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4WD SUSPENSION
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Leaf Springs
Leaf springs date back to the days of the horse and carriage, and are found on 4WDs with live axles. They are made up of a pack (several layers) of spring steel clamped to the axle with U-bolts at the centre and connected to the chassis rail either end. The leaf pack can be made heavier or lighter duty by changing the number, thickness and width of each leaf within the pack. More or less camber can be built in to adjust ride height. A combination of the above can be utilised to manufacture a heavy-duty suspension intended for carting heavy loads or highly flexible (light-duty) to maximise articulation for off-road use. The upper leaves, which run the full length of the spring, are thinner and therefore softer than the shorter, thicker leaves below. As more weight is added to the vehicle, the softer top leaves flatten, which brings the stiffer lower leaves into action, increasing the springs’ stiffness. In other words, they are variable-rate springs.
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GOOD POINTS
Leaf springs can vary their load-carrying capabilities depending on the number of leaves the weight is resting on. Suspension designers use leaf springs on the rear of many current-model 4WDs, especially those expected to carry heavy loads. The truth is that leaf springs are still superior over other spring types. Their ability to carry varying payloads while still providing relatively good wheel travel and reasonable ride quality is why so many utes still use them. Another benefit of leaf springs is their ability to locate the axle housing to the chassis. This saves the need for extra locating linkages, leaving more room under the rear floor to fit larger fuel tanks. Leaf springs are fairly strong, but if they break, they are relatively easy to repair in the bush and are easily modified.
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BAD POINTS
Leaf springs are heavy and bulky. Their sheer size limits steering lock and makes for wide turning circles when fitted to front suspensions. The separate leaves slide relative to one another as the spring moves up and down through its travel, creating friction between each of its leaves. This inter-leaf friction provides a degree of self-damping because it resists the spring’s ability to move freely. This feature, combined with their variable rate, makes it difficult to finetune shock absorbers to control the spring movements and has led to leaf springs being phased out of use in most modern front suspensions where handling is more critical. Leaf springs resist sideways movement of the axle, which contributes to their poor ability in allowing the axle to articulate. While this reduces the need for a swaybar to limit body roll, it means that as one front wheel rises up into the wheel arch, the front wheel opposite is not as free to move downwards. In order for a leaf spring to maximise articulation, the leaf pack should contain fewer and thinner leaves, and the rubber spring eye bushes must flex. It is not wise to replace the original rubber spring eye bushes with the less-flexible polyurethane alternatives because they compound this lack of articulation. Leaf springs locate the axle but still don’t control its lateral movement as well as multi-link coil-sprung systems do.
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