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Fifth (5th) Wheeler Stability

Fifth Wheeler Stability
 

There have been concerns over fifth wheeler stability so the following is reproduced to assist fifth wheeler owners to gain a better understanding of the issues involved.  The article also goes some way to explain the difference between fifth-wheeler and conventional caravan stability.

 

Lets-Getway.com has no relationship, commercial or otherwise with Collyn Rivers or Glenn Portch.

Reproduced with kind permission from Glenn Portch N10690 and Collyn Rivers W8054. This article was originally printed in the June 2009 edition of The Wanderer.

‘One of the strong advantages of fifth wheel caravans is that the better designed and engineered units are a pleasure to tow.  They are potentially more stable than conventional caravans.  This achievable stability is not just a qualitative (touchy feely) perception.  It is achievable both theoretically and practically.

To achieve this, they must, as do semi-trailers, have most of their weight over their rear wheels.  Given that, they act (when subject to side winds etc) very much as do pendulums, i.e. remove the disturbing side force and swinging automatically dies down.

Designed correctly, a fifth wheel caravan does this inherently, but no matter how well designed, a conventional caravan cannot and does not self-correct once beyond a certain speed (specific to each rig).  With the latter, a disturbing force can and sometimes does act much as a fuse with dynamite.

Once triggered the effect continues and may escalate if the energy needed is available (which with a caravan is kinetic and related to speed).  Above a certain critical speed, swaying will not only continue but build up and may (and sometimes does) overwhelm the rig even after that initial disturbing force has ceased.  It is now in what is known as a positive feedback loop.

Like turning down the volume on a suddenly howling PA (public address) system, the only way out is to reduce the energy source that keeps it going (speed).  But this is not always humanly possible in time.  Fortunately, the effect at which this occurs is usually above legal speed limits.  But not always by that much.

The above is also not unlike the relation between ocean temperature (the energy source) and cyclonic build up: the cyclone dies down in cooler water.

 

Suspension

Well designed fifth-wheelers also behave differently over bumps.  Suspended at either end, they rise and fall on the rear suspension of the tow vehicle (at their front), and their own suspension (a their rear).

Here again, conventional caravans are different.  They are like a dumbbell supported on a pivot a bit ahead of their middle.

Road irregularities not only cause them to rise and fall on their own and the tow vehicle’s suspension, but to rock fore and aft around their axle/s (or more technically correctly – their centre of mass).  Side winds too cause them to sway around, much as do sailing boats about their respective (sails’) centre of pressure.

Some fifth wheelers do however pitch fore and aft in a manner that manifests in the tow vehicle being subject to an uncomfortable motion and that their owners call ‘shunting’.  In extreme cases the rigs sway as well.

Knowing this to be so, and of the various solutions (that like many pills address the effect but not the cause) I was pleased to receive the following article from the respected Australian fifth-wheel designed and builder, Glenn Portch.

Glenn puts into plain English just what may happened to prejudice a fundamentally superb concept.  I feel his article will interest and influence fifth wheel owners and builders alike.

Glenn is a CMCA Member and owner of Fifth Wheels Australia (in Port Macquarie, New South Wales).  (I have no relationship, commercial or otherwise with Fifth Wheels Australia)’. 

Keeping them stable

Glenn Portch explains how

I first became seriously interest in fifth wheel caravans in 1995.  I’d long held a Heavy Vehicle licence and driven many kilometres and believed I understood the mechanical dynamics well enough to replicate the handling and roadworthiness of the load-carrying monster semi-trailers into a lightweight package designed for recreational use.

Having a manufacturing background, and owning the necessary machinery and premises, I began building them in limited numbers.

I subsequently spent all of 1998 in America, driving some 40,000 km in an American-made fifth wheeler towed by a Dodge Ram.  This proved to be a serious learning experience.  My travel extended well into Mexico on roads worse that I have every experienced in Australia.  I then drove from west to east across Canada, and then from east to west back across the USA.

My fifth wheeler was 28 foot long, with a rear kitchen.  In the first 200 km every plate, cup, saucer and anything that even looked breakable (and was stored in the rear kitchen cupboards) was destroyed.  And that was on Highway 101 along the Oregon Coast: quite a good road by Australian standards.

Shunting

In particular, I noticed a great deal of input into the tow vehicle from the trailer.  It was a sort of ‘shunting’ action (and is commonly referred to as such in posts on web site forums).

This puzzled me at the time.  I had not experienced this problem with fifth wheelers back home.  The Dodge Ram 3500, a one tonne model, was well within its load capacity for my fifth wheeler, but even so, it was (as many others report), an uncomfortable driving experience.

The cause is almost certainly that of an axle location similar to conventional caravans: i.e. approximately half way along the main body of the trailer.  Again, as with conventional caravans, this causes the body of the trailer to pivot around its axle centre (centre of mass) – ‘much like that of a sea saw’.  The resultant ongoing movement reacted on the tow vehicle, resulting in the aforementioned ‘shunting’ effect.

Weight Aft

If you look at heavy articulated vehicles (semi-trailers), you will see that the axles’ are well towards the rear of the trailer.  This enables the front of the trailer to move up and down on the sprung suspension of the tow vehicle, and the rear of the trailer to do the same on its own sprung suspension.

The rig thus traverses road irregularities as one cohesive unit rather than the trailer rocking fore and aft around a central pivot (centre of mass).  In practice, the rule of thumb design for articulated trailers dictates that the centre of the axle group be about 75% to the rear of the total length of the trailer.

The reason why some fifth wheelers have a more forward than desirable axle placing is simply weight.

Many fifth wheelers are designed to be towed by the readily available three quarter one tonne trucks such as the Ford F Series, Chevrolets and Dodges. 

For these trucks to be legally able to tow a large heavy fifth wheeler – yet not exceed the rear axle loan rating of these tow vehicles, the forward weight or ‘king pin weight’ of the wheeler is reduced by placing the axles forward.  This ensures more weight is borne by the trailer, and less (via the hitch) by the tow vehicle.

Dynamic Effects

But while this reduces the rear axle loading of the tow vehicle, it necessitates engineering compromises that introduce undesirable dynamic effects.

As noted previously, with a close-to-centre located axle assembly, the trailer and tow vehicle no longer ride as one cohesive unit: in some cases the trailer seems almost to be ‘fighting’ the tow vehicle, causing the shunting effect.

By more centrally locating the axle assembly, the trailer’s rear overhang is increased – with a real risk of the rear end scraping on the ground when negotiating uneven surfaces, driveways, etc.

To relieve this, extended spring hangers and other modifications are necessary to raise the chassis height.

But this in turn raises the centre of gravity, and may lead to instability to the extent that a single oscillatory hitch may be necessary to transfer the now-increased roll couple to the tow vehicle – despite the trailer’s inability to flex to accommodate this.

The higher chassis also increase the number of steps needed to access the accommodation area.  Frequently five steps are used instead of the usual two or three. 

Seven-headed dragon

The above leads to the common scenario of the seven-headed dragon: solving one problem may introduce many.

It has also created a market for products that are otherwise less necessary, or possibly not necessary at all, except with very hard sprung tow vehicles.  These include fifth wheeler towing aides including air and/or rubber pin boxes, and airbag hitches.

It can be argued that these are used in part to overcome shortcomings in the original design concept: a semi trailer has a prime mover of six-ten tonne that handles its loaded trailer (of 20 to 30 tonne) with far less shunting than many a recreational fifth-wheeler

I believe the original problem – of too much weight on the rear of the tow vehicle – is brought about by excess weight, yet is necessitated for it to be legally towable by lightweight vehicles that may be driven by holders of a car licence.  This can only be done with heavyweight construction by forward axle placing.

The major problem is thus heavy construction – many fifth wheelers are built from materials that have been used since the early nineteenth century.

It is common for 19 mm plywood floors to sit atop massive steel l-beam chassis rails, but strength and weight are not necessarily the same thing at all.

Weight begets weight.  To support thick plywood and steel you need heavy-duty axles, wheels, tires, floor framing and so on.  And this brings another seven-headed dragon to life!

The solution is to build lightly.  Today’s hi-tech materials, e.g. composite wall and floor materials, lightweight alloys reduce weight yet increase overall strength.  Consider the boating industry.  Kevlar and carbon fibre masts, composite hull materials and metal alloys are now the norm and the products are stronger and lighter than ever before.

I have personally been building fifth wheelers with three dimensional alloy chassis since 1995.  I am now using a second-generation plastic composite for floors, walls and roofing.

I should emphasise that I am far from alone in this.  Various other innovative Australian and overseas manufacturers of recreational vehicles also use various fibreglass, plastic and metal alloy composite panels with the ultimate goal of building light, strong, long lasting vehicles for Australian conditions’.

There is another article ‘Fifth Wheeler Hitches – April 2009’ by Collyn Rivers in the (April edition) of the CMCA ‘The Wanderer’ which covers the topic of single oscillatory hitches and some problems being experience by some owners of 5th wheelers.  If you own an imported 5th wheeler with a single oscillatory hitch we suggest it would be in your own interests to obtain a copy of the Wanderer and read the concerns expressed.

 
 


 

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