design is deemed as older technology. From a performance point of view, Handling characteristics can be quite limited in it's application. 

Beam Axle suspension 

Beam Axle suspension set ups are normally deployed in FF drive cars and it is a relatively simple designed system. The so called beam runs across under the car width, with the rear wheels attached to either end.

Spring/shock combination units or indeed struts are bolted to either end and normally located in the car body or chassis. The beam has two integrated trailing arms built in inplace of the separate control arms, which are found on solid-axle coil spring suspension system. Again there are other variations on the beam axle design and they can have either separate springs and shocks, or the combined 'coil-overs.

One main difference with other designs is the track bar (panhard rod). Basically a diagonal bar which runs from one end of the beam to another point, either in front of the opposite control arm or diagonally up to the top of the opposite spring mount. This bar's job is to try and stop side to side in the beam and help make the system more efficient.

Another variation is the twist axle which is identical with the exception of the panhard rod. In this design the axle is designed to twist slightly, which in effect creates a semi-independent system and a bump on one wheel is partially absorbed by the twisting action of this beam design.

From a Performance point of view, beam Suspension is not the optimum choice. Due to the construction, this design has a large unsprung weight compared to other suspension set ups. Also when the suspension is under load, each individual wheel can not operated independently. Similar to a car with a too stiff Anti Roll Bar / Sway bar at the front. Anything which affects one wheel, will transfer to the other.

This is far from ideal for a turn in on a Racetrack, also another disadvantage is the lack of Negative Camber under load. This results in poor Handling compared to other Suspension set ups. Normally when the Suspension is compressed, Negative Camber will result, aiding turn in response

This design does have some advantages in off Road applications, also due to the lay out lends itself well for Pick Ups and Van applications, due the way it can deal with varied load weights. Another good point is the impact it has on Internal space, due to it's compact suspension design and the dimensions will increase load carrying capacities.