Comment: Suspension

Hi
We have just had our Ranger 4x4 2007 chassis fixed after it completely broke. Reason -in 2010 we bent the chassis because of air bags fitted . The vehicle was fixed in WA but 2014 broke. The first fix was not in line. Please be careful and be aware you can void your warranty on the chassis of your vehicle when fitting air bags with leaf springs.
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Reply By: Member - Phil G (SA) - Tuesday, May 13, 2014 at 21:29

Tuesday, May 13, 2014 at 21:29
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
AnswerID: 532419

Follow Up By: John and Regina M - Tuesday, May 13, 2014 at 21:41

Tuesday, May 13, 2014 at 21:41
Yes, a well documented issue with the latter model twin cabs.
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Reply By: Ross M - Tuesday, May 13, 2014 at 22:07

Tuesday, May 13, 2014 at 22:07
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.
AnswerID: 532425

Follow Up By: tom 2 - Tuesday, May 13, 2014 at 23:22

Tuesday, May 13, 2014 at 23:22
gday ross do you have or know were I can get any further info on these d40 chassis issues
just purchased 2014 d40 to tow a 18 ft van and had considered air bags but that's obviously a bad idea
thanks
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Follow Up By: Nomadic Navara - Wednesday, May 14, 2014 at 00:01

Wednesday, May 14, 2014 at 00:01
The chassis always break in a downward direction. If you fit WDH this relieves the downward stresses on the chassis. I am yet to hear of chassis breaking when towing with WDH.
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Follow Up By: Ross M - Wednesday, May 14, 2014 at 10:30

Wednesday, May 14, 2014 at 10:30
Nomadic Navara
Earlier Navaras used to break the towbar OFF the chassis until they changed the mounting system.

Tom 2
An 18ft van correctly loaded and the ute the same should be ok.
I would make sure you have rear springs which cater for the load and try to avoid airbags. That will then retain some or all of the ride height and give you a suspension which isn't closed up and likely to bottom.
Bottoming will bend a chassis quite easily as the van load hits.
The idea is to have compliant suspension with sufficient upward travel, which controls the dynamic load forces.
The OE shocks will most likely be not much good at the control of the van forces. Something not noticed on a smooth road but in rough the OE shocks will show their deficiencies.
Compliance and absorbing of forces is the name of the game.

The D40 seems to bend about 300mm rearwards of the front spring hanger, close to the shocker upper mount point.
It is there the forces are concentrated and IF too much it will bend like most others.

Another very obvious thing is, many towbars used with vans have too much of the tow neck sticking out the rear of the actual towbar
That increases the load on the chassis, springs, axle and lifts the front more, than it should. So short as possible behind the axle for the ball position is best.
I have redrilled quite a few so the ball load isn't being MAGNIFIED into the chassis. Some of the tow system sold are simply madness in the way they don't consider all the forces.
They are certified of course, but that is no good after the crash.

I have recently been involved with D40, bent from new, NOT LOAD RELATED at all, and it appears to have slight depressions on the inner chassis face close to the shocker point and the chassis tails are uneven in their relative heights.


I agree with Peter D, if a WDH is used sensibly and the towbar forces are correctly spread by it's attachments, then it applies the load in the opposite direction to the normal bending, rear downward forces.
It all makes the chassis work harder than normal though.

There is a dualcab D40 near me and it's tray is very down at the back, even the dog would slide off. Just a load has caused the bend because to much weight applied behind the axle and the middle goes up.
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Follow Up By: Nomadic Navara - Wednesday, May 14, 2014 at 12:02

Wednesday, May 14, 2014 at 12:02
Quote "Earlier Navaras used to break the towbar OFF the chassis until they changed the mounting system."

Ross, I think you have it the wrong way around. The problem was the tow bars. They only had 2 cap screws holding them. Two fasteners was insufficient to hole the bar, one screw broke each side and the force or the bar being held by that one screw bent the chassis. The chassis all had 6 captive nuts to accommodate up to 6 screws so it was the poor design of the bar and not the chassis that was the problem.

I have one of those early D40 chassis. My ARB supplied protection bar and hitch receiver used all 6 holes. I also have WDH. There is no sign of chassis problems, the vehicle has traveled nearly 200,000 km of which probably over half of that has been towing a 2 t weight van.

The failure of the early Nissan OEM bars (particularly on the Pathfinder) was what lead Nissan to declare that they do not recommend the use of WDH with their tow bars. It was cheaper to condemn the use of WDH than admit their bars were RS. Their new bars are more in tune with the likes of Hayman Reese ones. If you have any concerns with your OEM bar carrying the stress of WDH then change it to one where the manufacturer will warrant the use of their product with WDH. What ever you do, make sure you have WDH fitted for open road use, this will take most of the sag out of the rear suspension and compensate for the downward force from the hitch.


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Follow Up By: Nomadic Navara - Wednesday, May 14, 2014 at 12:12

Wednesday, May 14, 2014 at 12:12
Bugger, no edit feature. The second paragraph should commence:

Ross, I think you have it the wrong way around. The problem was the tow bars. They only had 2 cap screws holding them each side. Two fasteners each side was insufficient to hole the bar,

Also down lower where I am talking about 6 captive nuts, that should read 6 captive nuts each side. The 2 paragraphs should thus read:

Ross, I think you have it the wrong way around. The problem was the tow bars. They only had 2 cap screws holding them each side. Four fasteners was insufficient to hold the bar, one screw broke each side and the force or the bar being held by that one remaining screw bent the chassis. The chassis all had 6 captive nuts each side to accommodate up to 12 screws so it was the poor design of the bar and not the chassis that was the problem.

I have one of those early D40 chassis. My ARB supplied protection bar and hitch receiver used all 6 holes each side. I also have WDH. There is no sign of chassis problems, the vehicle has traveled nearly 200,000 km of which probably over half of that has been towing a 2 t weight van.


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Reply By: 671 - Wednesday, May 14, 2014 at 19:03

Wednesday, May 14, 2014 at 19:03
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.
AnswerID: 532459

Reply By: tom 2 - Wednesday, May 14, 2014 at 22:21

Wednesday, May 14, 2014 at 22:21
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
AnswerID: 532470

Reply By: Echucan Bob - Wednesday, May 14, 2014 at 22:29

Wednesday, May 14, 2014 at 22:29
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
AnswerID: 532471

Follow Up By: 671 - Thursday, May 15, 2014 at 09:36

Thursday, May 15, 2014 at 09:36
I recently had reinforcing plates welded to my chassis to overcome this problem.
.........................

Bob
Even doing that may not solve the problem. There was a DVD on Utube recently that has since been removed that showed an F250 tow truck in the US trying to pull a Land Rover out of a bog. Another truck was pulling the Ford from the front. Had the towie attached a chain from his chassis back to the Landie, it would have worked. Instead he used the cable from the top of the jib on his truck. As he pulled, the cable pulled the top of the jib back like a giant lever resulting in the front wheels of the truck lifting high off the ground. Its chassis slowly bent just forward of the front hangers on the rear spring while you watched it.

I saw exactly the same thing happen on a car race track in the 1960s when a tow truck attempted to pull apart a couple of crashed cars that were locked together. He ran the lifting cable back from the top of the jib and pulled. The front wheels lifted way up into the air and the chassis bent immediately in front of thousands of cheering people.

These two cases involved extreme forces and instant bending. Tail heavy utes take their time. As they drive along rough roads, the rear end of the chassis is flexing up and down ever so slightly until metal fatigue sets in and the next bump results in a permanent bend.

It does not matter what type of rear suspension you have, the axle becomes the pivot point when the rear of the chassis goes down under excessive forces and the front goes up.

To be sure the chassis is not going to bend, an owner would need the chassis design blue prints in front of him and the knowledge to calculate the exact forces involved to be certain everything is ok.

Without that you are on your own and you will not know if the chassis is at risk until it bends.
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FollowupID: 815771

Reply By: gbc - Thursday, May 15, 2014 at 05:38

Thursday, May 15, 2014 at 05:38
How about some pics of how it was loaded and what pressures you usually ran?
AnswerID: 532480

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