What Is a MacPherson Strut?

What Is a MacPherson Strut?

Last Updated July 14, 2020 | Meghan Drummond

The MacPherson strut was originally developed by Earle S. MacPherson in 1945. This simple suspension system is still used for many modern vehicles, including the Ford Mustang. There are many variations on the classic MacPherson strut design, but the fundamentals remain the same.

History of the MacPherson Strut

Earle MacPherson worked at numerous car makers before landing at General Motors in 1934. There, he was promoted to chief engineer for passenger car design.

As soldiers returned from World War II, GM predicted there would be an increased need for affordable and comfortable vehicles. This factored into many design choices. MacPherson, himself a veteran, worked hard to develop an inexpensive and simple front suspension. GM used an early prototype on a pre-production vehicle, but the project was canceled.

MacPherson, and his strut suspension, went to Ford. The first vehicles to ever feature a MacPherson suspension were the Ford Consul and Zephyr.

The beauty of MacPherson’s strut design is that it’s simple and inexpensive. At the time, the primary suspension system in use for cars was a straight axle front suspension, which used leaf springs.

The straight axle suspension prevented the front wheels of a vehicle from moving independently of one another. MacPherson struts allowed each front wheel to move independently. This offered more comfort than a straight axle system would. It also preserved suspension geometry, which is important for handling.

What Is a Strut?

A strut, like a shock, exists to dampen the movement of a vehicle’s coil spring. This helps to keep a ride from being too bouncy.

Struts are different from shocks in several key ways though. The first is that unlike a shock, a strut is a part of a car’s structure. While heavier vehicles, like large trucks, use shocks, most lighter cars are better suited for struts.

How Does a MacPherson Strut Work?

MacPherson strut assemblies can theoretically be used for all four wheels, but usually they’re only used for the front suspension of a car. The MacPherson strut, in effect, does several important things. It provides a point of stability, creates a steering pivot, and, of course, absorbs shocks.

A diagram of a MacPherson Strut with parts labeled

MacPherson Strut Components

To better understand how the MacPherson strut accomplishes these goals, it helps to understand each component in the strut assembly. Each component of a MacPherson strut has a purpose.

Shock Absorber

This is the part that people usually think of when they think “strut.” It’s also the part people are referring to if they tell you the strut needs to be replaced. Most struts use pressurized gas to absorb impacts.

In a MacPherson strut system, the strut is also where caster and camber angles can be adjusted. With performance struts, you can even adjust your damping rate.

Coil Springs

Springs help to absorb all the bumps in the road. If you only had springs, you’d have a much bumpier ride. Instead, springs work together with struts to give a smoother ride.

Struts and springs are sometimes replaced with coilovers, which combine the two.

Lower Control Arms

The MacPherson system only uses a lower control arm. This is part of how it preserves space. The control arm connects the wheel to the unibody and allows it to travel up and down thanks to a ball joint.

Steering Linkage

This serves as a way to steer the front wheels. One of the major advantages of the MacPherson strut system is the wide range of steering angles it allows for. This makes it a considerably better suspension system for drifting and other performance driving.

Common MacPherson Strut Variations

Though all MacPherson struts share a basic design and serve the same function, there are some variations. These variations have evolved over the years to meet different needs.

Control Arms

The MacPherson strut system will always only have a lower control arm (an upper control arm would make it a double wishbone suspension). But the shape of that control arm may vary. Most control arms will either be “A”-shaped or “L” shaped. Manufacturers tend to select one or the other depending on other design features of the car.

Two control arms, one L-shaped and one A-shaped

Sway Bars

One of the major advantages of MacPherson struts is that they allow the front wheels to move independently of one another. There are some situations where that can be a weakness though, and having a more solid connection between the two wheels is advantageous. A sway bar connects the left and right sides of the front suspension.

Sway bars help to provide stability and prevent body roll. While many vehicles that have a MacPherson strut system will come with a sway bar, they can also be added later.

Strut Tower Brace

A strut tower brace basically does the same thing as a sway bar, only by connecting the tops of struts. One of the great things about this modification is that it’s easy, and can be done from underneath the hood.

Some performance vehicles that have MacPherson strut systems will include a strut tower brace. The Bullitt Mustang is just one example of this.

Advantages and Disadvantages

MacPherson Strut Pros and Cons
ProsCons
Compact Not as comfortable as a double wishbone suspension
Allows for extreme steering angles Allows for more body roll
Better for collisions Only works for unibody designs

Double-wishbone suspension systems offer many of the same advantages as a MacPherson strut. They also offer a slightly more comfortable ride. But double wishbone suspensions require an upper control arm, which takes up space. They also tend to be more expensive than MacPherson struts.

Double wishbone suspension and MacPherson suspension side-by-side

MacPherson struts are often adapted by manufacturers looking to improve their vehicle’s front-end crash ratings. Because the Macpherson strut is so compact it allows for significantly more overlap space in the event of a crash.

Having additional space also allows for more cargo storage, a greater range of steering angles, and more room for modifications.

Because the MacPherson strut system is so simple, repairs are also relatively straightforward. It’s no wonder that so many modern vehicles continue to use a suspension first invented in the 1940s.

Image Credit: Creative Commons

What Is a MacPherson Strut?

Many modern cars use a MacPherson strut suspension system. Though this design was first implemented in the 1950s, it's still in use today. This is partially due to its simple and effective design, which is easy to work on and modify.