Air bags don’t cause bent chassis
Bent Ute chassis’ are becoming more common every single day, and almost every time a photo of one is uploaded to social media, people are very quick to jump on and blame airbags for the failure. This is almost always with no knowledge of the exact incident, but they just feel that airbags are to blame.
I’ve spent the better part of 15 years helping people to travel with a 4WD, and am very passionate about sharing information that is helpful, accurate and useful. Air bags do not cause bent chassis, and I want to take a minute to cover exactly why.
Now, I want to start with mentioning that a lot of vehicles with bent chassis do indeed have air bags. That means they must be the cause yes? On the surface you might think so, but correlation does not mean causation. Could it be that there are other, much greater facets that are actually the cause? Hint; why do people fit airbags in the first place?
This myth just keeps getting parroted
As I’ve eluded to above, there’s a lot of people who firmly believe that air bags cause bent chassis, and like lots of poor quality information online, they continue to write about it, and others soon pick up the rubbish and keep the circle going.
What’s the purpose of air bags?
Air bags are quite literally that; strong bags of air that you can pump up and down, to help with carrying a load. Trucks have been relying on them solely for years, and there’s even a few 4WD’s that run pure air bag suspension. They’re reliable (when done well), and allow you to adjust the height of your vehicle, and the load carrying capacity as your needs change.
For many people, they are a great solution to carry varying loads. What I mean by this is simple; if you have a family 4WD that you drive around town 90% of the time, but once a month you want to load it up and tow a caravan, air bags will allow you to do this comfortably and safely without changing suspension.
When you have no load, the factory suspension (or light duty springs) are comfortable and carry the vehicle just fine. When you load it all up, and hook a van on the back (tow ball weight) the suspension sags, and is very unhappy. However, if you have air bags to assist, you can just pump them up and use them sensibly and life is great.
Coil suspension vs leaf springs
Before we begin, I want to point out that there are two very commonly used suspension types on the rear of a 4WD. The first are coil springs, which are literally a normal looking spring that takes the load of the rear of your 4WD from one point on the chassis, and runs down to the differential.
You can get airbags for these, and they run inside the coil spring, and virtually nothing changes as your still applying load in exactly the same location. There aren’t many Utes that run coil springs, and its usually reserved for wagons, which will rarely bend a chassis because they have a physical car body coupling the whole thing together, and providing immense support.
On leaf sprung vehicles, the airbag is a support system that replaces the bump stop, and it goes in the middle of the leaf spring, between the differential and the chassis. This is adding a new load point, and when they are pumped up, it makes it harder for the springs to compress, which gives you greater load carrying capacity.
You change the point of load
On leaf spring vehicles, the rear of the vehicle’s chassis is supported by 4 places, under normal circumstances. The leaf spring has a front and rear hanger, and with one on either side, you end up with 4 points of load.
It’s worth noting that the bump stop is in the centre, which will get used and compressed (sometimes heavily) when the spring hits full compression. When you add airbags, you remove the bump stop, and the airbag is there in its replacement form. Now, you’ve gone from 4 to 6 points of load support, and people think that doing this makes the chassis bend. How?
If an air bag changed the load point to where its not designed in such a bad way, then you’d expect the chassis to bend at the air bag, correct? It never does. Without fail, it will bend just in front, or just behind the front leaf hanger.
What actually causes bent chassis?
A bent chassis doesn’t just happen. It’s a culmination of things that come together, and bite you in the backside. Ultimately, you need a certain amount of energy to bend a chassis, and by doing any of the below, you increase the chance of this happening.
Overloading
By far and away the primary cause for a bent chassis is overloading. Every single vehicle that comes off the factory floor has load ratings that you are legally obligated to adhere to, and that’s for normal, easy operating conditions.
If you head offroad with a vehicle that is at the absolute maximum weight, what do you think is going to happen? Some 4WD manufacturers recommend no more than 80% of the GVM when you are using a vehicle off road, and I can tell you that there’s a massive number of vehicles on the road running at 100 – 150% of their maximum load, and still being flogged down every 4WD track possible.
If you are using the full payload of your vehicle (and in many cases even more with GVM/GCM upgrades), you’re step one into improving the chances of a bent chassis.
Poor load distribution
The only thing worse than an overloaded vehicle is one that is badly loaded, and that happens all the time. Your 4WD will have a GVM, which is the tare weight and payload together. It also has front and rear axle maximum weights, and you can exceed these without going over your GVM.
I’m going to say that again; you can be over your rear axle weight without being over your GVM, and its done by poor load distribution. Lets say you have a brand new Ford Ranger 4WD, with a payload of around 1100kg. Fantastic.
If you head down to the local hardware store, and pick up a 1 tonne pallet of cement, and you ask for it to be loaded as far back as they can get it in the tray, what do you think will happen to the vehicle? The front is going to point to the sky, and you’ll be so far over your rear axle weight it won’t be funny.
That 1100kg is supposed to be evenly distributed across the vehicle, and if its not, your chances of bending a chassis go up so much more.
Dual cab utes are by far and away the most likely vehicles to bend, and all too often people don’t have the really heavy weight as far forward as possible. I’m talking about spare wheels, jerry cans, kitchens, water tanks, long range tanks and anything else that weighs a lot.
Leverage is a seriously scary thing, and to demonstrate, we’ll chat quickly about tow ball weights, below
Large tow ball weights
In many ways, this still falls under poor load distribution, but its not something people naturally think about, so it gets its own header. When you add a heavy tow ball weight, the front of your vehicle often rises considerably, depending on the suspension you are running, and the weights.
The major consideration though, is the distance from the rear axle to the tow point of your vehicle. On many dual cab utes this is nearly 1.5 metres, and that’s before you add in the crazy tow bar extensions that some people run. If you add 200kg of tow ball weight, at 1.5 metres out on an average dual cab ute (which is very common), you’re actually applying about 300kg on the rear axle.
This happens by removing around 100kg of weight from the front axles of your vehicle.
Tow ball weight can also be applied at the worst times when you are off road too, as the trailer is a few seconds behind the vehicle, and if your vehicle is wanting to go up at the same time as the tow ball weight is really applied, it’s a double whammy.
Now, add this in with an overloaded vehicle that has lots of weight really far back, and you’re going to bend a chassis at the first hump you hit.
Excess force
If you’ve been in the 4WD game for a while, you’ll know that they take an absolute beating. Whether it’s a wash out you didn’t see, or a series of whoops on a sandy track, its very, very easy to add a level of force into the mix that is well beyond what your vehicle is designed for.
I saw it in Cape York, when a vehicle that was probably not overweight hit a floodway faster than he should have, and it met a hole that you could not see due to water being over the top. We hit it much slower and it was still a shock for us, but our chassis remained straight, whilst his BT50 bent like a banana.
Excess force can come from the driver being stupid, but its also very common when an accident happens. You might not see something, or your timing is bad towing a trailer, and you do not need a badly loaded vehicle, or an overloaded one for excess force to bend your chassis.
Expecting too much from your vehicle
I guess the finale is this. 4WD’s are designed to go so far, and if you push them beyond this, you’ll have a problem sooner or later. Your GVM, payload and axle weights are not a target. They’re a maximum, under good conditions.
If you think you can load it all up (perhaps not that well either), plus tow your trailer and bounce your way along corrugated roads and 4WD tracks without something going wrong sooner or later, you’re dreaming.
Should you fit airbags?
I’ll close by saying that airbags solve a problem perfectly, when used as they’re designed for. If you have a vehicle that carries variable weights, are always under GVM and the relevant axle weights, has good load distribution and you’re driving carefully, airbags are brilliant.
I would always recommend trying to get the right springs for your weight, but when you carry 100kg one day, and 500kg the next, that can be quite hard to achieve.
If you are overloaded and/or have poor weight distribution, air bags are not for you. A diet (of the vehicle), and an adjustment of how you load your vehicle is.
Do airbags cause bent chassis?
It’s pretty simple. Airbags do not cause bent chassis. Overloading, poor load distribution and excess force do.
If you want to see more, have a look at the study done by Syngineering, which clearly shows zero change to the stress on any part of the chassis of a vehicle running air bags with a leaf spring setup, vs it not running air bags.
Air bags are perfectly fine, when used sensibly.
What’s your thoughts? Am I wrong?
