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Back in the 1970s, a 4X4 chassis was little more than a pair of thick 5-6mm C-section steel rails kept roughly parallel to each other via a few cross members attached via rivets and, if you were lucky, some done-by-an-underpaid-guy-in-a-factory-somewhere welds. They typically allowed a fair degree of flex and twist to help axle movement, which was limited thanks to the overly stiff and inflexible suspension set-ups of the day.
Jump forward in time and by the late ’80s and into the ’90s suspension improvements (from a handling point of view) like IFS and swaybars were being developed by just about every manufacturer under the sun. With the flexible chassis of yesteryear sending handling into a spin with these new improvements, they had to be stiffened up to take advantage of the newly available movement from the springs and shocks. Usually with box section rails, albeit made from thinner 3mm steel.
While talking strictly stock vehicles, this is actually a good thing, but what four-wheel driver ever leaves their vehicle 100% as when it left the factory floor? One of the most popular modifications on the planet is to throw some upgraded suspension under the frame – specifically, airbags to compliment the rear springs and take some extra weight over the rear of the vehicle.
And that’s where problems can arise.
Y’see there’s a bit of a misconception out there that airbags can cause the chassis of some dual-cab utes to fold like a pair of twos in a high-stakes poker game. I say misconception because it’s usually not the fault of the airbag at all… or the frame rails for that matter.
ARE FRAMES GETTING WEAKER OR SMARTER?
It’s no secret that weight is the enemy of performance and fuel economy. The lighter you can drive your 4WD, the better everything else generally is – it’s basic physics. With this in mind, manufacturers have long been trying to walk the fine line between strength and being light enough to not affect performance too much. From the C-section steel used in the early utes to the fully enclosed box section used in the more modern vehicles, to the full monocoque (Jeep Cherokee, Mitsubishi Pajero etc.) construction frames, they all have their pluses and minuses.
When it comes to dual-cab utes, they usually utilise either a C-section using thicker steel or box using slightly thinner steel but with extra strength (and weight). The C-section is more likely to flex a little (not necessarily a bad thing) in off-road situations while the box frames offer more rigidity, although usually at the expense of weight.
So which one offers better performance for off-road use and load carrying? Well, neither one is “better” than the other, as they’re both fairly stout and can support a lot of weight. The real devil is in the detail. Take the VW Amarok as an example. They’ve gone for a full box frame with no fewer than seven cross members to aid torsional rigidity and weight-carrying ability. Even the later model HiLux is sporting a thicker frame these days; are we seeing a swing back to thicker and heavier frames with the current crops of utes? Perhaps it’s the stories getting around of the previous generation of dual-cabs bending chassis that are making them go this way, or are they simply looking to get more practicality to go with the modern crop of ‘comfortable’ work utes?
DO AIRBAGS CRACK CHASSIS?
We’ve all heard the story. Somebody fits helper-airbag springs to the back of his ute to help out with the load carrying. They pile all their gear in the tray and hit the tracks… and come home on the back of a trayback with a chassis that looks like a banana – must be the bags’ fault right?
Not exactly, no. The idea is that the bags concentrate the forces on a small section of the chassis rails, causing them to bend or crack. While this may be true to a certain degree, it’s not the whole story.
You see, airbags enable you to level your ride, pretty much regardless of whatever loads you have in the back. And they can give a false sense of confidence and make you think that your vehicle is absolutely fine, despite being a ton or so over GVM.
So there they are, carrying way too much weight and travelling over tracks that would tax even the heaviest duty suspension – it’s really no surprise the chassis call it quits, but let’s be honest, it’s not really the bags’ fault is it?
SO YAY OR NAY TO BAGS?
Keeping your vehicle within its GVM and having the weight on board correctly distributed will more than likely see you getting home without a drama – and airbags can make the trip a lot more comfy and easier on your vehicle too. The problem with them is that folks see them as something that’ll allow you to load a few ton of gear on the back of the ute without a worry for the weight the vehicle is designed to handle or axle load capabilities. Although they are great for carrying loads, levelling out suspension systems and increasing ride comfort while heavily laden, they don’t magically make your BT-50 able to carry F350-type loads. And that’s where people get into trouble, especially when driving hard.
Are they worth it? Yes, absolutely, but not to sound like a broken record, if you’re exceeding your available payload you’re going to run into trouble – probably sooner rather than later.
Words by Dex Fulton
