Why are Australian caravan chassis so heavy?

Advances in caravan technology have been subtle rather than revolutionary over the years, but one steady change we have noticed is the move away from the time-honoured timber frame/aluminium cladding body fitted on steel chassis to a more integral (and lighter) structure with a strong, rigid composite fibreglass or aluminium body on a lightweight chassis.

Despite their stronger and sexier-looking composite body, these new-wave caravans still need a chassis.

But why is a chassis so important on a caravan -- and why are the chassis on Australian caravans a lot stronger and heavier than those on similar sized vans in Europe?

Related: Caravan chassis bling on the rise

What is a caravan chassis?

A chassis is a frame that supports the body and typically onto which things like the suspension, jockey wheel, stabiliser legs, water tanks and coupling are attached. You can’t have a caravan without some kind of chassis, but there are a few variations on the theme...

The most common set-up involves longitudinal chassis beams made from Rectangular Hollow Section (RHS) 150mm x 50mm steel, while the cross beams often are Square Hollow Section (SHS)  50mm x 50mm.

You’ll see this set-up (or a version thereof) on just about all Aussie-made vans. Other (lighter) vans, such as the European models, use a C-channel chassis main beam.

Many caravan chassis have extra strengthening in the form of A-frame gussetting, which is extra steel welded to the longitudinal beams. There’s also the A-frame reinforcement plate, which is additional sheet steel welded onto the longitudinal beams at what can be a weak point on a caravan chassis, where the A-frame beams meet the main chassis structure.

Risers are sections of RHS welded onto the top (or underneath) the parallel chassis side sections. This allows the body to have more clearance and as wheel wells are higher, there’s the opportunity for both a flat floor inside the van and more wheel travel off-road.

Most chassis are stitch-welded, which is a short run of welding at about 400mm intervals.

Lightweight European caravan chassis

The Europeans do chassis very differently. Generally tow vehicles in Europe are smaller, more fuel efficient and lighter then tow vehicles in Australia so, accordingly their caravans have to be designed with low mass paramount. 

This is reflected in their chassis design; today you won’t find any RHS steel chassis under a caravan made in Europe. Instead, there will be C-channel longitudinal chassis lengths, with the only cross bracing being the torsion beam suspension.

Many don't have the chassis cross beams used on Aussie vans for extra rigidity and strength simply because they don’t need it. The van’s rigidity is gained from the composite body and this chassis set-up is strong enough for the maximum 2000kg that these vans weigh.

Some European chassis are even bolted together rather than welded, to save some weight -- such as the the AL-KO chassis. 

While we’re not focussing on body construction here, it’s important to explain how a composite body can take over some of the chassis’ responsibility for strength. The walls, roof, floor and furniture in a composite body design are usually all interlocked in place and then bonded - providing a solid, unitary body structure.

What can’t a caravan body do?

The one crucial thing a composite, interlocked body can’t do is adequately support suspension, or provide a platform for the coupling to attach to. But who knows what future technology might serve up...

Meanwhile, as Aussie caravans have become larger and better equipped in recent years so too have they increased in weight. Interest is growing in shaving weight off caravans, including by reducing chassis/body weight. The Aussie manufacturers have begun to eye off what the Europeans have been doing for years and applying some of their chassis-construction methods.

Some manufacturers have done this by using C-channel steel cross-beams instead of SHS sections. Others have gone a step further by using C-section steel with cut-outs, reducing weight without compromising on strength.

You’ll notice that most of these C-channel beams and cut-outs are not cut straight but have a curled-over section at the cut-out openings or at the tips of the C-channel beams. This is done for added strength.

Another way to reduce weight has been to build a van with a strong composite body. As the chassis does not then have to do all the work, it can be built lighter.

However, as yet there hasn't been a huge weight saving by building composite vans with lighter steel chassis, especially when it comes to large (22 foot-plus) off-road tandems. The weight saving is about 200-300kg over a regular large Aussie van.

There are several reasons why Aussie van chassis haven’t made a wholesale move to Euro-like construction. Probably the biggest reason is that it's cheaper and easier to weld up a RHS/SHS chassis that can be used across a range of caravan bodies; it also doesn’t need as much engineering and varying changes in chassis size to account for high-stress areas of the chassis.

What type of galvanising is best?

DuraGal and SupaGal steel use galvanised zinc coatings to prevent the steel from rusting. Done before the chassis has been welded together, DuraGal is zinc coating applied only to external surfaces while SupaGal is zinc applied to both internal and external surfaces of the steel.

When the chassis is welded together, zinc can also be applied to the external welds -- internally though, the welds can corrode as they aren't zinc-protected.

Hot-dipped galvanising, which is a process of dipping the finished chassis into a tank of molten zinc, is the best way to rustproof a chassis. However, it does add a bit of weight to the chassis -- about five per cent, although that can vary according to how much of the coating remains in the hollow sections of the chassis.

Two large, identical chassis can be several kilos apart in weight as different amounts of zinc have settled in each.

You also increasingly see a spray-on polyurethane or a Raptor protective coating on the drawbar and chassis of upmarket Australian off-road caravans, which should be lighter than hot dipped galvanising but may require some touch-up maintenance if chipped.

Looking after your chassis

There isn’t much involved with chassis maintenance. Although there isn’t much to go wrong, the chassis isn’t an entirely set-and-forget item. It still needs periodic checking and a small amount of maintenance.

If you’re buying an older, second-hand van or if you have owned your van for a while it won’t hurt to check the A-frame for rust. Obvious signs of corrosion on the weld seam, and if tapping the box section reveals a hollow sound in some sections but not others, may indicate serious rust problems. Get it checked out by a caravan specialist.

If you’re spending a lot of time camping near the sea or if you’re towing on the beach, don’t just wash your van’s shiny side. The chassis should be rinsed with plenty of water, especially if it has been exposed to sea water on the beach.

The gas bottle holders don’t normally cause problems but with large 9kg bottles fitted you may need to occasionally check the frame’s stiffness and for any signs of broken welds.

The handbrake lever pin needs to be greased, and the handbrake cable needs to be checked for fraying.

The tow coupling also needs cleaning and re-greasing occasionally to provide a smooth trouble-free movement when coupling/de-coupling.

The spring holding the lock-down clip must be in good condition, or the safety margin given by the clip could become compromised. While an unlikely scenario, you don’t want to risk the caravan de-coupling on a bumpy road because the coupling release safety clip was broken.

The tow coupling head needs the securing bolts checked for tension - they should never be loose. If you're uncertain what torque to apply to tighten these bolts, then check with your caravan manufacturer.

Some tow vehicles do not tow some caravans very well, but a lot of that can be down to chassis set-up. Make sure that the A-frame is not sitting too low when coupled to the vehicle’s towball, or not only will the caravan not tow well but also the A-frame will scrape against the road on dips such as those when entering or exiting driveways.

Flipping the towbar shank over (where possible -- some shanks are not designed to do this) or replacing the shank with one that sets the coupling point higher can help deliver a more level, and safer towing rig.

In some cases a Weight Distribution Hitch will be needed -- if so, make sure there's space on the appropriate points on the van chassis' A-frame for the WDH collars to fit.

While a scrape or two won’t harm the A-frame (it can remove any zinc coating and encourage corrosion though), a solid thump against the road can cause all sorts of problems.

The chain links could become damaged and while the A-frame structure is usually very solid, it could transfer shocks through the towbar and the caravan, causing further damage. However, making use of the different height adjustments on a WDH can make the geometry more acceptable.

Lastly, safety chains should be inspected for wear to the links or any other indications of deterioration.