MacPherson Strut vs. Double Wishbone

In this brief article, we will discuss the designs of MacPherson Strut suspension, and the Double Wishbone Suspension. We will also discuss the advantages of the MacPherson Strut  suspension, and the Double Wishbone Suspension and the differences between these two types of suspension.

What are the differences between the layout of the MacPherson Strut and that of the Double Wishbone suspension

The Basic construction approach of the MacPherson is: 

  • To take the Double-wishbone front suspension and eliminate the Upper Control Arm. 
  • Replace The Upper Control arm with a Shock absorber Coil-over Strut that is securely bolted on to the Knuckle. 
  • Replace the Double-wishbone’s Upper Ball-joint with a bearing at the Coil-over Strut’s top Chassis mount.

Is the double wishbone better than the MacPherson strut?

  • The Double Wishbone certainly has the advantage over the MacPherson strut when it comes to adjustability of scrub radius and Kingpin inclination (KPI). In the case of the MacPherson Strut, the top mount position has to be altered in order to adjust KPI. The double wishbone’s KPI and scrub radius, in contrast, can be adjusted by moving the Upper ball-joint inwards or outwards, keeping the coilover top mount fixed.
  • The Negative Camber-gain characteristic of a Double-wishbone throughout the vertical wheel travel range is not achievable in the case of a MacPherson strut

What is a MacPherson Strut Suspension?

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.

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

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

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

The Double-Wishbone suspension, as the name suggests, consists of 2 wishbones, upper and lower, that are connected to the chassis. The upper and lower wishbones are connected to the Knuckle (or wheel carrier) through an upper and lower ball joint respectively.

The Basic construction approach of the MacPherson Strut is: 

  • To take the Double-wishbone front suspension and eliminate the Upper Control Arm. 
  • Replace The Upper Control arm with a Shock absorber Coil-over Strut that is securely bolted on to the Knuckle. 
  • Replace the Double-wishbone’s Upper Ball-joint with a bearing at the Coil-over Strut’s top Chassis mount.

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.

What is a Double Wishbone Suspension?

The ‘Double Wishbone’ Suspension is also known as “Double A-ams” and “Short-Long arm” (SLA) Suspension. The Double Wishbone’s design is intended to move the wheel with its rotational axis almost kept parallel to the chassis floor. The double wishbone provides the flexibility to fine-tune the wheel Camber, Castor and change in orientation of the steer axis while the wheel moves from bounce to rebound, which are essential parameters that influence the car’s handling.

The History of the Double Wishbone

The Double wishbone suspension first appeared in cars from the pre-world war II era like the Citroën Rosalie and Citroen Traction Avant models (1934). In North America, the Packard Motor Car  first used the Double-Wishbone in the Packard One-Twenty model (1935). 

The MacPherson strut type of suspension, in contrast, found implementation only much later in the Post-world war II era.

Double Wishbone Design Features

The Double-Wishbone suspension, as the name suggests, consists of 2 wishbones, upper and lower, that are connected to the chassis. The upper and lower wishbones are connected to the Knuckle (or wheel carrier) through an upper and lower ball joint respectively.

The most common configuration of a double wishbone is with coilover struts. In the past, however, like in old Ford trucks, the springs and dampers were separately mounted on the lower wishbone.

In a majority of the cars, the Spring lower mount is on the Lower Wishbone and the Spring Upper mount is on the Chassis or Body (depending on whether the car is a body-on-frame or Monocoque type of construction). In many other cases, like with Audi’s, the Spring lower mount attaches to the top of the knuckle.

Similar to the spring, the Damper in a double wishbone setup also mounts between the lower control arm and the Chassis/Body.

Advantages of the Double Wishbone

  • The Double Wishbone design allows easy tuning of the suspension parameters for optimizing ride and handling performance.
  • In the Double wishbone suspension, because of the parallel linkage formed by the upper and lower control arms, it is possible to keep the tire parallel to the car’s vertical line throughout the suspension travel.
  • One of the biggest advantages in the Double wishbone is the ability to control Camber Characteristics of the wheel while it travels up and down by adjusting the lengths of the upper and lower control arms. (Camber is the Angle between the Tire centerline and the line perpendicular to the road at the point of contact)
  • While cornering, if the outside tire moves parallel to the car’s vertical line, the outside tires will tend to have a positive camber to the road. In a double wishbone suspension, it is also possible to have a negative Camber gain characteristic all throughout from full bump to full rebound.( Negative camber means that the top of the tire leans towards the body or inwards. Negative Camber means more tire grip and is a desirable condition. ) This is an important characteristic to get the maximum grip out of the outside tires during extreme cornering especially in performance-oriented cars.
  • In some vehicles, a Double Wishbone is used in rear independent suspension systems as well. This approach is generally adopted in performance-oriented cars, where ride and handling are of great importance.

What vehicles use a Double Wishbone?

Some well-known and high selling cars had or still have double-wishbone front suspension are:

  • Mazda Miata MX-5
  • Honda Accord
  • Toyota LandCruiser, Prado, HiLux
  • Nissan Titan/Armada/Xterra/Frontier/Pathfinder
  • Ford F-Series: F150, F250, F350

Conclusion

In this brief article, we have discussed the designs of MacPherson Strut suspension, and the Double Wishbone Suspension. We have also discussed the advantages of the MacPherson Strut  suspension, and the Double Wishbone Suspension and the differences between these two types of suspension

In case of any questions or queries, please feel free to ask.

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