Do aerodynamics affect caravan stability?

Automotive aerodynamics became an established field not long after cars could drive faster than walking pace. The wind tunnel and, eventually, CAD software has been employed to radically improve a production car’s wind-resistance.

This has all helped to make the cars we drive every day perform better at speed, use less fuel, emit less pollution – and be more stable.

Yet most caravans, at least those built in Australia, are still basically the same shape they've always been – that is, like a shoe box on wheels. Often they'll have a slanted front to reduce wind resistance, but not much else to allow them to slip though the air.

Could a better caravan shape lead to not only less drag (and therefore better tow vehicle economy) but also better stability?

Teardrop ideal

The ideal shape for anything slicing though the air efficiently is a teardrop shape. Anything that is designed to go successfully fast or efficiently though the air, from natures’s own efforts such as birds to human efforts like aircraft or cars, have elements of this shape incorporated in them.

Not many cars, let alone caravans, are shaped exactly like a teardrop but you can even see these elements of aerodynamics in relatively early cars like the 1955 Citroen DS.

Not all caravans or campers are shaped like a box, either; some have also intentionally incorporated good aerodynamics in their DNA. The traditional teardrop camper – which has its origins in the 1930s – is probably the most aero, efficient and stable camper shape there is.

There are many other examples of aerodynamic style caravans but most of the larger aero shaped caravans have not been a big hit in Australia.

One example was the promising Aerovan, which appeared in 2014 but didn’t last.

Another example was the 2013 Desert Sands Cheetah, a fibreglass camper that looked like it would slice though the air perfectly but was also short-lived.

One of few aero shaped campers that has lasted the distance in Australia – for more than 25 years – is the Ultimate camper. As for anything bigger, like a full-size caravan, the aero shape, US-built Airstream has only been an expensive, boutique model selling by the handful in Australia.

However, the 10-year old Kimberley Kruiser has proved a minor hit with grey nomads wanting a stylish and aerodynamic Australian-built off-road caravan. When it was introduced in 2014, the NSW-built Kruiser's aquiline ‘nose’ was said to almost halve the wind drag of the blunter-nosed original Kruiser.

Factoring in stability

Any talk of caravan stability and aerodynamics can’t ignore the biggest influence of all, and that is speed.

You simply can’t have detrimental aerodynamics affecting stability if there isn’t enough speed to cause the air to be pushed around (and push at or be drawn away from the caravan).

While rolling drag – that is, the resistance of the axles and wheels – is fairly consistent, air resistance increases at the square of speed. So at 80km/h you have four times the drag as you do at 40 clicks!

The practical needs of touring Australia towing a caravan – even if you’re only camping somewhere local for the weekend – is that you need to hit the highway and travel at these speeds or higher. Anyone who tows much at all is affected by aerodynamics.

What do 'van manufacturers do to improve aero stability?

Local manufacturers of caravans largely follow practical camping needs over aerodynamics, but in Europe and the UK caravan manufacturers like Swift and Adria pay serious attention to aerodynamics.

When designing its new Astella caravan, Slovenian caravan manufacturer Adria used the expertise of light aircraft manufacturer Pipistrel to understand how to make the Astella more aerodynamically efficient and safe.

According to Adria’s Uroš Dvornik, the caravan’s front wall angle is not as relevant for aerodynamics as you might first think, due to the typically large tow vehicle in front of the van.

"We proceeded from the fact that caravans of this size are towed by fairly large cars, that is, often SUVs, which displace extensive quantities of air, so we assumed that the angle at the front wall was irrelevant,” he said.

Dvornik said that Adria had to therefore consider the caravan’s aerodynamics only in context of being towed, much like a train.

“Instead of dealing with the aerodynamics of the caravan on its own, it was necessary to analyse the entire convoy, that is both the car and the trailer. This concept is similar to a train composition, where the wagons are completely cut off and the locomotive is the one that displaces the air.”

Although the tow vehicle sliced though the air and the caravan always just followed, Adria realised that the air current around the van – and how it was manipulated – still played an important part in caravan stability.

Pipistrel’s aerodynamics expert Matej Andrejašic explained the different approach his company needed to assess good caravan aerodynamic stability for Adria.

“In an aircraft, the lift and drag are both important. But when it come to caravans, lift should be avoided, as should lateral forces. It is necessary to stream the air flow in a way that will primarily affect that component of the force active in the direction of travel, that is, the drag. The lift could drastically affect the safety of the vehicle by reducing the grip of the tires.”

Andrejašic explains that the caravan’s box on wheels shape, entirely conducive for efficient internal living and storage space, was also an aerodynamicist’s biggest challenge. Any aerodynamic improvements could only be incremental.

“Of course, the basic shape of the caravan was very close to that of a square due to the optimisation of usable space. In the field of aviation, such body shape is rarely encountered as it causes the greatest air drag, which is one of the worst inhibitors of aerodynamics.”

Trucks lead the way

The inspiration for better caravan aerodynamics and stability, Andrejašic said, came from another road vehicle industry, one that is very interested in efficiencies wherever they can be found – that is, commercial trucks.

“So one of the most notable innovations in the world of caravans essentially emerged from the world of trucks rather than aircraft. There are air flow guiding vanes on the front vertical edge of the caravan between the front and side surfaces, which drastically reduce the area of increased air pressure between the towing vehicle and the caravan while ensuring that the air flow on the side surfaces does not get detached.”

As was discovered in optimising aerodynamics with truck prime movers and their trailers, the key goal with the Adria Astella was to reduce the maximum contribution of drag resulting from air flow that gets detached from the surface. So, small design changes could make a difference to stability.

“If an airflow travelling around the body encounters an obstacle and has insufficient energy to bypass it, it can detach itself from the surface," Andrejašic said.

"One of such examples can often be seen on the edge between the front and the side of the caravan. Such a detached air flow is most easily detected by a series of woollen strips glued to the surface. The comparison of patterns in woollen strips created during the ride and the calculation simulations showed a positive match, which confirmed the effectiveness of the solutions used.”

Clearing the air on aerodynamics

While the effects of weight balance and uneven road surfaces in generating caravan instability is clear, the effect of aerodynamics is generally less understood. Wind gusts, other large vehicles passing or being passed and even small steering inputs from the tow vehicle can all influence airflow symmetry and create aerodynamic forces which could have bad dynamic affects.

Melbourne-based Colin Young, a qualified engineer and founder of the Caravan Council Australia, says that while he believes aerodynamic testing would improve caravan stability, it's hard to achieve without doing practical, real-world testing, with the tow vehicle hooked up.

“While streamlining the shape of a caravan will generally reduce the amount of air drag, it is very hard to design the optimum body contours that will produce the least air resistance," he said.

“Wind-tunnel testing of a small-scale model may well assist in determining the contours that provide the most-stable laminar air flows, while preventing the onset of sudden and undesirable disturbing turbulent flows.

“However, the vital issue is the overall air resistance of the tow-vehicle/caravan combination.

“Clearly the best way to determine - and to reduce - the various resistances is to conduct real-world on-the-road tests.”

Caravan crashes directly caused by high speed aerodynamic instability alone are not common, but understanding such effects – and designing caravans so that they're not badly affected – will only improve safety...

Related: Truckies back wheel at each corner caravan concept