Leaf suspension has, quite literally, been around since the horse and cart days. They’re simple, they’re versatile, they’re practical, and most importantly, they work. And when it comes to carrying a load, there’s not much else that comes close for similar money. It’d be a good bet that’s the reason why the majority of dual-cab utes still run them in the rear.
However, leaves are not without their drawbacks. They have a lot of steel in each spring pack so they’re heavy; they can be noisy and harsh over bumpy roads and when it comes to flex, they don’t do much of that either to be honest.
At least, those are the problems traditional leaf packs bring to the table, but luckily there’s a solution. Let’s dive into the ins and outs of parabolic leaf springs and look at why you should be running them on your tow rig.
There are a heap of factors that influence leaf spring performance. We could go down the rabbit hole of metallurgical properties, pack lengths, military wraps, pack thicknesses and about 100 other variables. In an effort to keep things simple, we’ll just keep it to factory spring packs vs aftermarket parabolic ones. As always, speak to your local suspension dealer or reach out through our socials if you have any questions specific to your set-up.
Pro tip: www.terraintamer.com is a great place to start.
A traditional leaf spring, or semi-elliptical spring as nerds call them, are made up of several (usually 6-9 or so) curved pieces of flat bar of varying lengths layered on top of each other that are designed to flatten out (bend) when a load is placed on them. Each leaf layer varies in length – the shorter ones being harder to compress and the longer ones being easier. The idea is that the initial part of the suspension travel is fairly soft, then as you get deeper into the range of motion, the springs become harder to bend. This is called a progressive spring rate (the ability to bear weight gets higher the more you compress the spring) and allows the leaf pack to support a large amount of mass. Perfect for utes eh?
Now, a traditional pack will have each individual leaf be made up of a uniform thickness and stacked on top of the shorter ones beneath it. It’s a simple system and it works for what it’s designed for – carrying loads without unduly upsetting the handling of the vehicle. There’s not much concession made in the name of comfort, off-road flex or unsprung weight (the weight of the vehicle that is not supported by the suspension – like wheels and brakes etc.). They’re all really just about supporting the load.
They’re designed to support the load too, but have a few extra tweaks in their design to, you know, make them good at that other stuff too.
Rather than being uniform thickness along their length, each leaf in a parabolic pack is thicker in the middle, and then tapers off towards the ends. So not only does the whole pack have a progressive rate, but each individual leaf does as well. This is an idea that has been carried over from the trucking industry, and it makes a whole heap of sense for both tow vehicles and off-road rigs.
In basic terms, the more leaves in a given pack, the higher the spring rate and the more mass that pack can support. With parabolic springs, however, each individual leaf is working more efficiently at distributing the load, which means fewer leaves per pack are needed. This frees up a bunch of unspring weight, which can now be used in more important areas, you know, like GVM.
In essence, you’re getting the same load-carrying ability from a lighter, more efficient spring.
As mentioned, a traditional leaf spring will have the individual leaves stacked on top of each other. Some factory and aftermarket springs will have a thin piece of anti-friction material between each layer, but they can often degrade or wear away before long. Then, each piece of steel is free to rub on the one above and below it, causing all sorts of noise.
By their very design, parabolic leaves are not in contact along their full length when unloaded, so they’re a lot quieter. When you’re on a dirt road, and the suspension is moving rapidly across corrugated tracks, this can make a huge difference. It also largely mitigates mud and grit getting stuck between leaves and causing rust build-up.
This lack of inter-leaf friction also means the pack can move more freely, providing increased ride comfort and allowing the initial suspension travel – the portion of travel that soaks up minor bumps in the road – to compress and rebound much more easily. This provides a marked improvement in ride quality.
The rigidity of traditional leaf springs means that, when in an off-road scenario, the suspension is less likely to tuck up into the wheel arches or droop down into depressions in the terrain.
In simpler terms: you’re likely to lift wheels and lose forward momentum pretty quickly.
Parabolic’s tapered design, on the other hand, distributes the forces more evenly across the spring, allowing each leaf to work independently to allow for more suspension movement.
In simpler terms: they flex better.
Despite being able to flex easier than traditional leaf springs, the genius of parabolics is that they can still handle the downforce from heavy loads. This means they work exceptionally well for tow vehicles or utes that are regularly loaded to the gunwales. The rate of spring compression allows for small bumps and undulations to be absorbed by the spring while still being able to support a fair whack of ball weight or a canopy stocked up with a packed fridge and camping gear.
There are a fair few different parabolic springs on the market, and some are better than others. As with any aftermarket choice, it pays to shop around to find the right fit for your vehicle. For our money, the range and expertise from Terrain Tamer is hard to beat. They have researched, designed and built their own parabolic leaf springs using modern technology and years of careful examination of what works and what doesn’t. They even went as far as to use their own special grade of spring steel to make what is one of the best products on the market.
You can find out more at www.terraintamer.com
