If you really want to bend your chassis then air bags certainly help but they are not essential. This Hilux does not have any but it does have the one thing that is common with all cars that bend their chassis and that is far too much material too far behind the rear axle.
When the rear wheels drop into a depression in the road, the chassis drops and is brought to a sudden stop by the bump rubbers. Whatever is behind the rubbers wants to keep falling. As it falls the forces generated increase and, I think from memory, it is by the square of the distance that everything is from the axle. This means the chassis tries to pivot on the axle and lift the front.
A split second later the wheels come up out of the depression resulting in the end of the chassis having to catch the falling material and heave it back up again. Imagine what this is doing to the chassis.
The same will happen when the front end drops. The chassis will go down and its rear end will go up. All the material behind the axle will have to be instantly pushed up placing tremendous stress on the chassis.
Whatever you put in your car will weigh something but it is only that weight while it is stationery. Things change dramatically when it moves.
All of this will flex the chassis a little each time then it will return to its original shape. It can only take so much of it though, particularly on rough roads, before metal fatigue sets in and the chassis bends permanently.
If anyone doubts this then try a simple experiment with a hammer. Hold it by its head then extend your arm and the handle straight out in front of you. Move your arm up and down a few times and note how it feels. Now hold the hammer by the end of its handle and move your arm again. You will very quickly feel a build up of stress in your wrist. If you bring your hand holding the hammer down firmly into the open palm of your other hand, the head will want to keep going down and the end of the handle will most likely move a little in your hand.
The weight (pull of gravity) of the hammer has not changed and the amount of material (mass) in it is still the same. The only change is you have moved the greater part of its mass further away from your hand. If you think of your open hand as a bump rubber, you will soon see what is happening to the overloaded end of a chassis.
A major problem with people loading utes is they do not understand the difference between mass and weight. The claim that it was under GVM but the chassis bent therefore it must be a crappy design is often heard. Take two identical dual cabs loaded in different ways for example. One has four large adults and a good sized teenager in the middle of the back seat. The fuel tank is full and the rest of the load needed to take it up to GVM is in the back with the heaviest items as far forward as possible. The other has a 60 kg driver plus a full tank and the rest in the back. One of those cars will be sitting low and level while the other will be on the bump rubbers at the rear yet both are at GVM. One driver will be smiling while the other will be complaining bitterly and rushing to the aftermarket industry to get the rear back up again. One has all the material in it correctly distributed while the other definitely does not.
You simply can't load cars anyway you want to. Everything must be distributed properly. If you can't get enough material into the front of a dual cab then you can't get it up to GVM without running the risk of damaging it.
You could argue that the manufacturer claims it will take 300 kg hanging off the end of the tow bar and that is a long way from the axle. It will but there may well be a few restrictions. The handbook in my Lux says a maximum of 180 kg on the ball but a WDH must be used for anything over 90kg. Toyota is saying the chassis will need a lift but a WDH, which is a lever and not a spring, is the approved way to do it. Others may so no WDH and you can fully load up the seats but don't put very much in the tub or tray.
A WDH lifts the rear end and transfers weight to the front wheels just like the handles on a wheel barrow. Heavier springs or air bags just lift the rear end up higher and don't transfer weight.
If you load your car and find it is not siltting up properly then you have done something wrong and it is time to make a few inquires to find out why. The best place is the manufacturer's customer information service which will be on their web site.
The other issue is air bags. Your springs compress at a predetermined rate. X number of kgs will compress them one inch. Double the weight and they will go down 2 inches. Triple it and they compress 3 inches. They will keep going down at that rate untill something stops them and that is the role of the bump rubbers. They are the last line of defence before the chassis slams into the axle. They will stop the chassis and do it very quickly because they are exponential springs, not linear springs. They will become prorgessively harder as they compress and the more you try and compress them, the more they resist further compression.
Your air bags are also exponential so installing them in your car is like sitting it on oversize bump rubbers. If you have enough pressure in them, they can become very hard and really start resisting further downward movement of the chassis before it would normally reach the stock bump rubber. This results in a better pivot point for the chassis as the back goes down and the front goes up and explains why a lot more air bag cars bend chassis than do those without them..
Heavier spings can do much the same. The Lux in the photo would have to have heavier springs with all that stuff out the back and it has ended up with the worse case of chassis bending that I have ever seen. They just get the chassis rocking up and down at each end a little earlier than it would on stock suspension.
Many will say their modified suspension is working fine and that may well be true but, if the car is not loaded corectly and is outside its design limits, there is always the chance that major problems are slowly but surely developing and it will just take a combination of the right speed and the right type of road conditions to change everything from smooth sailing to disaster.
