The MacPherson Strut (A complete guide)

In this brief article, we will discuss the MacPherson Strut, its design features, advantages and applications.

What is a MacPherson Strut?

The MacPherson strut is a type of front suspension system seen in cars, in which the top mount of the shock absorber also performs the duty of the upper steering pivot of the wheel (also called upper ball joint in the double wishbone suspension). It is widely used in the front suspension of modern vehicles.

A Macpherson Strut suspension’s main component is the Coilover shock absorber. The Coilover Strut’s lower mount is securely bolted onto the Knuckle. The Coilover Strut’s top mount has a bearing that allows the strut to rotate simultaneously with the knuckle. The MacPherson strut is relatively simple as compared to the Double wishbone with lesser number of components and joints and also occupies comparatively lesser space.

Why is the Macpherson strut suspension called so?

The MacPherson Strut suspension was first seen on the Chevrolet Cadet Prototype. The name MacPherson strut is in remembrance of Chevrolet’s then Chief Engineer of the famous Chevy Cadet Light car Project, Earle MacPherson. The Chevy Cadet was the first car to feature the unique and compact suspension design approach which later came to be known as the MacPherson Strut.

The History behind the MacPherson Strut Design

The Double wishbone suspension with upper and lower control arms had always been the standard independent suspension setup for any car from the post World War II era.

The Chevy Cadet post-war Light Car Project had the ambitious objective of producing a new mass-market car that was ‘Compact’ and fit within a $1,000 price tag. As a result, the Chevy Cadet’s Chief Engineer, Earle MacPherson tried out several unique space-saver design innovations. One of the many aspects he looked into was the design of the front independent suspension.

Even though the Chevy Cadet Prototype was successfully built, project fell through in its last stages and never saw production. However, Earle MacPherson joined Ford subsequent to this and saw through the implementation his MacPherson struts on the Ford Verdette, Consul and Zephyr. The Verdette, in 1949, became the first ever production car with MacPherson struts.

MacPherson Strut Design Features

The initial design of the Strut design drew inspiration, most probably, from Aircraft Landing gear strut design and some designs by FIAT from the mid-1920’s. 

The lower arm is a triangular metallic structure that constrains the wheel system in both the lateral as well as the longitudinal directions. The lower arm mounts onto 2 points on the front subframe of the chassis via 2 Rubber bushings on the inner side. The outer edge of the control arm has a ball-joint that connects to the bottom of the wheel-hub carrier.

The bottom tube of the shock absorber strut has 2 welded legs with bolt holes through which the strut gets rigidly fixed to the upper part of the wheel hub carrier. 

The Upper mount of the Strut mounts onto a cavity in the body shell of the vehicle (called the ‘Shock Tower’) on either side of the engine bay. The mounting contains a heavy-duty bearing that allows the strut to rotate and, since the wheel-hub carrier is fixed to the strut, rotates the wheel hub as well in the process. 

The line from the center of the top mount bearing of the strut to the Lower Ball-joint on the lower control arm gives the steering axis inclination.

The strut is a coilover that has the upper and lower coil spring mounts within, and the shock absorber at the center. Pre-assembly of the coil spring assembly onto the strut is very simple and can be done with minimal equipment. Due to its simplicity of design and ease of manufacture, the MacPherson Strut has become a constant with almost every economy car manufactured around the world.

Many modern versions replace the lower control arm with a wishbone. An anti-roll bar is optional, but is there on almost every car that has a MacPherson strut front because it helps reduce the spring stiffness in the interest of ride comfort. The anti-roll bar is usually ball-jointed to the strut with a drop link in between.

Advantages of the MacPherson strut

  • The biggest advantage of the MacPherson Strut was space. The MacPherson Strut’s top mount occupied less of space, that would have otherwise been occupied by the engine bay
  • Another advantage was the reduction in the total number of major suspension components, namely the Upper Wishbone and the Upper Ball Joint
  • With the elimination of the Wishbone and Ball-joint came the reduction in the cost associated with replacement due to wear out issues of the upper control arm and the upper ball-joint
  • The MacPherson Strut design allows easy tuning of the suspension parameters for optimizing ride and handling performance.
  • In some vehicles, a MacPherson Strut is used in rear independent suspension systems as well. This approach would eliminate the need for an upper control arm and thus result in a packaging advantage

What vehicles use a MacPherson strut?

  • In the initial years of the post-war era, the MacPherson strut was used primarily in compact cars where overall size vs. packaging space was the challenge.
  • In the later era, as cars moved from the body-on-chassis approach to the more compact and efficient Monocoque design, the MacPherson struts were the preferred choice since 
    • Monocoques provided higher rigidity between the strut mounting points,  
    • MacPherson struts were more cost-efficient, and
    • MacPherson struts help in maximizing the packaging space advantage that monocoque design offers
  • Nowadays, apart from only compact hatchbacks, the MacPherson strut has seen more widespread application in mid-size sedans as well
  • Over the years, the MacPherson struts application has only widened across the spectrum of car categories. Between 80-90% of all passenger cars sold worldwide have a MacPherson strut suspension.
  • Contrary to the general impression that the MacPherson strut is a compromised design meant for economy class, several performance oriented cars have MacPherson struts on them. 
  • A few of the notable examples of performance car models with MacPherson struts are the Porsche 911 GT3, the Audi A4, the Ford Mustang GT350 and BMW 1,2, and 3-series.

Is a MacPherson strut a shock absorber?

Yes, a MacPherson strut is a shock absorber,  meaning, it deos the work of Damping road vibrations that come in through the tires. The MacPherson Strut is a Coilover, which means that the shock absorber and spring are both mounted within a load bearing column called a Strut with spring mounts inbuilt. Apart from being a Shock Absorber, the MacPherson also performs two other functions of-

  • Absorbing road loads through the coilover spring
  • Providing a Steering axis to the knuckle through the Strut Top mount bearing

Conclusion

In this brief article, we have discussed the MacPherson Strut  suspension, its design features, advantages and applications.

Leave a Comment