Why Modern 4WD Frames Keep Bending


Hint: It’s not the vehicle’s fault!

We’ve all seen the pictures of (mainly) dual-cab utes on the internet that have banana’d frames with the cab and the tray looking like two sides of a triangle, and you just know from looking at them that it’s going to be an expensive repair. But take a closer look at the pics. Notice anything? Yep, almost all of the utes that have bent their chassis’ are all uniformly loaded up to buggery, are usually in an environment that sees heavy corrugations or washouts (deserts and beaches are popular) and more often than not are towing a trailer too.


With that said, the first thing people blame are helper airbags in the suspension. While they can be a contributing factor (when misused) they are not the actual reason why frames bend. The actual culprits are basic physics and the vehicle owners not understanding (or caring) how their fourby is built to handle.

Yeah, you heard me. I’m blaming the owners; not the airbags nor the fact that “utes these days are soft” nor the trailer in tow nor the huge washout that was hit at a buck-twenty. The problem starts and stops with the owners.


Let’s look at why.


REMEDIAL PHYSICS 101

See how this ute (which is designed to carry big loads) has the axle positioned in the middle of the tray?

…and this one doesn’t? Fancy that!


Take a look at just about any modern dual-cab ute, in particular where the back axle is in relation to the tray. See how pretty much all of them have the axle right at the front of the tray directly behind the cab? This means that when the tray is fully loaded the majority of the weight will be behind the axle. That’s bad. Yes, this is mitigated somewhat by the design of leaf suspension (which most dual-cabs run in the rear) but the weight all being behind the axle – even more so when towing – introduces leverage onto the suspension, making it sag. Suddenly there is less available travel in the suspension to soak up bumps. When the suspension runs out of travel all that force is then transmitted through the chassis. Multiply that over several hundred kays of corrugations and it’s a matter of when not if.


Basically, if you’re picking up what I’m putting down you’ll have already guessed what I’m about to say: WEIGHT IS THE ENEMY!


More than that, knowing how to distribute it is key. Keep it between the axles as much as possible. And don’t overlook your trailer either. Water tanks, gas bottles, boats, fuel and storage boxes can all have an impact on ball weight and trailer handling depending on where they’re placed.


AIBAGS DON’T CAUSE BENT FRAMES, YOU DO

Airbag helper springs have to be one of the most maligned accessories out there. Much like the high-lift jack, they’re incredibly useful, but so dangerous when used incorrectly.

Essentially, airbags are a band-aid for heavy loads, and they do that job seriously well, but problems arise when folks make the mistake of thinking their airbags have raised their ute’s GVM. They load the bejeezus out of their trays, maybe have a few hundred kilos of canopy on the back, hitch up their 25ft plate boat so that they’re well over their GVM (or GCM) and notice their suspension is sagging – so they pump up their airbags until it isn’t anymore.


The thing is, you can’t argue with gravity. The weight being supported by that chassis and suspension is still way more than it was ever designed to take and the airbags, instead of helping spread the load as they’re designed to do, have now introduced a stress point on the frame – which is where it’s likely to bend.


When it does, the bags cop the blame, when really it’s the guy who thought carrying 4T of gear behind the back axle while towing his monolith of a caravan with his Navara was a good idea. Ok, maybe an extreme example, but take a look at the weights you’re hauling before your next big trip. It’s not hard to go over the GVM.


On that note…


TOW RATINGS – THEY’RE FULL OF S#!T

A full-size ‘truck’ may be your only option if you want to tow heavy

You know when you bought your ute and the sales guy told you “this baby will tow 3500kg no worries!”? Yeah, he wasn’t lying, but he left a fair bit out of the whole story. In fact, I’d even go as far as to say modern tow ratings are a crock.

Let’s look at a case study of the Holden Colorado (purely as an example): It has a GCM of 6000kg; a GVM of 3150kg; a kerb weight of 2150kg and a payload of 1000kg.


What does this mean in the real world? It means that when towing the claimed 3500kg, the Colorado only has a payload of 350kg. Go ahead and weigh up your whole family, a couple bits of barwork, a full tank of juice and a loaded fridge and tray… it’s really not that difficult to be over GVM, so kiss your insurance goodbye in the event of an accident or breakdown, and welcome to driving illegally.


When you factor in the rigours on your frame when driving off-road the story just gets grimmer and the forces that are doing their best to put a nice big kink in your chassis rails just get higher.


Interestingly only Land Rover drop their tow rating down to 1000kg when driving off-road. Nobody else does. It’s a recipe for frame-bendiness if ever I’ve heard one.


HOW TO AVOID IT

Notice the stretched chassis to better handle the weight of the canopy. For this particular purpose, it’s a smart modification

There are several ways to avoid a banana’d fourby. And the most effective ones are even free. As much as I despise the current road authority rhetoric of speed being the root cause of everything wrong with the world, in this case it’s actually true. When bombing over corrugations, whoops and washouts the best thing you can do for your vehicle is slow down. Less speed equals less force acting on your suspension and frame. You ain’t BJ Baldwin and your 2in-lifted HiLux sure as hell ain’t his trophy truck. So ease off the accelerator and enjoy the scenery for a little longer.


Next thing is to pack smart. Try to keep the majority of your weight as central and low as possible and be aware of the handling effects from an incorrectly packed trailer. Keep the weights down too - you probably don’t need 300L of spare fuel for a trip to Fraser Island, so why take it? Less can be more here, learning to pack economically is an essential skill for the off-road adventurer.


From there we can get a bit more serious and speak to your local workshop about frame-strengthening plates, GVM upgrades and even chassis-extending to get the axles better placed for load distribution. We’re talking serious money here, but if you’re carrying heavy and doing big kays through the outback, it may be your only option.

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