As
more and more fifth-wheeler owners join the CMCA it seems timely to run
another Tech Notes column relating to their on-road characteristics.
In particular I am surprised that rarely do neither owners nor vendors
of these vehicles mention what is probably the strongest case for their
use: and that is their superior dynamic stability. The latter difference
is so great that people are now querying why, this being so, were not
all caravans built like this from the start?
Some were of course, but the answer is that originally there was less
need. The issue relates more to changes in the design of towing vehicles
rather than anything to do with trailers (the design of which remains
almost Edwardian). Early motor vehicles had little rear overhang (i.e.,
distance from rear axle to towing bracket). If there was much luggage,
passengers would travel in the Royce, luggage would go with Jeeves in
the Bentley. With cars like the T-model Ford, it was assumed owners were
too poor to own luggage anyway. If there was luggage it was strapped
to the running boards.
There being no luggage boot to speak of, there was no need for rear overhang
so a trailer would be typically attached a bit over half a metre behind
the rear axle. Engines were then mostly big lumps of cast iron with holes
in various places for pistons and things so there was a lot of weight
on the front wheels. But two trends evolved: softer suspension; and largish
rear luggage boots. The latter necessitated lengthy rear overhangs.
Now, trailers were attached at ever-increasing distances behind the
rear axles – onto a tow bar that had equally increasing up and
down movement as a consequence of longer travel, softer rear springs.
And often some sideways movement as tyre sidewalls became more horizontally
compliant.
So began an inherent effect. Any trailer so attached has an inherent
tendency to pitch and snake. From the mid-1930s to this day, caravan
magazines, and now websites, report jack-knifing and (not infrequently)
overturning as a consequence. Curiously, the physics of this has barely
been investigated (my original paper on which this article is based
is still the only literature – except for a heavily mathematical
one published by the US military). Here, in hopefully less obscure
terms, is what happens and why.
Push a child on a swing only a few times and let go, friction and wind
forces will quickly slow that swing. But continue to give even a little
push at the critical time and swing angle will increase. Once past
a certain point, movement is difficult to stop. If pushing still continues,
the swing may ‘go over the top’. This so-called ‘positive
feedback’ does not just happen with swings. It is true of monetary
economies, early Porsches and VW Beetles, sailing boats, and public address
systems. Unless that disturbing impetus is negated (eg, a yacht turned
into the wind, a PA system turned down) the effect rapidly escalates.
This can spectacularly happen with caravans. Given ongoing ‘pushes’ from
any number of causes, a trailer attached via an overhanging hitch may
build up this movement. Unless resisted or damped, escalating pitching
and snaking may overwhelm. A pitching caravan pushes down on the rear
overhang of its towing vehicle. This causes the front of that vehicle
to lift. It’s like pushing down on the handles of a wheelbarrow:
the front wheel lifts. It makes no difference whether the wheelbarrow’s
legs are sprung, or are totally rigid – push down on the handles
and the front end lifts. It is a simple lever effect.
This is exactly what happens when a caravan pitches: the down force
on the overhung rear lifts the front of the towing vehicle. Stiffening
the towing vehicle’s rear suspension makes no difference. The effect
is innate: it is that of a ‘first class lever’, originally
described by Archimedes in 260 BC. Despite clear visual evidence to
the contrary and readily demonstrated simply by standing on a tow bar,
many caravanners and builders deny it exists!
Snaking is an ongoing lateral sway usually triggered by one trailer
wheel encountering a dip or hump. This slightly slows that side of
the ‘van,
causing it to skew. Much the same, only worse, can happen if one ‘van
wheel runs onto soft ground whilst the other remains on the bitumen (the
suddenly increased drag brakes that wheel). Skewing is also initiated
by wind gusts, including from passing trucks. Things may get serious
if pitching and snaking occur simultaneously, because pitching forces
reduce the grip of the towing vehicle’s front wheels, at the same
time that snaking forces try to spin the vehicle. If snaking forces exceed
the grip of the towing vehicle’s tyres, the rig will begin to
jack-knife. The friction as the tyres slide sideways dissipates some
of the snaking energy, but whether it is enough to prevent total jack-knifing
is now in the hands of Archimedes, Sir Isaac Newton - and whichever
gods look after rubber technology. This combination of events is the
known cause of many caravan accidents, some of which happen on straight
suburban roads.
A major problem with convincing drivers there is a problem is that
big end-heavy rigs seem exceptionally stable in normal circumstances.
Their rigs behave like ocean-going tankers in calm seas and so strongly
resist minor pitching and swaying that their owners may boast of the
apparent stability. But these rigs can suddenly become an unstable
nightmare if, for example, crossing a humpback bridge too fast – or
cornering too fast on a bumpy and slippery road. Strong, cyclically
gusting winds, or passing road trains too may induce seriously dangerous
snaking. Pitching and snaking forces are not linear. They increase
by the square of the distance that any mass is in front of or behind
the centre of mass (in practice the latter is a point a bit ahead of
the axle/s. In plainer English that means that weight at the ends has
very much greater adverse effect than weight near the middle. Caravan
builders know their products are less unstable if front-heavy and they
usually have about 10% of the weight on the tow bar. But when that
caravan pitches, a 150 kg mass one metre away from the centre of mass
pushes down with a force equivalent to 150 kg; if three metres away
that 150-kg mass pushes down with a force equivalent to over a tonne.
Pitching forces can be partially counteracted by using a weight equalising
hitch. This acts like a full-length semi-rigid beam that forms a single
and common virtual chassis for both towing vehicle and trailer. The
rig becomes the dynamic equivalent of an ultra-flexible three-axle
truck. This reduces but cannot eliminate the full effects of pitching
but it reduces them by a worthwhile amount. Snaking pushes the rear
overhang of the towing vehicle sideways. Because the force is applied
behind the towing vehicle’s centre of mass it attempts to spin that vehicle
in the opposite direction, and around that centre of mass. The build
up of snaking energy can be resisted and partially absorbed by friction
or cam mechanisms, but it must always be ultimately resisted by the grip
of the towing vehicle’s tyres (particularly front tyres) on the
road. If that grip is lost, the vehicle is almost certain to jack-knife.
There is less cause for concern where light caravans are towed by heavier
vehicles, but undesirable effects are inherent when a heavy trailer
is attached by a tow bar any appreciable distance behind the rear axle
of a towing vehicle.
The basic problem with caravan stability is thus not so much the caravan.
It is rather that modern passenger vehicles, especially cars, have
developed long overhangs. This introduces the lever effect that causes
the problems: of undesirable forces that affect stability. The greater
the overhang, the greater the problem. (The ultra-short overhang DS
Citroen was a wonderful towing vehicle). But if the trailer’s weight is carried by a coupling
located over the towing vehicle’s rear axle, pitching and snaking
forces are reacted by that axle and tyres alone. The front of the towing
vehicle still lifts under acceleration but that is rarely an issue.
The above configuration is dynamically stable.
But if the coupling is behind that axle, the dynamics are that of a
conventional caravan. The argument is not whether it is stable or otherwise.
It is inherently unstable. The only argument is ‘by what degree’.
Will a millimetre or two rearward make any measurable or perceivable
difference? Measurably? Certainly, given scales of adequate repeatability
and resolution. Perceptibly? Probably not - except in an extreme situation.
There simply has to be a defined hitch location. The current two ‘centimetres
in front of the axle’ makes good sense but an allowable tolerance
of (say) minus one centimetre would allow for measurement discrepancies.
Whilst the CMCA’s policy regarding hitch position is sometimes
criticised, it is that positioning, and that positioning alone, that
differentiates a fifth-wheeler from a conventional caravan hung onto
an overhung hitch.This is not an argument such as ‘how many hairs
constitute a beard’ – this is an issue of engineering absolutes.
There is no logical/engineering difference whatever between a caravan
with a raised draw bar being attached onto the deck or chassis of a
towing vehicle and behind its rear axle – and that caravan being
hitched to the tow bar of that same vehicle. In virtually every respect,
both have ‘caravan’ not fifth-wheeler dynamics. But if
(say) that modified Jayco poptop is attached over the towing vehicle’s
rear axle it becomes, by definition, a ‘fifth-wheeler’.
Archimedes and Newton explained most of the above centuries ago. I’ve
just attempted to put it into an RV context.
Collyn Rivers, W8054.
