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What (and where) causes a bent chassis?

ByPat CallinanMarch 13, 2020
5 MINUTE READ
What (and where) causes a bent chassis?

Having criss-crossed the country on some of the roughest of tracks, I’ve seen plenty of bent chassis…even bent a few myself. Here’s why (and where) they bend.

The bent chassis phenomenon was definitely not a thing 30 years ago. Our thirst for more accessories and longer vehicles, coupled with the auto industry’s attempts to make lighter (read: cheaper) 4X4s, has caused a problem.

And then there’s towing. We love our accessories, so we like to spread them over a couple more wheels, hence, we tow stuff.  Not to mention towing stuff over rugged terrain. As my kids would say, it’s a bad day to be a chassis.

I should highlight that I am not an automotive engineer. I’m a ‘cause and effect’ kinda bloke. And I’ve been on many, many trips where chassis have bent. Patrols, LandCruiser 100s, Navaras – thankfully not Amaroks. Not to mention I’ve spoken with plenty of people who know better than me, like Sam Barnes from Birdsville Auto. So below, is my distilled knowledge on the topic of bent chassis.

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CAUSE NO.1 – The Towing Conundrum

If the Simpson Desert were a WWI chassis battleground, it would be Gallipoli.  1100 sand dunes ranging from three metres high to 30 metres high (Big Red) crossed by more than 30,000 vehicles each year.

No graders, soft sand and often inexperienced drivers create a worrying concoction of giant wheel-swallowing holes. Mix in a wee bit of momentum to allow your 4X4 to get over the dunes, and you have a seriously challenging obstacle.

Your suspension will cycle from full droop to full compression in less than one second, which is fine, because that’s what it was designed to do. But that suspension on your vehicle, can, on occasion, cycle at precisely the wrong time if you are towing. For example, if your 4X4s rear suspension is just at the start of its rebound cycle, but your trailer hitch is on its compression cycle, only your chassis can solve that problem. Two, massively opposing forces are colliding, and if the chassis lacks the strength in that split second, it will bend. Something has to give. You can successfully traverse literally hundreds of sand dunes, but when the holes are in just the right (or is that wrong!) spacing apart, the opposing forces are just too great. Additional weight (in the form of heavy rear bars with multiple spares) tends to exacerbate the problem.

Day 11 18

A well setup towing rig. The Amarok’s chassis is built strong with seven cross members where most dualcabs only have five.

In my experience, the worst offenders seem to be D40 Navara, Tritons, and GU Patrols. It’s rare to see 79 Series LandCruisers and VW Amaroks with a bent chassis. The latter has seven cross-members in the chassis, as opposed to five cross-members on most the other dual cabs. Oh, and my 650mm extended chassis Amarok has nine crossmembers.

CAUSE NO.2 – The Airbag Issue

When the rear suspension of a vehicle is designed, a pretty smart engineer usually designs it. He or she calculates all sorts of loads and stresses that the suspension and chassis is designed with which to cope.  And then along comes the aftermarket, with a far smaller budget, and says ‘We can do better’.

And often, they can, because their components (shocks and springs) are designed for a different, more robust purpose. But sometimes, things don’t quite work, as they should. Let’s take leaf springs for example. They have two mounting points on the chassis, and those points are designed to shoulder almost any load the springs can throw at them. Enter airbags. They sit directly under the chassis and smack in the middle of the leaf springs. If the chassis is robust, this can even be a good thing. The weight of the vehicle is spread over three points, rather than two points. That’s great! And often, they work just fine.

In some cases though, airbags can lead to chassis failure:

  1. The airbags are inflated too high (like 60psi rather than 8psi). So the airbag is carrying the entire load of the rear of the vehicle. So rather than spreading the weight, you’re concentrating that weight into one central point – a point that was never designed to carry that weight (unlike a coil sprung vehicle which could easily have coils interchanged with truck-like airbags);
  2. The leaf springs are lightened off (numerous leaf springs removed), and used merely to locate the axle, and truck-like airbags are inserted, thereby concentrating the load on one central point; and
  3. There is simply too much weight on the rear of the vehicle, and an airbag concentrates that weight in the wrong place. Overloaded vehicles can cause this, as can excessive ball weight on your caravan or camper trailer.

Now there are plenty of 4X4s that have successfully used airbags like Polyairs over the years. If I were to use them though, I would do plenty of research into your specific model of vehicle, and ask if others have had any issues with their installations. Silverados, 79 Series, Amaroks would be no real problem, but other vehicles with lighter chassis may be an issue. You won’t discover that issue up the highways and byways, but in the rough and tumble of a desert track.

CAUSE NO.3 – The Tracks Are Getting Worse

The 4X4 and touring scene is booming, and it’s set to soar in 2020. We can thank a plunging Aussie dollar and a certain virus for that. I mean, who’s jumping on a cruise ship to China this year – not me.

Yet increasing traffic on the roads that we like to drive (Anne Beadell Highway, I’m talkin’ ‘bout you), means the conditions will deteriorate. Rough and unmaintained tracks like the French Line, the Canning Stock Route and the Telegraph Track in Cape York will get rougher again. And harder touring tracks and heavy loads equals chassis meltdown.

190709 Pctv Ep5 6 Day 12 (63 Of 70)

In 2019 after the Big Red Bash, I was blown away by how rough the QAA Line had become. In the space of a few years, it had gone from ‘bad’ to ‘rough as guts’. That’s the technical term, anyway. I, and our convoy, was thrilled to turn onto the comparatively smooth and gorgeous Hay River Track. Of course, we only saw one other convoy on the way, and that was Ron Moon with his 4X4 Australia team. No traffic, no worries.

OMG! It’s bent, what do I do know?

In terms of what to do in a remote location if you do suffer a bent chassis, here are a few options.

  • Lighten your load dramatically. Remove the trailer (if you can convince some other bugger to take it!), remove your spare tyres off your rear bar, do anything you can! Lighten lighten, lighten! Usually the vehicle can then be limped into the next town and towed. The chassis bending doesn’t usually affect the driveline, so provided you inspect the chassis and its not actually cracked (only bent), you may be able to limp out;
  • If you have full off-road recovery cover, you may be able to make a claim here to get a flat-bed tow to the next town. Keep in mind, recovery cover is not roadside assistance, and it will merely get you to town, not necessarily home to a capital city;
  • Big towns like Alice Springs see chassis bending all the time. Friends of mine had their Patrol chassis strengthened and straightened in Alice for $2500. I had a Patrol that landed at a city smash repairer who wanted $13,000 for the same job. The owner paid the former job. The insurer paid the latter; and
  • Speaking of insurance, are bent chassis covered by insurers? Most of the time, they’re not. Usually the insurer calls it ‘wear and tear’ as opposed to a ‘single incident’ (which is insurable). It’s debatable, but good luck trying to get your insurer or their underwriter to pay.

Full Disclosure

As I’m saying very positive things about Amarok chassis here, I feel I should ‘ring the sponsors bell’ and let you know that I’m sponsored by VW. I certainly am – have been for six years now. And that’s why I know weird facts about their chassis. But having bent a few chassis over the Simpson Desert, yet never an Amarok, I reckon credit where it’s due.

Their chassis does have two more cross-members than all other dual cab utes, and it’s hydro-formed too. It also costs about six times as much to manufacture than other dual cab chassis, which probably means that it will be ditched in the next gen Ranger / Rok collaboration. I don’t know that for sure, so I’m just hoping that Ford will see the value of that platform.