The Double Wishbone Suspension (What is it exactly)
In this brief article, we will discuss the Double Wishbone suspension, its design features, advantages and applications.
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. In 1951, the Ford Consul and the Ford Zephyr, got the world’s first mass-produced MacPherson strut suspensions.
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.
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 Double 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 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.
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
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
In this brief article, we have discussed the Double Wishbone suspension, its design features, advantages and applications.