The MacPherson strut system is the most popular and widely used solution in the front suspension of front-wheel drive cars. However, many people are not fully aware of the assumptions and design features of this element. If you delve into the idea of a MacPherson strut, it turns out that not every speaker is a MacPherson.
The inventor of the MacPherson strut, as the name suggests, is the American engineer Earl S. MacPherson. Earl worked in the 1930s at the GM concern. After the war, he moved to the British branch of Ford, where he introduced his invention for the first time to the Vedette. He did not live to see its solution become widely used in front-wheel drive cars, as he died in 1960.
The Idea of The MacPherson Design
The MacPherson strut was supposed to be a cheap, simple and space-saving front suspension with only a few components. The design idea is to support a quarter of the car on a wheel attached to the body at two points – the upper mounting on the shock absorber bearing and the lower mounting using a transverse arm. The column in question is an assembly of a shock absorber and spring and a steering knuckle in one piece.
Thus, the MacPherson strut is an element that simultaneously performs a guiding, spring, damping and torsion function. The single or triangular lower wishbone is responsible for the guidance.
Suspension and damping are the role of the strut consisting of the shock absorber and springs. The steering knuckle is connected to the shock absorber strut. It is worth noting that the entire MacPherson strut spins on the top bearing while the driver turns the steering wheel.
To sum up, a MacPherson strut is one if all the following assumptions are met:
- It is the front suspension
- Shock absorber, spring and steering knuckle form one element after mounting in the car
- One control arm, attached to the lower part of the steering knuckle, is responsible for the guidance
- The shock absorber rotates in relation to its axis when we turn the steering wheel
If any suspension does not meet all of the above assumptions at once, it is no longer a MacPherson strut. So, it is definitely not the rear suspension of any car. Nor is it a suspension consisting of more than one control arm per wheel. The Opel HiPerStrut solution is also not a column, as the damper does not rotate when turning – a separate crossover is used here.
The MacPherson Strut Advantages
The biggest advantage of this solution is simple, compact and compact design, which is perfectly suited to cars with an engine located transversely in the front, driving the front wheels.
It is also a lighter structure than multi-link ones and very cheap to manufacture and service. The long and relatively thick shock absorber with the spring holding elements is placed almost vertically and therefore does not interfere with the creation of small cars.
MacPherson also has good kinematics for relatively simple and not too fast cars. The parallel wheel guidance is ensured despite the long suspension travel, which in turn ensures a very favorable suspension.
In addition, a negative swing radius is favorable from the point of view of stability and maneuverability. During braking, the MacPherson strut ensures adequate stability.
The MacPherson Strut Disadvantages
One of the biggest disadvantages of the MacPherson strut is that it is not quite suitable for high-powered front-wheel drive cars. First, wide tires cannot be used – large offset rims or spacers are required, which in turn increases the unsprung mass.
Secondly, when turning, by turning the entire column, the angle of inclination of the wheels relative to the ground slightly changes and there is unfavorable friction in the piston rod seat of the damper.
This causes a reduction in grip in the corner and disturbances in the transmission of torque to the wheels during acceleration and high forces transmitted to the steering wheel. In terms of driving comfort, the MacPherson’s disadvantage is also vibrations coming from the road. They are partially eliminated by thick rubber cushions in the upper socket of the shock absorber mounting.
Due to the two mounting points, the MacPherson suspension is also relatively fragile – it is easy to damage when driving over large unevenness.
Due to the above, manufacturers of sports cars began to create very similar solutions, based on the ideas of MacPherson, but not being a copy of them. Examples of such cars are the previous generation Ford Focus RS with RevoKnuckle suspension and some Opel models, including from the OPC family, where a solution called HiPerStrut was used.
Single or triangular swingarm?
To remain a technical purist, only a single wishbone solution with two mountings should be considered a true MacPherson strut. However, the MacPherson concept does not preclude the use of a triangular wishbone (with three mountings).
This is better because it prevents the rocker arm from moving along the vehicle during acceleration and braking. In single-arm designs, this movement is prevented by the associated sway bar, which often passes through its center.
It is worth noting that even the same car model may have both solutions depending on the engine version. For example, the Peugeot 205, which in weaker versions has a single wishbone with a stabilizer passing through it, while the GTI version already has a wishbone with three mountings, which increases the stiffness of the suspension.
MacPherson strut at the rear?
The MacPherson strut is a solution for the front axle suspension, which results not only from the assumptions of the designer, but also from his patent.
MacPherson strut for non-steered wheels?
Here, too, must be denied. The MacPherson strut is designed for steered wheels only. However, this does not mean that the MacPherson strut is used in the rear steering axle.
MacPherson strut for non driven axle?
Although the MacPherson strut was in fact widespread in front-wheel drive cars, Earl MacPherson did not point out that it is only dedicated to such cars. The Ford Vedette, the car considered the first to feature such a column, had rear-wheel drive. In addition, in one of the several patent sketches signed by MacPherson himself you can see that it is a non-drive wheel suspension.
What is a MacPherson or Pseudo MacPherson Design?
There is a trick in the naming, because due to the patent that belongs to Ford, the MacPherson strut is a solution that other manufacturers would have to pay Ford to use. Therefore, they cannot call their designs that, although they are very similar and based on the same assumptions.
Fortunately for them, MacPherson patented this solution for a rear wheel drive single-wishbone car, and all manufacturers use it on front-wheel drive cars, most often with a triangle-arm, so it’s not an exact copy.
The pseudo MacPherson is often referred to in the context of the shock absorber strut itself, i.e. the strut of the shock absorber and the spring in one element. This nomenclature is often used by mechanics, if only for the sake of convenience. For the record, it is incorrect.
Concluion
The MacPherson suspension is the most common type of front suspension today. One main structural component provides guidance and damping while allowing rotation. Designers eagerly use this solution because it is cheap to produce and operate, simple to build, and provides quite good driving comfort and steering precision.
The MacPherson guiding design is a shock absorber built with a spring, which is mounted in the upper point in an element called the bearing or shock absorber cushion, while in the lower point it connects to the steering knuckle.
A shock absorber is a typical wear element that wears out as the mileage progresses. The spring also loses its original characteristics over time – it happens that old springs break. Ignoring the need for replacement ends with a reduction in driving comfort, a decrease in the level of safety and further degradation of suspension components. It is worth noting that components such as shock absorbers and springs are always replaced in pairs within a given axis.
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