What do most modern small and medium-sized cars have in common? Well, nearly everything.
They are almost all front-wheel drive, with the four-cylinder in-line engine in the front. And almost all of them have MacPherson suspension. Prizes if you can think of an exception. In 2004 the market for small cars consisted of the Fiat Panda, Daewoo Matiz, City Rover, Skoda Fabia and Daihatsu Charade (among others). They all had MacPherson struts. Moving to the present day this is still true nearly all the medium-sized cars are so equipped.
The story of the MacPherson strut begins with Earle S. MacPherson’s work for GM in the USA in the 1940s. GM planned a small and economical car but the project was cancelled and MacPherson moved to Ford, taking his ideas with him. While Ford sometimes have a reputation for technical conservatism, it does not apply here: it got a new suspension design. The 1950 Ford Consul had fresh technology such as a new ohc engine, clutch actuated hydraulically and a monococque chassis. That last element made it ideal for MacPherson struts which work best when fitted to a rigid body rather than a body-on-frame vehicle.
The MacPherson strut is a development of the landing gear of aeroplanes: an outer tube which is attached to the hub-carriers at the bottom and to a seating cup at the top. That cup is attached to the body either rigidly or by means of a bonded rubber bushing. The tube contains a telescopic damper and that is the part attached to the seating cup. The lower part of the tube is located horizontally by the track control arm and a tie bar of an anti-roll bar that links across to the other side of the vehicle.
Every suspension type comes with advantages and disadvantages. It depends entirely on the application as to whether the right compromise has been struck. Even disadvantages in theory can be reduced by intelligent modifications elsewhere in the suspension system.
For the small Ford, the struts helped to maximise passenger and load space as they are primarily vertically aligned – the control arm and track rod are located horizontally to a low supporting member. Other varieties make more demands of the space in-board of the wheel which means a wider car or a more crammed engine bay.
The MacPherson system is also light thus reducing unsprung weight and total vehicle weight. If the car is small the benefits of a lower-unsprung rate counteract the tendency of such cars to have a choppier ride. The MacPherson strut is also tolerant of variations in its location, meaning a front sub-frame might not be needed to ensure precise placement of the elements (excellent if cost cutting is in order). And finally, the MacPherson system is quite cheap in itself and cheap to assemble. All of these factors are as true today as they were in 1950.
The MacPherson strut can be adapted to front wheel drive and rear wheel drive cars. For FWD the cross links must be raised slightly to clear the drive-shafts.
What are the disadvantages – why don’t sports cars make more use of them, for example? First the tolerance of variation implies a lack of precision. The vertical dimension works against a lower bonnet line (recall Rover’s frustration with Honda’s FDW architecture in the 80s). Lowering the car works against the tallness of the coil and shock structure. Wide wheels with a large scrub radius tend to produce more resistance to steering, especially during cornering. This factor is related to
the king-pin offset. MacPherson front suspension precludes centre-point steering. Citroen used double-wishbones for their legendarily precise GS (and as a counterpoint, Lancia’s Trevi was noted for its steering but had MacPhersons all-around). And finally, a simple MacPherson system is camber-sensitive. As the car rolls the MacPherson strut, in its basic form, tends to lead to lose of contact between the tyre and the road. Finally, the system generates more friction, the enemy of smoothness and control. And again a counterpoint: Peugeot have refined the system to produce cars of legendary smoothness.
As noted, most of the problems of MacPherson struts can be dialled out at the expense of adding expense. However, the general simplicity, space-efficiency and low cost mean that the MacPherson system is a respectable solution in affordable and moderately sized cars for routine use.