'hoping for the best' and ignoring critical items
Copycat RV builders flouting safety,
claims Caravan Council of Australia
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VERY few caravans built in Australia are
professionally designed, according to the Caravan Council of Australia.
"Far too often, they are simply copied from other caravans,"
the council's founder and general manager,
Colin Young (pictured), said.
He claims that manufacturers "hope for the best" and ignore basic but
critical technical items which control how their vehicles handle on the
road … especially in emergency situations.
"They should take heed of the need to design their basic crucial
components and systems so as to best provide safe and predictable road
holding characteristics under all conditions," he said.
Mr Young has now produced the following technical article in a bid to draw
attention to the problem.
Engineering Design of Caravans
How much genuine professional "engineering design" went into your caravan?
Was it developed using a professional CAD (Computer-Assisted Design) and a
high-level CAE (Computer-Aided Engineering) program?
Or was it just simply copied from a competitor's 'van that was quite
possibly merely copied from another 'van that was made many years before,
and was possibly sketched-up on the back of an envelope?
Was it purely concocted by the oft-used "she'll be right" method?
Or did a professional engineer actually calculate the stresses involved in
the various components - especially the critical running-gear elements -
and then incorporate reasonable safety factors to provide acceptable
safety, reliability and durability?
While a potential-buyer of a caravan does not need to be a "walking
encyclopedia" on caravan design engineering, it is still a good idea to
be reasonably informed regarding the "whys and wherefores" of the
fundamental technical issues.
If a salesperson cannot credibly explain why a particular caravan has, or
does not have, certain features you may want to question their
understanding of what they are actually selling.
Are the all-too-common and annoying problems - such as water and dust
leaks caused by the deterioration of adhesives and sealants and the
cracking and loosening of body panels and cabinetry - properly considered
and measures taken to minimise the potential for damage?
Are the chassis and the body designed to be extremely rigid, very
flexible, or a pre-determined compromise?
Q: What is by far the most-important design consideration for any
A: Without doubt, the caravan must be safe and stable on the road
when towed legally by a suitable tow-vehicle so that its handling and
stability remain steady and predictable, regardless of whether the 'van is
empty, partly-loaded or fully-loaded (up to its ATM rating).
In addition, the caravan must be designed to have a high "critical speed"
so it is unlikely to jackknife and rollover in the event of the driver of
the tow-vehicle having to make a sudden "animal-avoidance" manoeuvre.
Q: What, then, is the next-equal-most-important design consideration
for any caravan?
A: Handling smoothly and predictably on the road, as per the
evenness - or roughness - of the roadway surface on which it was designed
to be towed (in a responsible manner).
Suspension-system design for caravans is an extremely-complex subject,
where poor (generic or off-the-shelf) configurations can quickly and
dangerously impair the dynamics and road holding of the caravan … especially
when you need them to perform ideally.
There are definite reasons why caravans can - and do - quickly and
uncontrollably jackknife and rollover.
A tow-vehicle and caravan combination has a major inherent disadvantage
regarding stability when compared with a 5th-wheeler or semi-trailer.
All 5th-wheelers and semi-trailers have their coupling, or
articulation-point, virtually directly above the rear axle of the
Some tow vehicles have their coupling ball, or pin, an undesirable
distance behind the rear axle, which greatly worsens the possibility of a
caravan sway commencing.
This potentially lethal condition is exacerbated if the mass of the tow
vehicle is not appreciably heavier than the mass of the caravan.
Q: What is the next equal most important design consideration for
A: Ensuring that the "mass-distribution" - in a side view - for
both empty and loaded conditions stays within the acceptable range to
ensure the extremely-critical "ball loading" stays safely within the
required narrow range. This can only be determined by conducting
real-world professional on-road testing.
An important design requirement is to ensure the magnitude of the "polar
moment-of-inertia" is acceptable. Basically this means that all heavy
items need to be located as close as possible to the axle(s) and not at
the extreme ends of the 'van, especially not at the rear end.
All storage compartments not located close to the centre of the 'van need
to have warning labels stating the maximum mass of luggage that should be
Instructions should be provided to state exactly where aftermarket
accessories must be located, along with the maximum permitted masses.
Q: What is the next equal most important design consideration for
A: Ensuring that the "ball loading" stays as constant as possible,
regardless of whether the water tanks are empty or full. It is imperative
that water tanks are located as close as possible to the axle(s) and
The manufacturer must be able to advise their suppliers - hence their
customers - of the plus-and-minus variations in the ball-loading that will
occur at the two most-adverse conditions of the water tanks being empty or
The photo shows an unbelievably dangerous
set-up where all three water tanks are located in front of the axles.
The empty ball-loading ‒ with all tanks
empty ‒ was 360 kg! One
dreads to think of how many similar 'vans were, or still are, on our
roads. So much for "self-certification" and
grossly-inadequate regulatory manufacturer audits and vehicle inspections.
Caravan Handling & Stability:
Two internationally-respected Automotive Engineers have authored the
following Publications on the topic of Caravan Handling & Stability:
“Investigation of Car-Trailer Stability” - Jos Darling [P.Eng.] -
University of Bath (UK)
“Why Caravans Roll Over” - Collyn Rivers [P.Eng.] (Australia)
A chassis manufacturer can only build a (rolling) chassis as per the
information provided by the caravan manufacturer.
The caravan manufacturer must be responsible for confirming they are
physically able to build, and legally sell, any specific caravan. A vital
consideration is always to be positively certain that they can, in fact,
provide a caravan that fully satisfies all of the "intended" item, in
particular the intended tow-vehicle.
One would reasonably believe that a caravan buyer is not expected to have
a detailed knowledge of ratings and masses, thus needs to rely on the
supplier to absolutely ensure - or guarantee - that the intended caravan
can be safely and legally towed by the intended tow vehicle.
What are the relative advantages and disadvantages of single-axle and
What are the relative advantages and disadvantages of solid-axle and
What are the relative advantages and disadvantages of leaf-springs,
coil-springs and torsion-bars?
Are dampers (shock absorbers) required or recommended?
How are the roll-centres and roll-axes affected when the 'van loading
With the particular suspension fitted, is the "wheel-alignment" critical
for good safe handling and tyre life.
Does it vary much depending on the degree of wheel travel (on smooth and
bumpy roads) and is it readily adjustable?
Non load-sharing tandem axle suspensions:
What are the relative advantages and disadvantages of load-sharing and non
load-sharing suspension systems on tandem-axle 'vans?
For a non load-sharing suspension, is a 120 percent safety factor
incorporated in the design? If yes, how is it determined?
Is the axle-group rating at least 1.2 times the GTM Rating? In other
words, is the GTM rating not more than 83 percent of the axle-group
If the 'van has a non load-sharing suspension system, are the two front
wheel/tyre assemblies strong enough to withstand hard impacts?
The axle-group rating must not exceed the lowest of the rating of the
suspension, springs, axles, wheel-bearings, wheels and tyres.
Are these components evenly "matched", in other words, are the ratings
The suspension, springs, axles, wheel bearings and wheels need to be
strong enough to reliably withstand the continual vibrations and impacts
that they are subjected to and have very little effect on the "ride"
(smoothness) of the 'van.
The GTM - like the ATM - is a rating, not an actual mass. Both ratings
relate to the caravan when it is fully loaded to its legal limit, and have
no relationship to the empty ball-loading which, as the term infers, is an
actual mass and only relevant when the caravan is empty.
Effective Spring "Rate":
The ride-handling-stability characteristics of a caravan depend largely on
the (effective) spring rate of its suspension system.
Suspension design is a highly technical area, and a major aspect is to
realise that the "spring" is a series of two separate "springs" ... the
actual "spring" and the tyre.
They have to work in harmony to provide the optimum performance, and need
to be varied in relation to the wheel loading.
Ideally, the actual spring should have a variable rate, with an initial
soft rate to provide a smooth ride on a "cobblestone" road, and then
progress to a hard rate to perform satisfactorily on rough roads.
Does your 'van have dampers (so-called shock absorbers) fitted? Do they
provide the optimum damping effect in both the upward (bump) and the
downward (rebound) travel of the wheel?
Coil springs have no inherent damping action needed to reduce the
magnitude of the vibrations subjected on them.
Leaf-springs do have some damping effect due to each leaf rubbing on each
adjacent leaf, but unfortunately it is in the wrong direction!
Leaf springs provide much more damping on "bump" movements (when each leaf
is forced hard against another). It is more desirable to have a greater
percentage of damping on "rebound" movements.
The tyre specification (complete size and ratings details) selected for
any specific caravan is of the utmost importance.
In some cases, the selected tyres have been dangerously under-engineered,
so that when a 'van is fully-loaded, the loading on each tyre exceeds the
maximum load rating of the tyre!
In far too many cases, the selected tyres are hideously over-engineered,
so when the 'van is fully-loaded, the loading on each tyre is just a small
fraction of the maximum load rating of the tyre!
It is vital that recommendations of the Tyre & Rim Association of
Australia (https://tyreandrim.org.au/) are always followed. Their handbook
covers all available tyre specifications.
It is also vital that the recommended inflation pressures are in
proportion to the tyre loading at all times.
Different recommended inflation pressures are clearly needed for the empty
and the fully loaded conditions of the 'van to provide optimum
Unfortunately, in quite a few cases, the recommended inflation pressure
embossed on the tyre sidewall in relation to the maximum permissible
pressure when the maximum actual tyre load is nowhere near the maximum
permissible tyre load.
Many 'vanners rightfully complain that 'vans "wallow and sway", or "bounce
and skip" on both smooth and bumpy road surfaces.
Along with ensuring that the ball-loading is correct, the tyre inflation
pressure plays a critical role in a 'van's handling and stability.
Remember: If the basic design of a caravan is fundamentally flawed, you
cannot have high expectations that the rest of the caravan will somehow be
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