AIR BAGS / WDH. D-MAX

My understanding is that air bags on leaf sprung vehicles could cause the chassis to crack, especially with weight at the rear. You move the load point from the 2 spring hangers to one central point not intended to be load bearing. I think there are a couple of photos around the place.

Cheers

Mark

Warren
These may be of interest Link below
Super Spring

Site Link

I have a Colorado 2011 LTR & took measurements when new with the bottom of the towbar 345mm above GL
Fitted a long range tank 130lts, canopy & a small amount of gear in the back with tank full dropped the arse end by 35mm, now towbar sitting 310mm above G/L.
Fitted the super spring which has two settings (set on heavy setting) & with a full tank bottom of towbar measured 390mm.
My ball weight when towing is around 260-280kg which dropped the springs approx 40mm.
Just returned from a 2,000km trip with van on back & they performed brilliantly. The beauty is they only come into play when you put the weight on & don’t seem to affect the ride when empty. Cost $520 & fitted myself in approx 1hr.
After the springs had settled bottom of towbar is approx 380mm from GL.
So far pleased with the performance.
I have been told you can use them with a lift kit but would pay to check.

Cheers Mick

Warren

There has been no end of discussions on the net about air bags and bent chassis. Last week I found an article on their use in motor homes in a magazine in a barber's shop. It was written by former General Motors vehicle dynamics research engineer Collyn Rivers. It goes a long way towards explaining why they cause so much trouble.

They are an exponential spring. That means they can get very hard when compressed a long way. That often happens with heavy weights behind the vehicle's rear axle and in front of the trailer/van axles. A steel spring is lineal and has a "rate" which is a measure of how much weight is required to compress it 1 inch. A 200 lb spring for example compresses 1 inch with 200 lbs on it, 2 inches with 400 lbs, 3 inches with 600 and so on. As you add weight to your ute, the spring just keeps bending at its designed rate until it reaches the rubber bump stop. The rubbers are progressive rate springs which is why they are always tapered or hollow etc. They keep absorbing the weight in a progressive manner if you keep adding more. You now have the chassis sitting on six load bearing points and anyone who puts any more onto it after that deserves whatever happens.

The air bag just gets harder and is probably a bit like slamming the chassis down onto a block of wood if the wheels drop down into a deep depression or wash away in the road. This is when most chassis seem bend.The springs can't help much because the air bag has lifted a lot of the weight off them. The bump rubber is not there any more to assist. It has also created a new single load bearing point in a section of the chassis not designed for it.

The following extract is a forum reply from Collyn to an air bag question. The whole discussion is in the first link below.

The other three are also from Collyn and are on van loading and stability. You may find the answer to your problem there.

You have a lot of weight on your tow ball and it must be getting close to maximum for that particular car. Most manufacturers advertise the maximum weight a car can tow in good highway conditions. They usually recommend reducing it in off road conditions so that is something you could check on.

In some of those links Collyn refers to the difference between weight and mass. The weight on your tow ball is static. Mass comes into it when the car is bouncing over rough surfaces or the van is swaying or pitching. The forces generated far exceed the static weight on the tow ball.

Enter the words dual cab chassis bending or a similar combination of those words into Google and you will soon find a lot of discussion and photos on the subject.

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by Collyn Rivers » Thu Feb 24, 2011 3:49 pm
There appears to be any amount of confusion re the use of air bags.

They are really a substitute for other forms of springing, or a way of changing ride characterists (such for example, to adjustably soften the ride of an unladen 4WD or other ute - but they are not for fixing sagging suspension nor to increase ride height, nor to enable you to carry more weight.

Be aware that fitting them at one or other end only will alter vehicle handling.

Also be aware that their weight/deflection characteristic is exponential - whereas that of a coil spring is linear. This totally changes how the vehicle rides - and may cause very heavy impact loads if used close to their maximum travel.

They do not remotely, any more or less than any other form of suspension, substitute for a WDH. That is a lever, and air bag is a spring.

I am currently working on an article re this for trailer users. I already have one in the CMCA The Wanderer (this month), but mainly re motorhomes, that I shall place on my own website shortly.

Collyn
Author/publisher of books and articles in RV and solar areas.
Collyn Rivers
Posts: 1796
Joined: Sat Feb 14, 2009 4:01 pm
Location: Broome WA
Website

http://caravanersforum.com/viewtopic.php?p=214130&sid=eaeec33bd38ef004c414bc731d1aed59#p214130

http://www.aroundoz.com/a_clinic_archive/towing_couplings_levellers/towing_collyn_rivers.htm

http://www.caravanandmotorhomebooks.com/pdf/vehicle_dynamics_complete.pdf

http://www.noboundaries.com.au/Safety/caravan_stability.htm

G'day Warren B
It sounds like you have no or little suspension travel left after hooking up the van.
With all the weight you have on the ball it makes it even more necessary to have the springs upgraded so when the weight is being carried you still have enough suspension upward travel left to absorb the road conditions.

Now a bold statement.
NO Dmax or Couradoo ever made has good suspension as standard and although when std they will carry a weight, like most of these breeds, they won't carry the load of a van and have enough suspension travel left. SO YOU GET KICKED IN THE BUM.
You have to make the loaded vehicle ride higher in the first place with upgraded springs. No other way.
I own a Dmax and it doesn't really have suspension worth talking about.
Made to a price not a capability, like all the others.
If you get upgraded springs, make sure they don't increase the driveline angles of the unis joints. Many aftermarket springs increase the drive angles, by poor production and design, and this causes vibes in driveline and short life on unis.
Any brand features here.