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4WD SUSPENSION
Page 6 of 7
Macpherson Struts
Only used on independent suspensions, MacPherson struts are commonly used in road cars. Struts help locate the wheel. A strut is made up of a coil spring sitting over a shock absorber inside a strut leg, the bottom of which bolts to the knuckle (hub) in two places. The top of the strut bolts to the strut tower in the chassis via a roller bearing and rubber bush. In this system, the strut takes the place of the upper wishbone and bears some of the side loads applied to the wheel and tyre. Struts are not used on 4WDs with separate chassis because the body shell panels are not strong enough to support the strut top mount. Therefore, they are commonly fitted to monocoque, soft-roader AWDs.
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1) Spring seat welded to strut leg
2) Coil spring mounted over the shock
3) Rubber dust cover protecting shock absorber
4) The lower wishbone mounts to the sub-frame with a rubber bush inside an aluminium block
5) These panels can pull apart at the seams after hard landings
GOOD POINTS
MacPherson struts combine the spring, shock and linkage all in one unit. They save 4WD makers money because it is cheaper to manufacture this type of suspension. On smooth and rough roads, they provide good handling and keep the tyre sitting reasonably flat on the road.
BAD POINTS
Struts perform several duties in locating, springing and damping the wheel. The side loads they endure create friction in the bearings between the shock absorber shaft and the shock body. This friction, referred to as Ôstiction’, affects their ability to dampen the spring and creates heat and wear. Struts can bend or break when a wheel suffers a side-on impact or heavy landing, and the rubber top mounts and surrounding body shell can crack and pull apart at the seams after heavy use. Like double-wishbone suspensions, they haven’t got the wheel travel of live-axle 4WDs and never will.
