To try and understand what is happening here you have to first look at the difference between weight and mass. Weight is simply a measure of the pull of gravity. Mass is the amount of material in something.
A brick for example might weigh say 3 kilos. When you place it on the ground, it does not float away or move because gravity is trying to pull it down to the centre of the earth. If you place some scales between the brick and the ground, you will get a 3 kilo reading. That 3 kilos does not change.
The brick did not damage the scales but if you pick it up to head height and drop it onto them, there is a good chance that it will. Why did that happen you could ask because it still weighs only 3 kilos? The difference is the material (mass) in the brick was put into motion. It built up momentum and did not want to sop. When the scales brought it to a sudden stop, it hit them hard causing a lot of damage in the process.
That is what is causing all of these bent chassis. All of the material that you put behind the rear axle will fall and build up momentum when the wheels drop into a depression in the road. It does not want to stop anymore than the brick did. It takes the end of the chassis down with it but it is unrestrained because there is no spring between the end of the chassis and the road. The restraint is the springs and axle housing that are well over a metre further forward. In other words the end of your chassis behind the rear axle is sticking out like a diving board and just like a diving board, there is nothing supporting the far end of it.
When the wheels come back up again a split second later, the falling material behind the axle still wants to fall. It thumps the end of the chassis down hard as the chassis tries to catch it and instantly heave it back up again. This pivots the chassis on the rear axle bearings and tries to lift the front.
The springs can't help very much. They just compress and pivot with the chassis.
It is a bit like a see saw. You can load it up with a very heavy man on one end and a slightly lighter one on the other and one end will go down while the other will go up. If you placed a spring between the plank and the cross bar, it will compress but the plank will still go down at one end and up at the other.
Imagine what this is doing to the rear of your chassis, particularly on rough roads. It is constantly flexing the end of the chassis up and down and any chassis can only take so much of that before something drastic happens.
The reason why the front of the chassis does not bend, even with heavy bull bars and winches on it is because it is so short in length. There is no serious leverage effect.
Look at your car from side on. There is very little overhang in front of the front axle but there is a mile of it out the back. That distance out the back is really a long lever that further amplifies the forces generated by all of that moving material.
To see an example of that, look up the specifications for the Defender on Land Rover’s site. It says 150 kg on the tow ball will add 206 kg to the rear axle. That is because that 150 is back on the end of a lever. Imagine the forces going into the chassis near its pivot point when the 150 is in motion and constantly changing direction up and down.
Those forces are further increased when you add the weight of whatever is in the rear of the tray.
The reason why air bags seem to be blamed more often than not is they don't compress like a leaf spring. A leaf spring compresses in proportion to the load i.e double the load and you double the compression. An air bag is an exponential spring. It just keeps on getting harder. The more you try and compress it, the more it resists further compression. They work a bit like a linear spring for about 40% of their travel then harden up rapidly after that to the point where they are like rocks at around 70% compressed.
A leaf or coil will keep compressing until they reach the bump rubbers which are also exponential springs. An air bag is like an oversize bump rubber and is not the sort of thing you want operating in a location designed for leaf springs. There is a good chance that an air bag will stop the downward movement of the chassis before the leaf springs would have. That is why they can be so destructive. It explains why more cars bend chassis with them than without.
I have heard many people claiming the high carrying capacities up in the vicinity of 1 ton are unrealistic and you can't load them up to that level without having the rear dragging on the ground. It may sound impossible but maximum capacities require precise loading and it can be done. Each load bearing area must carry its full share of the load.
To load one to the maximum you must start with 5 large football player size people. This will give you around 500 kg in the cabin. A manufacturer has to design a car to carry large people. It is no use setting it up for five size 6 women. The next step will be to fill the fuel tank. That should add about 70 to 80 kg. The remaining 420 kg can then go into the tub out the back with the heaviest items as far forward as possible.
That will leave the car fully loaded and sitting at exactly the height that the manufacturer intended. If you have a family consisting of two average size adults and three toddlers, you may have a combined weight of only 200 kgs. You are definitely going to have the rear sagging if you then try and put the remaining 720 kg in the back.
All of these maximum loads are for good highway conditions only and should be reduced when in the bush. The editorial in the 4X4 Australia bent chassis story said about 30 to 40 % should be knocked off the maximum capacity. The best place for advice about this is the car manufacturer’s customer advice service. Contact details will be on their web site. Don’t ask a dealer.
We then have towing weights and once again the advertised weights of 3000 to 3500kg seem unrealistic but they may not be. The manufacturer states a maximum capacity but does not specify what you can or can not tow.
Have a look at this old Leyland Brothers DVD and stop it on 1 minute and 24 seconds.
https://www.youtube.com/watch?v=MPXKKFnhqdQ A modern dual cab could tow a trailer like that with a total weight of 3500 kgs but there would be no more than about 20 kg on the tow ball.
The manufacturer says their car can tow something weighing 3500 kg somewhere but they don’t claim it can tow anything that heavy anywhere. It is up to the owner to ask a few questions about both the car and their trailer before buying anything.
Another big killer of ute chassis is the tow ball weight. 4X4 Australia mentions this and says the trailer can jerk the back of the car up and down. It may not always be apparent to the driver but the chassis will notice it. This all comes down to reducing towing capacities as the conditions get rougher. Some manufacturers tell you to do this and the Armed Forces always do it but how many owners have simply looked at the maximum and gone no further thinking they were well within limits?
My ute has a maximum towing capacity of 1800 kg with a maximum ball weight of 180kg. If I bought a camper trailer of say 1250 kgs with a ball weight of 120 kg I should be fine but I would have to keep it on fairly smooth roads. The reason is the specifications also state that the end of the chassis must be lifted by a load leveling device (WDH) for any ball weigh above 90 kg. A WDH is not practical and can cause some damage in the type of conditions camper trailers are often driven in so I can’t use one. If I ignored that direction from the manufacturer, I would be 30 kg overweight on the ball and the chassis could be at risk.
These are just a few of the things that you have to look thoroughly into before you head off into the bush. You can not afford to have too much heavy material way out the back on the end of the chassis. The very best heavier aftermarket springs or air bags are not going to take it away. They will certainly hold it up a little higher but they can not stop the end of the chassis from flexing up and down.
It is also no use saying you have weigh bridge certificates proving you were 250 kgs or whatever under GVM so your bent chassis is the car manufacturer’s fault.
