The Wishbone Suspension
In this brief article, we will discuss the Wishbone suspension, its design features, advantages and applications.
What is a Wishbone Suspension?
By biological definition, A “Wishbone” is a forked bone found in birds and is formed by the fusion of the two pink clavicles. The name is derived from an ancient Etruscan tradition, wherein the collar-bone or ‘Wishbone’ was preserved as it was considered a sign or good luck to make your wish come true.
Because of the shape similarity, the suspension Lower Control arm got its name. The “Wishbone suspension” is a commonly used terminology that refers to the ‘Double Wishbone’ Suspension, also known as “Double A-ams” and “Short-Long arm” (SLA) Suspension. So, if anyone referred to a suspension setup as ‘Wishbone Suspension’, what they actually meant was “Double Wishbone suspension’.
The Wishbone’s design is intended to move the wheel with its rotational axis almost kept parallel to the chassis floor. The 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 Wishbone Suspension
The 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 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. In 1951, the Ford Consul and the Ford Zephyr, got the world’s first mass-produced MacPherson strut suspensions.
Wishbone Suspension Design Features
The 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 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 wishbone setup also mounts between the lower control arm and the Chassis/Body.
A 5-Link suspension, that is more common with high end cars like the Audi A6, A8, Mercedes Benz E-Class, S-Class and the like, is actually a slight variation based on the wishbone’s basic design. Here, basically, the Wishbone is split into two control rods that are attached to the Knuckle at two pivot points instead of one-point. The two-pivot approach allows the steering axis to move further outwards and achieve negative scrub radius.
What is the difference between MacPherson Strut and Wishbone suspension
The major differences between the Wishbone Suspension and the MacPherson Strut suspension are:
- In essence, the Lower control Arm or ‘A-arm’ of the MacPherson Strut suspension and the knuckle are the same in a ‘ Wishbone’ setup. The differences lie in the rest of the components.
- The Upper Control Arm of the wishbone front suspension is eliminated
- Early forms of wishbone has the spring and damper as two separate components, whereas in the MacPherson strut setup, these are combined into a strut
- The Upper Control arm is replaced by a Shock absorber Coil-over Strut with a strengthened bottom mount that is securely bolted on to the Knuckle
- The wishbone’s Upper Ball-joint is replaced with a bearing at the Coil-over Strut’s top Chassis mount. This bearing, along with the lower arm ball-joint does the work of steering, in the case of the MacPherson strut suspension.
Advantages of the Wishbone suspension
- The Wishbone design allows easy tuning of the suspension parameters for optimizing ride and handling performance.
- One of the biggest advantages in the wishbone is the negative Camber gain characteristic all throughout from full bump to full rebound. This is an important characteristic to get the maximum grip out of the outside tires during extreme cornering.
- In some vehicles, a 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 Wishbone Suspension?
Some well-known and high selling cars had or still have 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
Is the wishbone suspension better than the MacPherson strut?
- The 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 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 wishbone throughout the vertical wheel travel range is not achievable in the case of a MacPherson strut
In this brief article, we have discussed the Wishbone suspension, its design features, advantages and applications.