Camps ad




Manufacturers 'hoping for the best' and ignoring critical items

Copycat RV builders flouting safety,
claims Caravan Council of Australia

Colin Young Have your say

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 caravan?

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.

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 prime-mover.

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 any caravan?

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 stored there.

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 any caravan?

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 evenly spaced.

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 full.

Sagging caravan

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)

Running-Gear Issues:

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 tandem-axle systems?
What are the relative advantages and disadvantages of solid-axle and independent systems?
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 changes?
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 rating?

If the 'van has a non load-sharing suspension system, are the two front wheel/tyre assemblies strong enough to withstand hard impacts?

Axle-Group Rating:

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 fairly similar?

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.

GTM Rating:

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 ( 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 performance.

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 fine.


 No part of this publication may be reproduced or transmitted without
the prior written permission of Dennis Amor.

Copyright 2005 Dennis Amor
All Rights Reserved

Time To Roam ad  

Kui Parks advert

Caravan Council of Australia advertAdvertising