Navara D40 rear suspension.

"Not much weight on it"

What about the canopy and boat, Kirk? They are going to add some too.

Try airbags at your peril, I'd suggest. Talk to Dobinsons in Rockhampton, and get a set of constant duty spring packs, say 300-400kg. Might ride a bit rough empty, but should do the job loaded.

Bob

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Hi Kirk mate of mine had the same problem with his ford ranger does a bit of 4wd and tows a horse float put Air bags on it sits level and has had no problems in the last 2 years TJM fitted them.

I have done research on airbags and have checked out the cracked chassis problem and it seems air bags don't cause this but overloading or severe use.
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Kirk

This is what causes bent chassis. donkeyIt is not so much overloading but incorrect loading that does it. The problem is too much weight too far back behind the axle.


In these photos of carts lifting donkeys, you could put all the heavy springs and air bags that you can find between the carts and their axles and the back will still go down and the front will still go up.

If you replaced the donkeys with a Clydesdale horse, the horse would most likely remain on the ground. If you then pulled the cart over rough ground with it bouncing up and down, all of the heavy material behind the axle may eventually snap the cart in half at the axle. This is what happens to car chassis.

This is where so many car owners come unstuck. They think lifting a sagging rear end with an aftermarket product is all they have to do and their problems are over. Unfortunately the excess material down the back is still there and the chassis is still at risk.

The only solution is move some of the material forward or take it out of the car.

In your case what you have in the back of the car combined with that ball weight so far back behind the axle is the reason your suspension is sagging. If it was placed directly above the axle, the leverage effect would be eliminated and the car would be sitting up a little higher.

There is nothing wrong with the original suspension when the car is loaded correctly. When fully loaded the springs are supposed to be flat with the front eye of the spring down to the same height of the axle or lower This will then result in some degree of rear wheel steering in corners which increases stability.

The second last and last post with the small photo in this link roll steer covers it briefly.


This link explains it in more detail roll steer drawings It is on semi trailing arm suspension. Read the first section and look at the three drawings. Just about all cars are set up like the centre drawing, leaf springs included.

Kirk,
You will continue to have the problem while ever the load is so far behind the rear axle! It is the same with all the dualcab utes.
If you put heavier springs or airbags on it you will just stiffen the chassis from the front spring mount back but there is still all that down force BEHIND the axle. That down force behind the axle makes the chassis act like a see-saw and the corresponding weight comes off the front until the chassis bends. Simple physics stuff.
The correct way and I believe the only way to fix this is to put the rear axle under the load by having the chassis lengthened by about 500-600 mm. I know it sounds a bit extreme and expensive but I have been through this exact problem with a semi goverment Ford Ranger which was loaded just under it's GVM. It was a dog to drive! I drove a similar ute that had been lenghtened and the difference was amazing. All the jerking (as the chassis flexed uncontrolabely) was gone, the front wheels felt like they were actually on the ground and the rear springs could easily carry the weight because the front wheels were now doing their share.
From memory the cost to lenghten the chassis was about $6500 which included the engineering certificate.
You will lose a bit of ramp over clearance but not much.
As you are out and about on your daily driving keep an eye out for dual cab utes of any brand and you will see loaded ones that look a bit "bent" but maybe not as much as the one below.
A good example of the correct set up is the Landcruiser dual cab ute/trayback. The axle is under the tray, not right at the front of the tray.
Anyway, best of luck with what ever you do.
Ian

Before adding air bags, read this article.

No matter what anyone says about who loads what where and by how much, air bags put point loading on the chassis in a place that was never designed for it.

I have a a Hilux 2004 SR5, I've added a lot of accessories including full length Black Widow Drawers, Kaymar Rear Bar, ARB canopy plus a 6x4 loweline roofrack. I've just updated with Tough Dog Suspension. Leaves are 500kg constant with Adjustable Shocks with 9 settings with about a 700% range according to Tough Dog from 1~9...

Well I couldn't be more happier so far, had them on for 5 months. I had it fitted all around...


Cheers..

G'day. I have a 2006 D40 on which I had suspension upgraded in 2012 by Outback Suspension...unfortunately located at Factory 2/43 Vinter Ave, Croydon VIC 3136 [03] 9723 3809.

I went to them on the recommendation of my next door neighbour...an ex-truckie/truck mechanic. He had his first 4x4 done around 12 years ago & with much trepidation handed over a large slab of money. When he came to replace his 4x4, he booked the brand new replacement in for an upgrade before pick up, something which he has done with each replacement vehicle ever since.

Essentially what happens is:
1. rear bump stop cut off the axle & welded to the frame rail above the axle facing downwards.
2. a set of King Springs [made to Outback Suspenion specs...I believe they are what he calls '100 lb' springs] set over the rear axle aligned with the relocated bump stop.
3. front springs replaced with a 30/40mm lift King Springs
4. Koni 80 series shock absorbers fitted all round.

The cost was around $2200 but did not include replacing the shocks but did include some front end suspension parts which were damaged as a result of a previous suspension upgrade attempt. I already had Koni 90 series shocks fitted.

The first trip after fitment was from Maffra to Alice Springs via Binns Track; Binns Track to Katherine then to Kalkarindji [Wave Hill], Townsville, Rockhampton, Longreach, Winton, Blackall, Camerons Corner, Broken Hill, Mildura then home.

On the Alice Springs to Katherine via Binns Track leg, the couple with whom I was travelling rolled & wrote off their Tvan. I was already quite loaded but took a 6 foot x 4 foot x 1 foot bag of clothes [for the community op shop at Kalkarindji] from them as they needed to salvage stuff from the van. I can only assume I was 'near' [I suspect over] GVM but the Navara did not sag at the rear & handled flawlessly.

May be worth a phone call to get more details.

Regards
Ian