FORMULA, RIVAL Parabolic Leaf Spring Kits

A parabolic leaf spring kit replaces the rear spring pack along with the hardware that mounts it, rather than leaving half the assembly original.

The springs are the reason to buy, and the hardware is what determines whether the parabolic suspension performs the way it was designed to.

What a Parabolic Leaf Spring Kit Includes

A complete kit supplies the parabolic springs for the axle along with the U-bolts, fixed pins and bushes needed to fit them, and on some applications the shackles as well.

That matters because the fasteners involved are not reusable. U-bolts stretch permanently once torqued into their yield range, and fixed pins have been clamped under load for the life of the old pack.

It matters more on a parabolic kit than on a conventional one. These springs rely on free movement between the leaves, and a worn shackle bush or a tired spring eye bush reintroduces exactly the restriction the design exists to remove.

Replacing one component and leaving the rest means paying for a spring that cannot do what it was built to do.

How Parabolic Leaf Springs Actually Work

The name comes from the shape, and the shape is doing something more specific than it looks.

Each leaf is thicker through the centre and thinner toward the ends, with the thickness tapering along a mathematical curve rather than a straight line. That curve is doing the real work, because it distributes bending stress evenly along the full length of the leaf instead of concentrating it anywhere.

Early attempts at tapered springs used a linear taper and failed for exactly that reason, with stress concentrating at points along the leaf and cracking there.

Because the stress is spread properly, a parabolic leaf can be considerably longer and thinner than a conventional one carrying the same load. Fewer leaves are needed, and a parabolic pack can replace a conventional pack of twelve leaves or more.

The leaves are also spaced so they touch only at the ends and at the centre rather than along their length, with floating brackets holding them in alignment without clamping them together.

Parabolic Against Traditional Multi-Leaf Springs

The difference comes down to friction and what it costs you.

A conventional multi-leaf pack has every leaf in contact with the one above and below along its whole length, and that inter-leaf contact is what separates the two designs. Those surfaces slide against each other as the spring flexes, and that friction resists the movement.

That friction has one genuine benefit, which is self-damping. A multi-leaf spring resists its own oscillation to a degree a coil spring cannot.

The cost of that friction is everything else. The spring resists small movements before it begins to flex at all, which is the harshness people associate with leaf sprung utes. The contact surfaces squeak once they dry out. Water and grit get between the leaves and corrode them from the inside, which is a common failure point. The pack is also considerably heavier than it needs to be.

A parabolic pack removes nearly all of that friction. The spring responds to small inputs the conventional pack simply absorbed, the leaves do not squeak because they are not rubbing, and corrosion between leaves is far less of a problem because there is a gap rather than a contact surface.

Articulation improves at the same time, so the spring flexes further and keeps the wheel on the ground where a stiff pack would lift it.

Why Parabolic Springs Need Better Shocks

This is the honest part and it gets left out of most descriptions.

Losing inter-leaf friction means losing the self-damping that friction was providing. The spring now moves more freely in both directions, and controlling that movement falls entirely to the shock absorbers.

Factory shocks were valved for a spring that damped part of its own movement. Put parabolic springs under them and the vehicle can feel loose or float over undulations, which is not the springs failing but the dampers being asked to do more than they were specified for.

A parabolic kit fitted with appropriate shocks rides considerably better than the conventional pack it replaced. The same kit on tired factory shocks can ride worse.

Budget for the shocks when budgeting for the springs, or at minimum have the existing ones assessed before the springs go in.

Parabolic Spring Weight Saving and Unsprung Mass

A parabolic pack matching a conventional pack for load capacity weighs around 30 percent less.

On a vehicle with leaf springs at both ends, converting can remove a substantial amount of total weight, and almost all of it is unsprung mass sitting below the springs.

Unsprung weight is the weight the suspension has to accelerate and decelerate over every bump. Less of it means the wheel follows the road surface more closely and the shocks have less work to do controlling it.

There is a second benefit on a vehicle running close to its limits. Weight removed from the springs is weight freed up for payload, which matters on anything fitted out for touring or carrying tools.

Military Wraps and the Overload Leaf in a Parabolic Pack

Two design features worth understanding before comparing kits.

A parabolic spring could theoretically work as a single leaf, and most kits use at least two. The lower leaf carries a full military wrap at each eye, where the leaf wraps completely around the eye of the main leaf above it. If the main leaf fails, that wrap contains it and keeps the axle located rather than letting the spring separate.

Many kits also include a free floating overload leaf, which sits below the pack and makes no contact until the suspension is compressed under heavy load. Unloaded, it does nothing and the ride stays soft. Once the load goes on it engages and adds the support required.

That arrangement is how a parabolic pack delivers comfort when empty and capacity when loaded, rather than compromising between the two the way a single rate conventional pack does.

Parabolic Kit Constant Load Ratings and Raised Heights

Parabolic kits are specified by constant load the same way conventional Australian leaf springs are.

Ratings run in bands, commonly up to 300 or 400kg, then 400 to 700kg, then 700kg and above for the heaviest applications. Raised heights of around 40mm to 50mm are typical, with standard height options where no lift is wanted.

The rating refers to permanent load being added rather than the vehicle's weight. A canopy, drawers, a water tank and recovery gear reach the second band faster than most people expect.

Front and rear are rated separately on vehicles with leaf springs at both ends, with front ratings considerably lower since the front carries accessories rather than payload.

Choosing the band around what the vehicle actually carries is what determines whether it sits at the right height loaded.

Where a Parabolic Kit Is Not the Right Choice

Worth saying plainly, since parabolic springs are not automatically the better option.

A vehicle that is permanently loaded to its limit and never runs empty gains less from the comfort advantage, since it is sitting on the loaded part of the range the whole time. A correctly rated conventional heavy duty pack can suit that use better.

Extreme off road use is the other case. Some parabolic designs have clearance limitations around the second stage or overload leaf at full articulation, and a vehicle used hard in that way is worth discussing before ordering.

Cost is a legitimate consideration alongside all that. A parabolic kit is a more expensive purchase than a conventional pack, and on a work vehicle where ride comfort is not the priority that money may be better spent elsewhere.

Where parabolic kits genuinely shine is the vehicle that alternates between empty and loaded, covers long distances, and spends time on corrugated roads.

Torquing a Parabolic Leaf Spring Kit and Checking the Shocks

Fit the new U-bolts supplied rather than reusing the old ones, and torque them evenly and in sequence.

Check the fixed pin head seats fully into the axle locating hole before the U-bolts are tightened, since that head is what holds the pack in position on the axle.

Grease and fit the new bushes rather than carrying old ones over, because free movement at the eyes and shackles is part of what the kit is delivering.

Retorque the U-bolts after the first few hundred kilometres. A new pack settles as it beds in and the U-bolts relax, and that retorque is not optional.

Have the shocks assessed at the same time, for the reason covered above.

Fulcrum has been working on suspension since 1976, across 16 Service Centres and 200+ authorised workshops nationwide, and leaf spring work is a core part of what the network does.

Find Parabolic Leaf Spring Kits for Your Vehicle

Select your vehicle to see the parabolic leaf spring kits available for it, and check the constant load rating and raised height each one is designed around. Talk to your nearest Service Centre about whether your shocks suit the change. Warranty and a fitment guarantee come with everything we sell, supported by workshops in every state.