Polyair levelling against Weight Distributing Hitches.

Submitted: Wednesday, Jun 10, 2009 at 18:05
ThreadID: 69692 Views:5219 Replies:8 FollowUps:6
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Some months ago there was a thread which I cant find in which there was fierce debate of the merits of either.
I contended and still do that Polyairs dont and cant level a car with a van hitched up.
There was a poster who argued the opposite.

In issue 128 of Caravan & Motorhome on page 168 there is an article regarding this. The end of it all is that it supports my argument. The paragraph at the end of page 171 sums it up.
In the article they used a Landy ( whyever for) with self levelling suspension which does and can level itself via front and rear airbags.
The last paragraph states that it would not work nearly as well with AIRBAG ASSISTED as the front springs would prevent the height returning back to normal.
There are also graphs to show the effect of before, hitched without and with a WDH.
I rest my case







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Reply By: Tenpounder (SA) - Wednesday, Jun 10, 2009 at 18:31

Wednesday, Jun 10, 2009 at 18:31
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.
AnswerID: 369365

Follow Up By: Member - Cram (Newcastle NSW) - Wednesday, Jun 10, 2009 at 18:44

Wednesday, Jun 10, 2009 at 18:44
In relation to weight distribution hitches can they be used with tregg/trigg style couplings?
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Follow Up By: GerryP - Wednesday, Jun 10, 2009 at 21:56

Wednesday, Jun 10, 2009 at 21:56
Yes, they most definitely can. Just need to watch if you're going through creek beds and the like as there can be added strain put on the bars if articulation is severe - in these cases it's best to take them off in any case.

The only negative is that the extra downforce on the coupling tends to wear the block more quickly than would otherwise be the case.

I have used a WDH with a Treg hitch for years and it certainly makes it a much better rig to tow.

Cheers
Gerry
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FollowupID: 636834

Reply By: Tenpounder (SA) - Wednesday, Jun 10, 2009 at 19:10

Wednesday, Jun 10, 2009 at 19:10
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.
AnswerID: 369380

Reply By: jeepthing - Wednesday, Jun 10, 2009 at 22:04

Wednesday, Jun 10, 2009 at 22:04
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.

AnswerID: 369426

Follow Up By: Member - Graham H (QLD) - Wednesday, Jun 10, 2009 at 22:34

Wednesday, Jun 10, 2009 at 22:34
Its simply physics If you place a load on the towbar behind the axle the front MUST lift as it swivels on the rear suspension. Lifting the rear suspension height only does just that.
However applying a straightening force at the joint between the car and the van shifts some weight both onto the front wheels and a little to the van wheels.
Read the chart in the article regarding before and after on the weighbridge the facts are inescapable.
I have both and also have progressive springs in the rear.
I find it hard to believe you find no difference in the ride whether towing or not.
I certainly can with nearly 3 tonne on the back and it is correctly loaded and weighed.
Incidentally you are incorrect to say that it is lillegal to have more than 10% ball weight .
Ther is NO LEGAL REQUIREMENT it is only a recommendation.
You also dont bother to measure your front guard height and if you did you would find with only Polyairs the would not come back to the unhitched height
I have no wish to get ito further argument excpetto say none are so blind as those who do not wish to see.
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Reply By: jeepthing - Thursday, Jun 11, 2009 at 07:13

Thursday, Jun 11, 2009 at 07:13
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.
AnswerID: 369451

Reply By: DIO - Thursday, Jun 11, 2009 at 08:57

Thursday, Jun 11, 2009 at 08:57
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.
AnswerID: 369459

Follow Up By: Andrew-rodeo - Thursday, Jun 11, 2009 at 23:00

Thursday, Jun 11, 2009 at 23:00
Spot on DIO, the instructions in the box when you buy PolyAir bags states they are designed to support the load not to raise the vehicle. ie. pump them up, load the car, adjust pressures to suit. I would have thought caravans/trailers should be loaded with as much load as possible over the axle with weight distributed evenly so as not to have too much weight on the tow ball.
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Reply By: Alloy c/t - Thursday, Jun 11, 2009 at 09:03

Thursday, Jun 11, 2009 at 09:03
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.
AnswerID: 369460

Follow Up By: Member - Graham H (QLD) - Thursday, Jun 11, 2009 at 10:09

Thursday, Jun 11, 2009 at 10:09
It possibly will but I am talking about a vehicle with a dirty big weight holding the back end down.
Read the article the weights are published there to prove it.

And its not a theory its a fact.
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Reply By: Member - Graham H (QLD) - Friday, Jun 12, 2009 at 12:22

Friday, Jun 12, 2009 at 12:22
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.
AnswerID: 369657

Follow Up By: jeepthing - Friday, Jun 12, 2009 at 20:48

Friday, Jun 12, 2009 at 20:48
"That WDH levers the front of the car down again"

Yes you're right on that point because there's and upward force,and that lifting force occurs at the end of the towbar of the vehicle. Now you have said you were talking about a "dirty big weight holding the rear of vehicle down".

When you use WDH on their own to lift that dirty big weight there is enormous forces being exerted at the towbar or depending on the type of WDH being used on the towball. As I said before I have seen the bolt on a towball snap because too much force was being exerted by the WDH, they are not a secondary spring to lift the front end of van because it has been loaded with too much weight at the front and or the rear of the vehicle that has been overloaded.

They are a leveling device and have limitations with respect to the lifting of weight to achieve desired leveling.

The use of polyairs inside a vehicles coil springs has the same upward force and does, whether you wish to be blind or not and cannot accept you are wrong, transfer weight to the front end of the vehicle in a more balanced way than a WDH alone because the WDH is doing that from the centre of the rear of the vehicle.

You have no first hand experience in the use of polyairs yet you persist in making these spurious contentions. All you are doing is relying on something that is written on a caravan forum, those comments are wrong if they are saying what you are contending.

I can tell you this from experience and the measurements I do when I set up my polyairs. I take measurements at both the front and rear tyres from top dead centre of each tyre to the top of the wheel arch. I also do measurements at the rear of the vehicle. This is performed BEFORE I load the vehicle and hitch my van.

Then I raise the rear of the vehicle with van hitched by using the van's jockey wheel until I achieve those same measurements then I inflate the airbags and I now know from experience that 15psi is generally the right pressure. When I detach the jockey wheel those measurements are generally maintained at 15psi if not I do the exercise again and add a bit more pressure.

The fact of the matter is that the polyairs DO what you say they won't!!

After achieving this with the polyairs I add my WDH with enough force to only slightly raise the rear of my vehicle THEN I let a little air out of the polyairs to get back to those same measurements. I do this to ensure the WDH has sufficient upward force to work efficiently.

In addition I add the Sway Control. I have found from experience that the 3 systems work together to provide a very safe tow.

I didn't think you wanted to argue about this anymore but if you want to continue with your spurious comments I will continue to entertain you.

You are wrong!!





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Reply By: Member - Graham H (QLD) - Friday, Jun 12, 2009 at 21:34

Friday, Jun 12, 2009 at 21:34
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.




AnswerID: 369743

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