Comment: Suspension

More to it than just airbags. It's very common on many dual cabs. If I can add:
- too much weight behind the rear axle - need to keep the heavy stuff forward of the rear axle. Hard to do with a dual cab
- too much weight on the towbar
- heavy aftermarket accessories such as wheel carriers and long range tanks
- towing over desert sanddunes
- There is an inherent problem with the dual cabs in that the cab finishes forward of the rear axle, so the chassis tends to hinge at that point.
- Airbags are meant to "supplement" and fine tune the rear springs and are not a fix for inadequate leaf rear springs.
- Ridiculous pressure recommendations by some manufacturers of leaf spring airbags

Some D40 Navara's have been sold "new" already with a bent chassis, I wonder how they will get on in the future, especially if used for some serious work.
At least 2 before NCAT at the moment.

Fatigue happens to metal as well as humans.

Victor

As the other have said, there is more to it than just air bags. They make chassis bending much easier but plenty of cars have done it without them.

Single and extra cab utes have all been known to bend their chassis but it is much easier with a dual. They are really just a single cab with a back seat added and that is where the trap lies for an unwary owner. If you are putting a lot of weight onto a single, you would put the heaviest items at the front of the tray and over the axle. You can't do that properly with a dual so the result is often too much behind the axle and just a couple of light weight kids or nothing at all in the back seats. If you want to get a dual up to near maximum weight, you need the equivalent lof four rugby forwards plus the half back in the cabin with the rest out the back and even then the heaviest items should be as far forward as possible. If you have a large tow ball weight then reduce the weight in the rear and use a WDH.

The other thing with air bags is few owners seem to know what type of spring they are. They do not compress at the same rate as a leaf or coil spring. Those springs compress in proportion to the load on them. One could have a rating of say 200 lbs for example. That means a weight of 200 lbs will compress the spring 1 inch. 400 lbs will take it down 2 inches. 600 lbs will be 3 inches and so on.

This will continue until the chassis hits the bump rubbers where things change dramatically. The bump rubber must stop the chassis hitting the axle, no ifs buts or maybes. It does it because it is a progressive rate {exponential) spring. The more you try and compress it, the harder it gets and the more it resists further compression. An air bag is the same. It will start getting hard by the time it is around 40% compressed and will be like a rock between 60 to 70%. This is why they make it so much easier to bend a chassis.

A higher rate leaf spring still does not get you completely out of trouble if you have too much weight behind the axle as many owners have found out. When your wheels drop into a depression in the road, the chassis falls. It does not matter whether it is brought to a stop by stock springs and bump rubbers, heavier aftermarket springs or air bags, whatever is behind the axle is unrestrained and will want to keep falling. The forces generated increase by the square of the distance all the weight is behind the axle and will be well above the static weight. It slams the rear of the chassis down, rocks it on the axle and tries to lift the front. If all of this takes the chassis beyond its design limits, it will eventually bend.

In addition to spring "rate" there is spring "load" and "wheel rate". Spring "load" is a measure of the amount of weight a spring will support at a certain height. "Wheel rate" has a lot to do with the distance a spring is from the wheel. The further away a wheel is, the less the spring will compress when the wheel rolls up over a bump so all of this is taken into account when suspensions are designed.

You then have weight transfer in corners which can determine whether the car understeers or oversteers plus front to rear tyre slip angle ratios and heaven only knows what else.

The point is the factory engineers know exactly what their suspensions will and won't do. When an owner changes the design, it helps if they also know exactly what is going to happen in all aspects of the car's handling.

thanks all some interesting reading there fairly happy with they navs height loaded with wdh fitted and seems to tow well at standard suspension at present
but will be looking at where the tongue sits for sure
I have also had to turn the touhge upside down to gain correct towing height
if anyone has thoughts on that
perhaps some better shocks may be in the pipeline eventually

I think it is possible to bend or break a chassis even without airbags. Downward forces are applied to the chassis where the body rests on the chassis, and on the tow bar. Upward force is applied where the suspension attaches to the chassis. In a simple coil and shock arrangement this can be at a single bracket usually where the chassis bends up over the axle. At least with leaf springs the upward force is at each end of the spring. If you add an air bag this adds a third point of upward force. If the downward forces are great enough, and far enough away from where the upward force is applied, the chassis will buckle. I recently had reinforcing plates welded to my chassis to overcome this problem.

Bob

How about some pics of how it was loaded and what pressures you usually ran?