Anatomy of a Caravan

What exactly makes up a caravan? John Hughes takes a deep dive into caravan construction

At this year's Caravan of the Year event we evaluated some of the smartest designs and best builds of caravans currently on the market. We delved into a diverse range of vans with various construction methods which reinforced that there is more than one way to build a great van. We saw it as an ideal time to run through the key components of a caravan as well as different approaches manufacturers can take to build their vans, with some of the pros and cons.

Broadly speaking, all caravans feature running gear and a body. The running gear comprises the wheels, suspension system, chassis, and coupling. The body comes together with the floor, walls, and roof. Touring on bitumen roads is still how many vans are used but much of what is on offer now is influenced by the option for semi-offroad and full offroad specifications. 

Wheels

Sixteen inch rims have become the default size for many vans, with alloy rather than steel being the material of choice — the occasional van will feature 15in rims and you will see 17in rims from time to time on manufacturers going for a big and bold look. Alloy rims are popular as they are light and available in a variety of attractive patterns. The occasional hard core offroad customer will ask for steel rims as they can be bashed back into shape out in the bush if you damage one hitting a rock or similar. Alloy rims are virtually unrepairable if subject to this sort of abuse, but we are talking extreme circumstances. 

Four wheel drive utes and large SUVs make up a large chunk of the tow vehicle market. Owners like the look of all terrain tyres and sometimes mud tyres on their rig and this flows through to what we see fitted to vans. The reality is standard on road tread pattern would be perfectly suitable for many applications. The most important aspect is to have a tyre with a light truck construction. This means the tyre has a stronger carcass and is rated for heavier loads.

Suspension Systems

The most traditional type of suspension system is leaf spring, and it has been used on vans since the inception of the caravan industry in Australia. It is robust, simple, and cost effective. The ‘spring’ is made by stacking a number of strips of flat steel together that flex when travelling over undulations. In this system the leaf springs on each side of the van are connected to a solid beam axle, so if one wheel moves, the opposite wheel on the other side of the axle is affected, which works fine on smooth roads. 

Spring rates are important as they affect how the van travels. If a spring is too lightly rated, the suspension can bottom out, but if it is too heavily rated, the suspension will not respond and the van will bounce, causing stability issues and impact loads on the van. 

Independent suspensions systems allow for each wheel to move up and down to do their own thing, as each wheel is attached to its own independent axle. This is better for travelling over rough terrain because one wheel can deal with the terrain at hand without being disturbed by the other. The most common form of independent suspension is trailing arm. The wheel, axle and suspension system are mounted on an arm attached to a pivot point on the chassis forward of the wheels — the pivoting arm is ‘trailing’ behind the vehicle when moving in a forward direction. Historically, trailing arm suspension systems tended to be heavier than leaf spring systems as there are more components, but recent developments have seen this even out. The spring in a trailing arm independent suspension is often a coil spring. More upmarket systems have air springs which have greater adjustability through varying air pressure.

Suspension systems are available in touring, semi-offroad and offroad/heavy duty. They all work in a similar way, being as you work up the scale the springs and shock absorbers are designed to cope with larger and higher frequency dynamic loads and the supporting components become more robust.

Chassis

Chassis can be broadly classified into two types — RHS section and C section. The traditional Australian chassis consists of main beams running longitudinally constructed from rectangular hollow section steel (RHS). This style is inherently stronger than a C section as it takes more load to deflect a closed profile. The standard size in Australia tends to be a 100mm high profile and a 150mm profile is classified as heavy duty. All things being equal, a 150mm profile will be heavier than a 100mm profile as it has more steel.

C section is an open profile with less material which saves weight but is not as strong. However, C section profiles can be more than strong enough for the right application, such as road touring or where clever engineering is used to build strength in other ways — examples include bonding the floor to the chassis and integrating a strong body. Some manufacturers will also cut holes in selected profiles to achieve weight savings.

Risers are worth mentioning as they are often associated with offroad vans. Risers are typically another layer of RHS welded to the chassis to raise the caravan body higher. This can provide more clearance for wheel travel and eliminate wheel arches protruding internally, creating a flat floor and increase fording depth. The downsides of this is increased top heaviness, which makes the van less stable and increasing the roof height.

Many chassis are galvanised, which is a process of adding a thin layer of zinc to the steel for improved corrosion protection. This is achieved by dipping the steel into a bath of molten zinc — hence the term ‘hot dip gal’. Sometimes the entire chassis is fabricated from untreated steel and then dipped in the zinc bath by the chassis manufacturer or affiliated supplier, but other time the chassis is fabricated from steel that has been galvanised by the steel manufacturer — in this instance DuraGal or SupaGal may have been applied. In these processes the RHS is hot dip galvanised and then an additional coating to protect against white rust* and improve paint adhesion is applied. This coating is applied on the exterior only for DuraGal and both the exterior and interior in the case of SupaGal. 

Some Australian chassis manufacturers offer aluminium variants. Aluminium is substantially lighter than steel but more prone to stress cracking if not engineered correctly. This is not to say it can’t be done right, but the take up in Australia to date is limited.

*White rust is zinc hydroxide caused by exposure to hydrogen and oxygen (water or hydrogen dioxide). It may or may not cause structural issues.

Couplings

The most traditional and cost effective coupling is the humble 50mm ball. It is simple, reliable and gets the job done in many situations, particularly for on road touring. In recent years more sophisticated and expensive offerings such as pin couplings have grown in popularity, as they offer far greater articulation up and down and side to side which is good for offroad. A pin coupling will allow the van to pivot much further than a ball coupling relative to the tow vehicle on uneven ground without the risk of uncoupling. 

Pin couplings also offer benefits for on road applications. When unhitching, a standard ball coupling will often bind on a slope, but a pin coupling releases more easily under load. The infinite lateral articulation of a pin coupling can even mitigate the risk of car roll over if a caravan rolls over. 

Floor

Floors are made from three different kinds of materials — ply, foam core composite panels, and polymer honeycomb core composite panels. No matter the material, the floor works hand in hand with the chassis, as when the floor is bonded to the chassis it adds structural strength to the assembly.

Ply is a proven material used by many caravan manufacturers. It is relatively inexpensive, and at around 13mm it is one of the thinner materials and has limited thermal insulation properties. Ply serves as a good reference point for newer materials. Foam core composite is much thicker (30mm+), has much better thermal insulation properties, and is significantly more expensive than ply. Both ply and foam core composite panels are similar in weight. 

Honeycomb core composite panels, meanwhile, are significantly lighter than both ply and foam core composite panels. Honeycomb core composite sits between ply and foam core composite in terms of thickness at around 17mm. Honeycomb core composite is a little less expensive than foam core but still more than ply. An important point is foam core composite is structurally stronger than ply and honeycomb meaning chassis cross members can be spaced out further saving cost and weight in steel.

Walls and Roof

Walls can be divided into framed and frameless construction methods. Frames are similar to structures inside plaster clad walls in many houses, and Meranti timber frames are the traditional caravan construction method with many vans still made this way. Timber is relatively inexpensive and easy to work with. However, timber is at risk of rotting if water leaks occur, which is why quality manufacturers place a great deal of emphasis on sealing their vans well.

Some manufacturers build their frames out of aluminium, which has the advantage of not having to worry about rot. However, caravan manufacturers have to invest in more expensive equipment to work with aluminium which can impact the price of the van. 

Both timber and aluminium frames are generally covered with a relatively thin aluminium cladding. The cladding can have a profile in which case it will be just aluminium with a coating. The profiled sheets are not that high and are attached to the frame overlapping each other to create a wall. In other instances, the sheets can be a flat 3–4mm composite material with a very thin inner and outer aluminium skin with foam in its core. These flat sheets tend to be larger than profiled sheets and therefore have less joins. Framed wall vans will generally have foam insulation sheets placed in the void during the caravan manufacturer’s construction process.

Frameless walls are achieved by using a thick foam core composite sheet as described in the floor section of this article. Also known as sandwich panels, the foam core makes up the bulk of the thickness and provides thermal and acoustic insulation properties. The inner and outer layers are then made up either of a fiberglass or aluminium flat skin. The composite panel strength comes from selecting the right kind of foam and adhesives to bond it all together. The composite panels are one piece with no joins and are attached to the floor and roof to create a strong box-like body which also helps with the strength of the chassis. The box-like body can be further strengthened by intelligent engineering of the furniture. By strongly securing the furniture to the floor, walls, and roof it become a series of internal braces tying the whole structure together.

Fibreglass skins have the advantage of being relatively easy to repair if a wall is damaged. They can be patched and colour match painted the same as a fibreglass boat would be repaired. Aluminium skinned walls are not as easy to repair and may require another sheet of aluminium to be applied or the whole wall replaced.

Holes are simply cut out of the composite panels to fit doors, windows, and appliances. The flat surface makes it easier to form seals in these cut outs. In the event of a water leak internal damage is far less likely due to the relatively impervious nature of the materials. We have tacked roofs onto the end of this section as they can be viewed as similar to walls in that they are generally made from either sandwich panels or aluminium clad frames.

More Than One Way

We have taken a high level view of the major components that make up a caravan. It shows there are many ways to build a great van. Some construction methods reduce weight, some reduce cost, and some stand up better to water leaks. Some approaches are more suited to offroad and heavy duty applications. Hopefully this information will help you to ask more informed questions if you are in the market for a new van or if nothing else join in the campfire banter with conviction about what makes a great caravan.