Why Is Double

Jan 17, 2024

There are reasons why it's seen as an ideal, but you shouldn't dismiss MacPherson struts.

There are a lot of accepted truths in the automotive world. One is that where performance is concerned, double-wishbone suspension is superior. Most race cars use double wishbones and a lot of the best road cars do too. Yet, if you were to draw up a list of the best-handling cars of all time, you'd find that many use MacPherson struts. All 911s but the newest GT3 and GT3 RS; every Boxster and Cayman; all BMW M3s; the new Honda Civic Type R; Cadillac's Blackwing siblings; the Toyota GR86/Subaru BRZ; the list goes on.

It was our recent comparison test between the Mazda Miata and the new GR86 that got me thinking about this. While a car is so much more than its spec sheet, I do like taking a good look at the mechanical makeup of a car to try and explain why it feels the way it does. In my mind, the double wishbones at the front of the Miata make it a more "pure" sports car than the GR86, with its MacPherson strut front (a consequence of its distant mass-market-car roots). Yet, the GR86 has excellent handling and proved to be a very worthy adversary to the Miata. Some digging was required.

To understand why a double-wishbone setup is held up as some kind of platonic ideal, let's start with the basics of independent suspension. Terry Satchell, a former engineer for both Penske racing and GM, defines it well in Race Car Vehicle Dynamics, one of the best books on the subject. "For an independent suspension, be it front or rear, the assemblage of control arms is intended to control the wheel motion relative to the car body in a single prescribed path," he wrote. The job of an independent suspension is to control—or as Satchell puts it, "severely" limit—five directions of tire motion: Bump/jounce, rebound/droop, lateral, longitudinal, and camber. To control each, you need a link of some sort, and a suspension wishbone is essentially just two connected links. Two wishbones plus a tie rod is five links.

In the early days of independent suspension, auto engineers employed control arms of equal length, but soon discovered this led to excessive tire scrub across the road surface, as the track width changes as the body of the car rolls in cornering. Also in cornering, the outside tire gains camber and the inside tire loses camber….which is the exact opposite of what you want. All of this results in less grip, and using a longer lower arm and a shorter upper arm solves both of these problems. With this so-called short-long arm suspension an (SLA) suspension, the track width remains stable in cornering, while the outside tire gains negative camber and the inside tire either stays neutral—tread parallel to road surface—or gains some positive camber. This is why you often see race cars running a lot of negative camber. When the car rolls onto the outside tire, you get a wider contact patch, resulting in more grip.

The problem with a double wishbone layout is that while it's a dream for a driver, it's a nightmare for a major car company. There are so many parts to manufacture, and they take up a great deal of room. They are hard to package on any car that prioritizes cabin space, double hard on a compact car, triple hard on a compact car with front-wheel drive. A double-wishbone might be ideal, but it comes at a cost.

If I've learned anything in my near 10 years of writing about cars, it's that making cars is expensive. Automakers are always looking to save a bit of money wherever possible, and thus, a simple, cheaper solution is almost always preferable. Earle S. MacPherson's simple, cheap strut suspension—which used a spring-damper unit to locate the front wheels, doing away with the need for an upper arm—proved to be revolutionary. While working at GM in the Forties, he developed a version of what became the MacPherson strut for the Chevrolet Cadet, a small car that was canceled after a feared post-War recession never materialized. MacPherson took his talents to Ford in the Fifties, and soon, the automaker started using MacPherson struts on many of its European models. Getting rid of upper arms helps automakers cut costs, and brings packaging advantages. The use of a strut makes the suspension package taller than a double-wishbone setup, but much narrower above the lower arm.

By the Sixties, many European cars adopted MacPherson strut front suspension while American cars followed in the decades to come. (MacPherson struts don't really work for body-on-frame cars, so it wasn't until the Seventies, with gas crises forcing American automakers to build smaller, unibody cars that they began to ditch double-wishbone suspension.) For many automakers, the choice to go with MacPherson struts over double wishbones was obvious. It does the same basic job, for less money. Additionally in front-wheel drive cars, the space above the lower arm also makes packaging driveshafts much easier.

A mainstream car company like Toyota, seeking to bring a niche sports car to market for as little cost as possible is going to look for any way it can save money on its design. Why pay to engineer an exotic double-wishbone setup when conventional struts would do? For the GR86, with its wide flat engine pushing out the frame rails to the edges of the engine bay, the tight packaging a strut provides certainly can't hurt its case. However, in simple terms of maximizing tire grip, the MacPherson strut is inherently compromised.

"What do you do about a strut? It's not a link. How do you extend it to construct an instant center and a roll center?" says Doug Milliken, who along with his late father William authored Race Car Vehicle Dynamics, in an interview with Road & Track. "Well, what you do is, perpendicular to the strut axis, you create a link, which has to be infinitely long, and that is the equivalent link for the strut. What that gets you is an extremely long upper link, which is not the way to do camber compensation at all."

In cornering, you simply don't get as much of the good camber behavior of a double wishbone. Milliken adds that strut suspensions also tend to have a lot of friction within the system, another consequence of not having upper links. "Unless you make a strut with linear ball bearings in it instead of the normal bushings, you've always got some amount of friction for any kind of ride motion," he says. "Not only do you have a ball joint at the bottom for the lower link, but you've also got this slider, which is not loaded along its axis all the time. It might be under certain circumstances, but normally, it's got side load on it, especially when you're cornering." This translates to more load variation on the tire, and tires perform at their best when held at (or more likely, near) constant loads.

Yet, what we're talking about here is extreme circumstances. In normal everyday driving, these sorts of things don't matter nearly as much as when you're tearing up your favorite backroad or when you're lapping your favorite road course. These things also matter in motorsport, which is why the only race cars you'll see with struts are based on street cars, and even then the MacPherson-strutted 911 has long used double-wishbones in racing variants.

Despite the inherent limitations of MacPherson struts, good engineering means they aren't an anathema to good handling. "At Toyota, [the engineers] probably played with different strut angles and strut geometry, trying to get a best average to get the loads as axial as they could," Milliken suspects of the GR86. "The strut axis, for example, often isn't the steering axis—it's slightly inclined. Then the spring may be inclined relative to the strut, to take some of the spring load and point it down at the contact patch rather than pointing it along the axis of the strut, which puts bending on the strut back from the tire forces.

"These are all little details that matter, and it's the sum total of them that tell you how good the thing is going to be overall. I'm sure, for example, that someone or a group—it's almost certainly a group of people—who really work on a strut car, will probably get better performance out of that strut than a thrown together SLA. I think there's room in there for overlap."

My learnings are a fun reflection on both the Miata and GR86. The Miata is great in part because Mazda went through the trouble of giving the car a bespoke double-wishbone front suspension; the GR86 is great because Toyota (and Subaru) worked really hard to make something amazing from its strut suspension. Yes, in terms of getting the most grip out of a tire with the fewest compromises, double wishbones are king, but the goal of a performance road car is not maximum grip all the time. Especially so in something like the GR86, which prides itself on having approachable limits that allow more drivers to play right in that fun zone between grip and slip.

Even though we're dealing with physics in the car world, things are never black and white, never binary. Saying, "double wishbones are better" isn't wrong, but it's a vast oversimplification.

A car enthusiast since childhood, Chris Perkins is Road & Track's engineering nerd and Porsche apologist. He joined the staff in 2016 and no one has figured out a way to fire him since. He street-parks a Porsche Boxster in Brooklyn, New York, much to the horror of everyone who sees the car, not least the author himself. He also insists he's not a convertible person, despite owning three.

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