Are aluminium caravan frames better?

Not long ago, the full composite fibreglass interlocking caravan body was the latest and brightest thing to arrive, offering a strong, simple, well-insulated -- and with a lighter chassis this design affords -- ultimately a lighter RV design.

You could allow hail to pelt down on it, stand on it - some manufacturers even parked a car or another van on top to demonstrate strength - and this new interlocking structure remained unaffected.

Yet at the recent Sydney Supershow, many manufacturers were touting the benefits of an aluminium frame, particularly for off-road vans. Aluminium frames are not a new concept by any means, but does aluminium do the job better than either a timber or full composite structure?

Aluminium has its champions

The aluminium frame caravan first popped up on the local scene in the 1970s with the likes of Viscount and Millard adopting the construction method instead of the traditional timber frame.

Millard still uses aluminium frames today, but so do a growing number of other manufacturers including Australia’s most popular caravan brand Jayco (albeit with a composite skin) but also the likes of Sunland, Bushtracker, and new brand Roadhouse.

The main argument for aluminium is that it's a light and strong material.

The aluminium used in caravan manufacture is either C-channel or box section. Off-road caravan manufacturers tend to go with box-section aluminium. For example, Bushtracker says it uses aluminium 25×25mm and 50x25mm structural box section exterior wall frames with 2mm and 3mm wall thicknesses.

Box-section aluminium is heavier and costs more than C-channel lengths but it's stronger.

Interlocking frames add strength

Aluminium vans usually have interlocking frames, adding to strength. The frames can be pop-riveted, secured with high-tensile pin punches or welded together (the last two providing the most strength).

Aluminium is more expensive as a raw material than timber and takes more of an investment by the caravan manufacturer to set up. The manufacturer needs to allow more room in the factory to make and prepare the large frames and there’s the cost involved in an aluminium welding set-up if it’s used.

Yet the cost of machinery and skill required for making a fibreglass or aluminium composite caravan body is not cheap or simple either.

What can make or (literally) break a caravan frame is just how well designed and built the chassis and suspension are. These components can and do have a significant influence on body structure performance.

Real world performance

That’s all the theory behind aluminium, but how does it cope in the field? There are few reports of problems with newer aluminium vans, especially those that are not used off-road, but it’s a fact of life that like any metal, aluminium will suffer from metal fatigue if given a hard time.

So with regular corrugated dirt road work or off-roading, metal fatigue can cause problems. With vans up around five to 10 years old that have been given a regular hard time, in some cases welded or riveted joints will crack and loosen and come apart. The compromised structure then, of course is much more likely to allow leaks.

Just because an aluminium frame can’t rot like timber, doesn’t mean the wood around the aluminium won’t. If ply is used in an aluminium-frame van (for inner walls, for example), that could rot if water gets to it. Some vans also use timber for shaped sections that again will rot if there’s water ingress. Even so, at least the van’s core body structure is not affected.

Extensively metal-fatigued aluminium frames are much harder to fix than a small amount of timber rot and a well-built timber van that has some ‘give’ can make for a good off-road van for that reason -- it doesn’t transfer road shocks as easily, absorbing them instead.

Because caravan manufacturers don’t all apply the exact same construction methods it’s hard to generalise and say that aluminium is the best body frame to have. Yet you can see why its popularity isn’t waning -- for a manufacturer that wants to avoid the complexity and expense of fabricating composite bodies in-house (or buying expensive panels in from a supplier, and either way, having to pass on that cost to the buyer) and wants a rot-resistant, lighter and more rigid caravan than available with timber, aluminium is an enticing option.