Polyair levelling against Weight Distributing Hitches.

Yes, I remember that thread. Obviously, Polyairs are good at lifting the back end, whether the load comes from the drawbar or within the vehicle. Equally obviously, if there is too much weight on the back end through excessive towball load, it makes more sense to shift the weight to the front axle of the tow vehicle via a hitch.
Some people seem to be willing to go to extraordinary lengths to avoid actually finding out what the load is on their drawbars (not to mention the real ATM of their vans (not the fictional value used by others including van manufacturers).
You are absolutely right: it is basic mechanics that if there is too much weight on the back end, lifting up the rear suspension cannot possibly have any impact on weight distribution or the rear axle loading, or on the light front end that so often sows the seeds for a disaster.

Yes, that's a very important question. I have seen views ranging from "certainly not" to "definitely yes". My own ears have told me that a hitched rig starts to protest, even with a 'normal' 50 mm ball, at quite modest degrees of articulation. But I must let others who know better respond on this point.

I use both polyair bags and wdh as a means of levelling my caravan and to support my vehicles suspension particularly on some of the disgusting roads we have. (The Warrego Highway to mention one it is a disgrace.)

However my experience is that if the van is level the vehicle will be level PROVIDED that both are not overloaded.

My belief is that a WDH is a means of levelling the van not the vehicle by reducing the drawbar load. However, you need to be very careful how much tension is placed on those hitches, if there is too much tension they will not flex very much and there will be immense tension on the vehicles towing hitch and ball.

I came across a fellow a few years ago who had snapped the towball bolt simply because he had placed too much tension on the WDH to get the vehicle level. Needless to say his van was overloaded with more than the recommended 10% on the drawbar

My understanding of the WDH is to place more weight on the front tyres WHEN the vans drawbar load is 10% of the van's gross weight, that is, the weight stamped on it's plaque. If the van is overloaded then technically those specs have been breached and the tow is illegal. It would also be likely that the vehicle's tow ball weight is also exceeded.

So it is very important to take note of the vehicles tow ball weight specifications and that includes both the van's draw bar load and the load placed in the vehicle. These added together should not exceed the vehicle makers carrying capacity.

Polyair emphasize that point and also state that the vehicle should not be raised by inflating the bags to normal levels. Measurements are taken of the height of the unladen vehicle at the rear bumper to ground behind both left and right rear tyres. The van should be hitched then raised so that the previous measurements are achieved, I use the jockey wheel, then inflate the air bags to around 15psi then lower the jockey wheel until the van is supported by the the vehicle.

This is generally enough to achieve the previous unladen measurements if not I repeat the process until this is achieved.

The fact of the matter is that my experience is that polyairs WILL level a vehicle to it's unladen measurements without any need to inflate them with high psi. Additionally they support a vehicles suspension without the need to replace standard springs with heavy duty ones which do nothing more than give the vehicle a harsh ride when unladen.

After this process I hook up the WDH with light tension thereby lessening the upward load on the towball and the vehicles towing hitch.

As far as I'm concerned my experience has been that both the polyairs and WDH complement each other and I do not agree with the findings in the article you refer to. I have used them on 2 vehicles now with WDH and have had the same experience.

When I deflate the airbags after towing the ride and handling is the same as before. In fact when towing the ride is of the same quality, soft and smooth and the handling first class.

That article is no different to vehicle tests some come up with findings that another magazine will not find and for that matter the person who decides to buy a vehicle where there have been negative findings have said that it is the best vehicle they have ever driven.

I'm afraid I don't take any notice of these sorts of articles I'd rather find out for myself.

Yes and with the greatest respect to you there is none so ignorant as those who make comments about subjects they have had no experience with.

The fact of the matter is I do measure the front height and it is the same as normal, so you clearly do not know what you are talking about mate.

I have used PolyAir bags for the past 20 yrs. My understanding of them is that you should inflate them to maximum pressure BEFORE you place a load on the rear (towbar) and once connected adjust your WDH and then the air pressure in the air bags until the rear suspension is at level as possible. I was told by PolyAir that even though the bags are probably able to lift a loaded vehicle it is not recommended.

None so blind as those that wont see ,, your theorem was flawed then and still is now , lift the BODY of a vehicle at the rear with polyairs / airbags transfers weight to the front , no it does not transfer UNSPRUNG weight , rave on all you like about pivot points ,the facts are that lifting the SPRUNG weight at the rear transfers a % of that weight forward.

For the non believers read tis from the Caravan Forum on Myths about towing



MYTH: Heavy caravans do not need weight distributing hitches if the tow vehicle's rear springs are stiffened.

False. This shows a total and dangerous misunderstanding of basic mechanics. The effect of that weight on the car is the same as pushing down on the handles of a wheelbarrow: the front must lift. There is every argument for stiffening that rear suspension and fitting better dampers, but it makes no difference at all to the lifting effect. That WDH actually levers the front of the car down again.


Exactly my point.

Once again you are wrong as I have both Polyairs and Progressive springs and have tried the method you advocate and have been to a weighbridge and have found that although the vehicle appears level with only the polyairs it has not returned the original weight to the front wheels due to the resistance of the front springs holding the reduced weight up.
I did not write the last post It was cut and pasted from the Caravanners forum and reputedly written by an engineer who understands physics.
You do what you like but I will stick with my proven safe method.