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TRAIL SAVVY: LEAF vs COIL SPRINGS

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The leaf versus coil debate has been going on since Land Cruiser 70 Series drivers got jealous of the comfy ride in coil-sprung Land Rover Defenders. We joke, but both leaves and coils have a range of benefits and drawbacks that suit different vehicles and needs. How you value these attributes could affect your next choice of vehicle…

The task at hand
Your vehicle’s suspension system is possibly the hardest-working, least understood, most abused and under-maintained aspect of your rig. The average bakkie is expected to safely carry a ton, but remain comfortable and stable when empty on the school run. Suspension needs to be flexible enough over rough terrain to keep all four wheels planted while in an axle twister, but at the same time contain body roll when speeding along a twisty mountain pass. The fact is, a relatively simple arrangement of springs and shock absorbers must cope with a huge variety of situations while still being reliable, safe, and cheap to produce.
These are among the reasons compromises must be made by manufacturers, because no standard suspension system can be perfect for everybody. It’s also why there is such a thriving aftermarket for upgraded suspensions. But, dialling back, why do some manufacturers use leaves and others coils? Why are most modern bakkies fitted with coils in the front but retain leaves in the back? And what are the fundamentals of these systems?

LEAF SPRINGS
An incredibly simple solution that’s been doing duty since the first sprung wheeled vehicle appeared, is that leaf springs consist of a stack of semi-elliptical (curved) spring steel plates. These are connected at their centre to an axle and suspended from a vehicle’s chassis by shackle hangers attached to eyes on each side of the longest leaf. This arrangement allows the spring pack to flex. As the axle moves up and down, the flexing of the leaves absorbs bumps in the road, so this movement is not directly transferred to the vehicle’s passenger cell. Adding more (or heftier) leaves increases load-carrying capacity, while a lighter-duty or thinner leaf pack will provide a softer side – but may well sag under load.

Due to their simplicity, leaf springs have been used for hundreds of years in everything from the first ox-carts to the latest Toyota Hilux. As technology has progressed, leaf springs are more comfortable due to advances in steel types and optimised spring rates, plus far better shock absorbers – but the basic principle of their design has not changed. Today, the only modern one-ton bakkies on the SA market not to use leaf springs are the Ford Ranger Raptor and the Nissan Navara. Why is this?

Pluses
Due to their very basic design, leaf springs are relatively cheap to manufacture out of sheets of spring steel, making them an attractive option for manufacturers. Leaf springs are also very strong for their weight and provide a more stable platform than coils, which need trailing arms and Panhard rods to keep the rear axle in place – all adding weight, complexity and cost.

Thanks to the widely-spaced spring base on the rear axle, with the leaf packs mounted directly under the primary chassis members, there is decent vehicle stability and good control over body roll. The Amarok scores even better on this point, as the leaf packs are mounted on the outside of the chassis rails.

If more weight is likely to be regularly added to a vehicle, extra leaves can easily and cheaply be added to a standard leaf pack. This beefed-up pack will be able to support more weight without sagging. If an individual leaf breaks, it can also be cheaply and easily replaced. Due to the basic anatomy of a leaf spring, they can support relatively large vertical loads, making them ideal for commercial vehicles.

Minuses
Leaves do have their downsides and remain a rather rudimentary system of shock absorption. They do not exhibit particularly good control over small, high-frequency bumps. This problem is exacerbated if more leaves are added, making leaf sprung vehicles handle poorly on loose and rutted surfaces, especially when unloaded.

Another factor is the friction caused by the various leaves in the leaf pack rubbing against each other. This friction gets worse as spring packs age and more grit and dust gets caught between the leaves – often causing annoying squeaking. To minimise this, leaf packs should be regularly cleaned and lubricated and, if necessary, stripped down to have their inter-leaf liners replaced.

In extreme scenarios the shackles that connect the leaf spring to the chassis can move past their designated axis of movement, causing the leaf to get stuck in an inverted position. This is what bump stops seek to avoid.

On powerful modified vehicles, heavy acceleration can cause ‘axle wrap’, when the entire rear axle twists forward on its axis, potentially breaking the universal joints that connect the prop shaft to the differential and/or transmission. Neither of the above problems occurs with coil springs.

COIL SPRINGS
Coil spring suspension has been around in four-wheel-drive vehicles at least since the Unimog was first manufactured in 1948, and the Stey-Puch Haflinger not much later in 1959. Here a coils-all-round formula was key to the exceptional off-road agility of these vehicles. It wasn’t until the Seventies that coils became commonplace in less commercially-oriented vehicles, pioneered by the Range Rover Classic. In this instance, coils offered exceptional ride comfort and refinement – a formula copied across the SUV spectrum since then.

Coils are much more compact than leaf springs and therefore work effectively in tight spaces, like inside the wheel arches in a vehicle with independent front suspension. Where high degrees of comfort and refinement are the brief, then they work well in the rear as well, though the strut towers do present some additional packaging challenges.

Conventional coils are made of wound sprung steel and provide a mostly linear spring rate as they are compressed or stretched. Progressive rate coils are also available, with their closer windings towards the base offering higher resistance to deal with heavier loads and big hits, while the looser-wound top section deals with small-bump compliance in greater comfort.

Unlike leaf springs that act as their own stabilisers in terms of fore and aft movement of the axle, coils need to be located. In a solid-axle set-up, trailing arms linking the chassis and the axle tame front/rear movement, while side-to-side movement is typically held in check with a Panhard rod. All this adds complexity and cost.

Pluses

The soft ride coils provide is the most obvious benefit, but when off-roading another advantage is the ability of coils to provide large amounts of wheel articulation, keeping a 4×4’s wheels planted on rough ground. Coils can stretch far more than a leaf spring pack, and even dislocate from an axle in extreme situations with no ill effects.
Coil setups (on independent suspension at least) are also far lighter than leaf spring designs. Because coils don’t physically rub against anything except their top and bottom location points, they generally require little or no maintenance, so last longer and don’t squeak or rattle. With the right tools, they are also easy to swap out or replace.

Minuses

The extra components needed to locate a coil spring setup makes them more expensive to produce.
They are less able to take a heavy load, and when overloaded or subjected to extreme 4×4 abuse, they are more likely to fail than a set of leaf springs.
Coils allow the vehicle to sag under a heavy load because of their linear spring rate, whereas the heavier-duty leaves on a leaf spring pack will stop this tendency up to a higher load level. Of course, the aftermarket does offer progressive-rate coils and those able to take a heavier load from the outset.

THE PRO VIEW
Ironman 4×4 director and suspension expert Mic van Zyl weighed in with these points…

1. Coil springs tend to be more comfortable than leaf springs because there is no internal friction in the compression of the coil spring. When leaf springs are compressed, the leaves rub against each other and the inter-leaf friction adds to the inherent spring rate of the spring pack. Inter-leaf rust can exacerbate this.

2. Leaf springs found on the rear of vehicles, typically pickups, have two distinct stages. The main pack does the work of carrying the vehicle weight and light loads with a degree of comfort, and the second stage or overload blades assist with heavy to GVM loads. While this is sometimes replicated in coil springs with progressive rate designs, they are typically not as effective in carrying loads where leaf springs are more effective. A case in point is the Nissan D23 Navara. Despite its comparable specifications, it just does not do as well with loads as other leaf-sprung pickups. In Australia, you can have either, and the leaf spring models are almost always preferred over the coil models by tradesmen.

3. The spring rate of leaf spring packs can be altered by adding or removing blades, which is not ideal but can work. Coils need to be replaced with a different design or an air bellow added inside the coil spring to alter the combined spring rate.