Leaf Springs for 4WD
Leaf springs were originally termed carriage or laminated springs and their history predates the invention of the automobile.
The use of a leaf for springing can be traced back to medieval times and many detractors of leaf springs would suggest that is where they belong, pointing to the benefits of newer technologies like coil springs and independent suspension.
Load Carrying Ability
There are millions of 4WD vehicles on the road still equipped with leaf springs and many still being manufactured with leaf springs as standard equipment.
This type of technology is hard to better when it comes to load carrying ability. The load in a leaf set-up is spread over a greater distance than the single point of coil springs or airbag suspension and by utilizing multiple leaves the suspension becomes progressive as more weight is applied.
Most leaf springs sold today are available as multi-leaf or mono-leaf (parabolic) semi elliptical units. Older vintage-era vehicles may have quarter-elliptical springs fitted. Conventional multi-leaf springs consist of one main leaf that bolts to the frame of the vehicle. Beneath it are a series of progressively smaller and lighter springs, the spring pack is often complimented at the base by one or more thicker ‘helper leaves.’
As load or compression is applied to the leaves they flatten and slide against each other - each leaf coming into play as more force is introduced. Each leaf supports and is supported by the leaf above and beneath it.
Old wisdom suggests lubricating the individual leaves to reduce friction and facilitate the ‘sliding’ between the leaves, however petroleum based lubricants can prove degenerative to newer steel compounds and should be avoided.
The profile of a parabolic leaf spring is similar to the ‘pack’ of leaves in a multi-leaf unit. The steel used in the leaf is tapered with the thickest section at the centre of the leaf. Very simply, this means a parabolic spring will have a similar function to that of the multi-leaf spring in providing progression as load is applied.
Parabolic springs have a more supple characteristic than multi-leaf springs and may provide a softer ride. The use of less steel in the manufacture means they are lighter. The trade off comes in load carrying capacity and a big, heavy 4x4 carrying lot’s of weight may be better suited to a multi-leaf set up.
All springs have a finite life and if pushed to the limit will eventually break. The composition of multi-leaf springs means that if a leaf fails and breaks the remaining pack can support it. A single parabolic leaf cannot offer the same supportive feature even though it is possible for a single leaf to accomplish the task of suspension. Parabolic springs are often sold as a unit containing two leaves for safety
The suspension of a conventional vehicle consists of two primary components, springs and shock absorbers. While most people are aware that shock absorbers wear and require replacing many people are unaware that a huge performance gain can be achieved by replacing worn springs
All springs suffer continual flexing and straightening and like any metal they will lose their resistive characteristic. Because it happens over time the sensation of a loss of suppleness or ride height is not obvious. Rate of wear will vary depending on the type of spring, the loads it has carried and the terrain it has covered.
Indicators Of Worn Suspension Componentry
- Reduced ground clearance.
- Nose diving under brakes.
- Excessive body roll when cornering and turning.
- Rear end drags or bottoms out when loaded.
- Unstable towing characteristics.
- A Hard, rough ride.
- Slow, unresponsive rebound over bumps and holes.
Many people fit a suspension lift-kit to their 4WD to gain more ground clearance and then re-fit the tired old springs. If you are continually carrying excess weight and you suspect your springs may be due for some attention then it is possible to fit new springs with an upgraded capacity. New springs can often be specified to give up to 2 inches of lift to the ride height of the vehicle.
Spring Rate or 'Firmness'
Selecting the correct spring rate is a balance between ride comfort and loaded weight. Be mindful that 4x4 suspension configured for heavy loads will be quite a firm ride when the vehicle is unladen.
Because most weight is carried towards the rear of a vehicle, especially in utilities, the rear suspension is generally the first area to show wear and demand attention. Care should be taken not to unbalance the ride characteristics of the vehicle by fitting heavy duty suspension to the rear alone and ignoring the front components. While the rear of a 4WD does carry the bulk of the weight, the load is still shared by the front and the 4WD’s overall suspension needs to work as a unit.
The springs of a vehicle are the foundation of the suspension system and components like shock absorbers and torsion rods are ancillary to the springs whether they are leaf or coil type.
Any suspension upgrade or modification should begin with the springs and progress from there through the remaining suspension components
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