The MacPherson strut: How modern suspension is rooted in 1940s tech

MacPherson Strut diagram

Suspension technology in automobiles has evolved at a glacial pace. Many modern cars can trace the technological roots of their suspensions back decades. Once such example is the MacPherson strut front suspension, which is near ubiquitous on front-wheel-drive vehicles (and the Porsche 911). But who is this MacPherson guy, how does his design work, and why is it still around?

Thank Earle Steel MacPherson, born July 6, 1891 outside Chicago, Illinois for this enduring bit of technology. After receiving his mechanical engineering degree from the University of Illinois, MacPherson chased his interest and moved east to the budding automotive center of Detroit, Michigan.

MacPherson served in the U.S. military during World War I. Upon his return, he worked at the Liberty Motor Car Company before moving to the Hupp Motor Car Company.  In 1934, serious conflict among stakeholders at Hupp drove MacPherson to seek a more stable gig, and his knowledge and experience helped him land a position at GM.

Earle MacPherson
1951 Ford Consul

Over the next decade, MacPherson worked up the corporate ladder to the position of Chief Engineer of Passenger Car and Truck Design. Chevrolet worried that after the war ended, the auto market might tank and GM needed an economy car ready for production. In 1945, MacPherson headed the design team for this advanced unibody “Light Car” concept, for which he personally tackled the new front suspension. Later dubbed the Cadet, the vehicle never reached production, but MacPherson’s innovative suspension would forever leave its mark on the automotive world.

His design incorporated a vertical strut attached to the wheel hub. The strut was composed of a coil spring with a tubular shock through the center, which acted as the upper anchor point for the wheel hub. The design differed radically from the standard straight axle front suspension, which often utilized leaf springs and knee-action shocks that required significantly more space and did not allow the front wheels to travel up and down independently.

With the strut acting as the upper anchor point, a lower suspension arm locates the bottom of the steering knuckle, and a steering link turns the knuckle. With no upper A-arm or spring in the middle of the operation, there is plenty of space for driveshafts, which helped encourage the proliferation of front-wheel and all-wheel-drive platforms. The geometry of MacPherson strut system is also very stable through its travel and easily adjustable. With fewer parts, this system gives very similar performance to more complicated systems. With minimal modifications, it can also give very high steering angles—desirable among performance and drift drivers.

MacPherson filed a patent application for the new suspension design in 1947. By 1949 he filed another featuring a more refined iteration of his design, which has held up to the test of time; many modern cars employ the MacPherson strut suspension.

What else is so great about this 70-year-old suspension design that it’s still in use? Low part count, simplicity, lighter weight, and compact packaging. All those details make it cheap to build, as well—an easier pill to swallow for the corporate bean counters.

Still, the overall package is does require unibody construction, as the top of the strut needs to anchor to part of the chassis. A body-on-frame vehicle does not have structure for the top of the strut to mount to, and the body itself can’t handle the suspension forces. Because modern cars are constructed almost exclusively in unibody form, and taller engines with dual overhead camshafts are now the norm, this time-proven solution is the suspension engineer’s longtime best friend.

The MacPherson strut got it right 70 years ago. No reason to fix what ain’t broke.