Grant and Linda test out new suspension

PARABOLIC sPRINGS

By Grant & Linda Hanan

When old suspension technology has been revitalised for the modern 4WD, Grant and Linda test whether it stands up in today’s world of 4WD touring.

Out with the old leaf spring pack and in with the new parabolics.

Owning a utility, and in particular, a dual cab utility, has been winning in the popularity stakes in recent times. And why not, when straight from the showroom floor they’re roomy and come with many luxury creature comforts. Besides taking a family of passengers, it’s easy enough to customise them to your heart’s content with a range of accessories and carry a load at the rear. Whether this means a fully fitted out canopy for work or play, or simply all the family’s get-away gear tied down in the back, the variety of options and flexibility is making them the go-to choice for many. When it comes to carrying the loads, let’s look at the suspension system. While we’re just covering the rear suspension here, you need to keep in mind a vehicle needs a complete front and rear tuned suspension that works in harmony together. For many of the popular utilities you see on the road today, their rear suspension has all been designed using the conventional leaf spring pack. This suspension style dates back to the horse and cart days and for good reason. By its design alone it can carry loads very well when set up correctly. Dual cab utes today can be used for pretty well anything. It could simply be the family car for the day-to-day grind going to and from work, or even just Mum’s taxi for the school run. Then on the weekend, it’s easy enough to load them up for a family get-away, or better still turn them into a modern-day touring escape machine.

This can mean the load carrying capacity can vary, from totally empty i.e. light loads, through to fully loaded to maximum GVM and extremely heavy. The conventional leaf spring pack as supplied OE has some range with its kilogram capacity, but it generally falls short. This results in changes to the ride and handling characteristics from comfortable and smooth, to downright awful. It’s one of the many reasons why aftermarket suspension systems are sold. But if you get the aftermarket suspension set up wrong, then you’ve spent a lot of money for little to no gain in comfort and handling. Let’s explore this further.

TRADITIONAL LEAF PACKS

Everything is a compromise when setting up any suspension system, and a rear solid axle leaf suspension used on the majority of the dual cab models is no exception. By way of its design alone, one adds “leaves” to the leaf pack when heavier loads are required to be carried. Many companies talk about their leaf packs being designed for a range of weights – these generally start with a constant load of 300kg. From here, they then might have leaf pack models with more added “leaves” and rated for between 400kg and 500kg. These bigger leaf packs are now getting heavy.

But they need to be big in order to carry the laden weight. This can result in resistance when it comes to overcoming the friction load in the spring before they start flexing. Only then will the spring do its job of evening out the ride and absorbing the road conditions when it comes to suspension travel. Take this approach and you can easily find yourself with an over sprung vehicle when the load isn’t applied. What we mean here is the suspension doesn’t travel or move, and the ride feel and handling is harsh, hard and very uncomfortable. If you take the opposite approach and have a lighter spring pack with less leaves for day to day use, that will be fine. However, it will be a different story the next time you load up your vehicle. Those lighter load springs will leave you with no suspension travel at all other than to rely on the shock absorbers. They’ll be worn out before you know it, and way short of what would be a reasonable service life. This can lead to breakdowns and a less than satisfactory handling vehicle. While you could add other aftermarket options like airbags or helper springs, these shouldn’t be relied upon to replace or compensate for poorly designed load carrying suspension systems in my opinion.

PARABOLIC SPRINGS

Whilst parabolic springs have been around for a while, it’s only been in recent times that companies are now offering a range to suit the more popular utility models. Before these became available, you had to be prepared to accept compromise when fitting out an aftermarket multi-leaf spring pack. Parabolic springs aren’t new in the way of suspension system design, and in fact have been widely used within the trucking transport industry. Big trucks with trailers are loaded to the maximum allowable loads so they can carry goods and supplies across country. It’s here where parabolic design leaf springs have

The Terrain Tamer 3 leaf parabolic has a military wrap spring, friction and silencer pads.

been operating for good reason, as their very design allows a wider range of load carrying weights. They cover weights from heavy to light and everything in between. Before jumping into the details of parabolic springs, let’s just clarify something. A conventional multi leaf spring pack that’s designed to carry heavy loads is going to have some 10-12 leaves of varying lengths packed together with clamps to hold them in place. All those leaves are required to move against one another so as to straighten out and flex up and down to act as your suspension. But when looking at a parabolic spring pack, it differs from the multi spring pack in a couple of ways. Beginning with the most noticeable feature, let’s start with the base spring at the bottom of the pack. The spring thickness at the centre of this spring is a lot thicker and decreases in thickness the further you follow it to its outer edges. You’ll find this results in a tapered profile. This single leaf alone can cope with a high degree of load and varying force applied to it, compared to a multi leaf spring pack that distributes this load over multiple leaf springs. Additional leaf springs are then added to a parabolic set up to assist when a higher constant load is required. These additional leaf springs have what is known as “military wrap” at the fixed end of the spring where they are rolled around. The idea here is that it provides improved safety for off road conditions. Inter leaf poly blocks and silencer pads are then added to reduce the friction between leaves to offer a softer ride and less noise. When manufacturing parabolic springs, companies can use high chromium steel that undergoes special heat treating and stress peening processes using a parabolic tapering machine. This results in a spring that is less prone to sagging. It also allows the spring to flex more readily and gives improved traction plus a smoother vehicle ride. As parabolics are a smaller leaf pack in size (2/3 leaves), the pack is much lighter. So there’s weight saving gains there. It’s also thinner in the pack width when compared to a 10/12 multi leaf pack, so we get benefits in ground clearance as well. That’s a good thing for an off road 4WD.

REAL WORLD PARABOLICS TESTS

After deciding to change our rear suspension from the old leaf pack, we initially fitted a three leaf rear parabolic spring set to our touring 79 series dual cab LandCruiser. We then proceeded to take it on a 10,000km test across bitumen, sand, dirt and rocks, with our camper trailer in tow for the majority of the time. Things were noticeably different as soon as we left the fitment centre, and this was just highway driving. We found the ride was smoother over minor road imperfections because of the inter-leaf flexing. This is a big plus for

We originally replaced the old 10 leaf pack with a 3 leaf parabolic version.

anyone using their vehicle as a daily drive who would regularly change their payload from a little to a lot. But the big test with the parabolics was still to come. We headed up the bitumen of the Stuart Highway and across the dirt of the Buchanan Highway into the Kimberley. From here it was a run along the Gibb River Road, down to Broome and up the Dampier Peninsula. We ended up going home via the Great Sandy and Gibson Deserts.

FOUR LEAF PARABOLICS TEST

THREE LEAF PARABOLICS TEST

We found highway touring was a breeze and not too dissimilar to our initial thoughts after leaving the fitment shop. But the real test was to come once we went off-road and lowered tyre pressures. It was here the parabolics copped a real good workout. We found the ideal average cruising speeds of between 70 and 80km/h was the sweet spot for a comfortable smooth ride over corrugations and graded roads. We also stopped occasionally to test the shock absorbers with an infra-red thermometer to ensure they weren’t overheating. Generally, the readings were between 45 and 60 degrees. The areas where we did experience some issues with this first set of parabolic springs was at some of the washouts and large sudden dips in the roads. At these times, the rear suspension would bottom out. The shockers would then take all the load as the spring set was just too light. We knew in the long term the shockers couldn’t continue being the defence to this bottoming out issue. Overall though, we liked the improved ride and handling. There was a noticeable difference with the flexing parabolic springs when driving over rocks and in sand, and they smoothed humps and ruts with ease.

After getting back from the 10,000km test, we upgraded our rear parabolics given the bottoming out issue we had experienced at times with the three leaf set. Swapping out the three leaf pack for a four leaf pack took us next to no time in our shed. With the vehicle all cleaned up, serviced and ready to head out again, we loaded the vehicle and camper similar to our previous 10,000km test. But this time, our testing ground would see us head up the bitumen of the Stuart Highway and turning off to head across to Birdsville via the Simpson Desert. We would then make our way down south to tackle the Googs Track before heading west and returning home via a combination of the Nullarbor and coast roads. It was a good mix of sand, gravel and rocky tracks with some bitumen highway driving.

We stopped occasionally during testing to measure shocker temperatures.

The results were very similar to the three leaves when it came to highway driving. The Simpson crossing was a breeze, and the suspension absorbed the ups and downs of the sand dunes quite well. Gravel roads were fine too. We had a couple of occasions where we bottomed out along the Googs Track, but we put that down to having the camper in tow and the condition of the track. It was very rutted out in places and some deep sandy holes made the camper act like a bit of a boat anchor at times. We disconnected the camper at one stage to do some exploring and noticed a remarkable difference in suspension travel, with no bottoming out observed. Clearly the extra leaf assisted in carrying our load and although a bit stiffer than the three leaf, it was still a long way better than our original ten leaf pack. Getting the right spring/shocker combination can be a challenge when you’re asking it to accommodate a wide range of load carrying capacities, along with multiple types of terrains and surfaces. And if you own a solid axle rear end, you’ll find yourself having to make trade-offs between a suspension that can carry heavy loads when needed, but at the same time offer a comfortable ride and handling. For us and our way of thinking, a parabolic spring set up is a great place to start. Then following it up by tuning it with perhaps a remote style adjustable shock absorber might just be the next thing we visit to improve its overall performance and range.

The parabolics soaked up the Simpson Desert sand dunes without any problems. Additional friction pads were provided with the 4 leaf parabolic set.

Join us on a 4WD tag-along adventure tour, taking in the raw beauty of WA’s landscape and history. We are family-friendly, environmentally conscious and respect the land on which we travel as well as its history, features and landscape. Our goals are simple; to lead and support people to discover the splendour of Western Australia and to provide you with a safe adventure-filled experience. Our overall goal is to make sure you see things that otherwise you might not, go to places that otherwise you might not visit and meet some new friends and enjoy their company in a friendly, safe and secure surrounding. Why not join us? If you are new to four-wheel driving, don’t worry, we will provide you with support and guidance whenever you need it.

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