I've heard about dual cabs and bending chassis. From the research I've done I have concluded that many of these cases have been the result of overloading and airbags.
The main problems are incorrect loading, leverage and the forces generated by mass in motion. As an example, imagine you have a 2kg axe in your hand. Hold it by the head with the handle sticking out in front of you in a horizontal position. It will be very easy to hold it that way. Now turn it around and try and do the same thing while holding the end of the handle. This time it won't be easy. Your wrist will be under a lot of stress and you most likely won't be able to keep it horizontal. The weight of the axe is still only 2kg but you have moved the greater part of the material (mass) in the axe i.e the head, further away from your hand. In other words you have put it out on the end of a lever
Now go one step further and stand on the bottom step of a staircase while still trying to hold the axe by the end of the handle. Step quickly off the step and note the increased strain on your wrist and arm as your feet touch the ground. That is the result of you and the axle falling but while you stopped suddenly, there was nothing under the end of the axle handle to stop the heavy head from wanting to keep going down.
That is what happens to the end of a ute chassis when material is placed behind the rear axle. All of it is sitting on a lever which increases in length the further each item is from the axle. All of this is fine while the car is sitting still but everything changes the moment it moves. As the front and rear wheels rise and fall over undulating road surfaces, the material behind the axle is constantly being lifted rapidly or stopped suddenly when it falls. Springs. air bags or even a block of wood between the axle and chassis can't stop the stress on the chassis in the area around the rear axle. Its end is flexing up and down like a diving board and there is nothing back there to support it. The springs or air bags just compress and tilt with the chassis while its front and rear end go up and down as it see saws on the rear axle. If the forces generated by all of this exceed the design limits of the chassis, it will eventually bend.
The other issue is loading. Most owners forget a dual cab has been designed to carry five people in the cabin. That means a fully loaded ute should have upwards of 450 kg in the cabin. There is no point in the manufacturer designing the cabin to carry no more than the combined weight of say an 80kg man, a 60 kg woman and three little kids under eight. A family like that may not even get to 200kgs. It has to be five heavy adults plus all of their clothing etc.
This means you can't take the car up to its maximum capacity unless you can fully load the front with either people or people plus something else. This often results in many owners having too little up front and too much down the back resulting in the chassis bending even though the car was under its maximum weight.
Sagging: Don't make the mistake of thinking a near flat rear spring is sagging as you load the car, it is supposed to do that. They are all designed with the front section short and mounted down low and the rear long and up high. As the loaded car leans into a corner, the springs change the angle of the axle to assist in stability. It is a form of rear wheel steering. This link covers semi trailing arm rear suspension but the same principle applies to live axle leaf sprung cars.
https://www.civilengineeringhandbook.tk/vehicle-technology/1082-suspension-roll-steer.htmlOff road towing: It might not be a bad idea to discuss what you want to tow and where with Mitsubishi. This email is a few years old but the information will most likely apply to current models and other makes.
We acknowledge receipt and thank you for your recent enquiry regarding the Mitsubishi
Triton.
Please be advised that Mitsubishi Motors Australia Ltd (MMAL) only release towing
specifications for vehicles being used under normal conditions such as highway driving. We
would not recommend towing this amount while driving on 4WD tracks or surfaces of this
nature. The 3 Tonne towing capacity is a statement of the maximum permissible towing
weight possible for this vehicle.
Thank you again for your enquiry.
Kind Regards,
Simon
Mitsubishi Customer Assistance Centre
Mitsubishi Motors Australia Ltd
GPO Box 1851 Adelaide SA 5001
These popular volume selling utes are built to a price and are not bordering on being indestructible like heavy trucks. Their maximum towing and carrying capacities are usually for good sealed roads only and should be reduced as the conditions get rougher. If you ignore this and turn to the aftermarket industry to upgrade things, you might get away with it but many don't.
