Of Wishbones And Struts

Our mechanical engineer explains the differences between double wishbones and MacPherson struts.

THERE is no mystery in the “wishbone” suspension being named after part of a bird’s anatomy – a car suspension’s triangular wishbone arm bears a strong resemblance to the furcular (actual name), which is a strut that links a bird’s shoulders. A bird’s wishbone is critical in helping the animal stay airborne, while a car’s wishbone is designed to keep the tyres planted on the ground. Compared to all other suspension setups in the front or rear of a vehicle’s chassis, the double wishbone is technically the most sophisticated.

Some chassis engineers will be quick to argue that the latest multi-link arrangements are superior, but in reality, these are all derivatives of the double wishbone. The classic double wishbone suspension layout is formed by two rigid arms mounted roughly parallel to each other. Each triangular wishbone is mounted at two pivot points on the chassis, with the third pivot point (at the apex) on the wheel hub carrier. Together, they form the lower and upper links. The wishbones are often also referred to as A-arms.

The macpherson strut may not be equal to the double wishbone, but it has served perfectly well in many cars, including the porsche 911.

To this end, the main task of a car’s suspension is to control the wheel movements relative to the vehicle body, so that grip, directional control and driving comfort are maintained at their optimum at all times. To this end, the double wishbone off ers a wide range of geometrical “tuning” to achieve the objectives. Wishbone lengths, pivot axis angles and mounting locations can be tailored for the desired characteristics.

In addition, the wishbone emulates a three-member truss-like shape, making it inherently stiff for positive location of the wheel in the transverse plane (cornering) and longitudinal plane (braking/accelerating). It’s not surprising, then, that Formula One cars continue to employ the classical double wishbone arrangement at both ends. So do most purpose-built racecars. But this does not mean that other types of suspension arrangements are inferior. As a matter of fact, the other more widely used and well-known name in car suspensions is the MacPherson strut.

The name comes from Earle MacPherson, the engineer who designed this relatively simple independent suspension unit. It can be used at the front or rear, with front- or rear-drive. Used in almost every frontwheel- drive car, MacPherson’s strut is made up of one lower arm and a telescopic springdamper unit that not only links the hub carrier to the top of the wheel well, but also provides the steering pivot. The MacPherson strut does not off er the design flexibility of the double wishbone, but its relative simplicity, ease of assembly and low cost are its advantages and also the reasons for its popularity in the global car industry.

The arrangement also occupies less of the engine bay than a double wishbone. The use of MacPherson struts in the rear of the chassis is less common because the telescopic spring-damper unit encroaches into the backseat/boot area. A more compact alternative setup is therefore used in the rear instead. Ford was one of the first automakers to incorporate front MacPherson struts extensively in saloon cars, including the first Escort model. The single lower arm provided the track control, while the anti-roll bar served as the lateral locating link.

The original Ford Escort was one of the earliest massmarket saloons with MacPherson strut front suspension.

Although it provided long suspension travel and hence good ride comfort, wheel location in both planes was heavily dependent on the rubber bushes at the single point where the lower arm was mounted to the chassis, and where the anti-roll bar connected to the trackcontrol arm. The bushes did not last very long, causing wheel misalignment and annoying steering wobble. Both the Mk 1 and Mk 2 Escorts suff ered from this ailment, but it was cheap and easy to fix.

Interestingly, today’s MacPherson strut suspension employs a wishbone or L-shaped link as the lower arm, which provides good, positive longitudinal location. The bushes used at the mounting points are asymmetric in design, which means they are suffi ciently flexible for vibration isolation, yet stiff enough to resist movement if geometric integrity is compromised. In engineering theory, the simple MacPherson strut may not be equal to the more sophisticated double wishbone, but it has served many cars perfectly well, including the Porsche 911.