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 fiberglass, 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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