Potential buyers of caravans need to be extremely careful to ensure that their proposed caravan is, in fact, fully compliant with Australia’s vehicle safety regulations, as stated in the national Australian Design Rules. The Federal government’s Vehicle Safety Standards branch has recognised the problems in the caravan industry. It is introducing a new Road Vehicle Standards Act in July 2021. This Act replaces the Motor Vehicle Standards Act, which has been in force since 1989.
Caravan/Tow Vehicle Behaviour & Suspension
For decades both caravan owners and vendors have emphasised the need for a laden tow vehicle to at least weigh as much as the laden caravan. Length and caravan stability, and where weight is distributed over that length, is now being seen as more significant.
Making caravans more stable is readily possible by design, loading, and tow vehicle use and choice. This article by Collyn Rivers explains how. It also provides practical guidelines for buying a caravan and tow vehicle, their loading and on-road usage. For a full technical explanation of why rigs can be unstable please see my Caravan and Tow Vehicle Dynamics/. See also Why Caravans Roll Over/
Wheels falling off trailers, wheel studs breaking and wheel nuts loosening still happens. Here is why it happens, and how to prevent it. Wrecked wheel bearings and fractured stub axles are also common. That fastenings such as wheel nuts may be caused, not just permitted, to loosen is rarely covered in engineering training. The causes and prevention are however well known. This referenced article by Collyn Rivers explains how and why in rigour and detail.
Should I grease my tow ball is asked by caravan owners worldwide. A recent poll in Australia’s Caravanners Forum.com showed that slightly over half do so, but primarily to reduce wear. That seemingly overlooked by almost all respondents is that tow ball friction plays a vital role in reducing caravan sway. Those who grease them unwittingly prejudice safety for the possible need to renew the towball every ten or so years.
In the early 1900s, trailers with central axles, towed by trucks with overhung hitches, were unstable. This escalated as towing speeds increased. Fruehauf (USA) realised hitch overhang imposed lateral forces on tow vehicles. As trailers yawed clockwise, that overhang caused tow vehicles to yaw anti-clockwise. And vice versa. The longer that hitch overhangs, the greater the effect.
This article is a précis of my discussion with Sergeant Graeme Shenton (Orbost Police Station) about two years ago. Sergeant Shenton is a caravanner and is taking a leading role (via ongoing road side checks etc) in establishing accurate data on the extent of overladen RVs on Australian roads. Most rigs checked are caravans – as there are about six times more of those than campervans and motor homes.
The current (February 2018) Australian RV rules and regulations are outlined here by Collyn Rivers of RV Books. Those for trailers under 4500 kg (9920 lb) will change in late 2018 but details are not yet available
Australia has two main and seemingly interdependent caravan industries. One makes caravans of varying stability. The other makes devices (of varying effectiveness) intended to increase that stability. Both are curious. Not all makers appear to understand the more basic laws of physics involved. Or perhaps assume immunity from them. So presumably do owners of ultra-long caravans. With a few rare exceptions, there is a caravan design need for change. And for caravan owners to realise a caravan’s inherent stability issues.
A weight distributing hitch is not needed if the laden weight of the tow vehicle is equal to or exceeds that of the laden caravan. Nor is a weight distributing hitch needed for any trailer under about 4 metres.
Caravan suspension has requirements that are very different from tow vehicles. This is only too often misunderstood. Here’s why and what it should be.
Caravan and motor home tyres take a far greater beating than those in general use – an industry report noted that such tyres are subject to major abuse greater than any other form of use. In particular, stated the report, caravan and motor home tyres are often grossly under-inflated and overloaded.
Tow ball weight pushes down the rear of the tow vehicle – thereby increasing the weight on its rear tyres. A WDH, in effect, is a semi-flexible springy beam that levers back up the rear of the tow vehicle and levers down its front. In doing so, however, it reduces the imposed load on the tow vehicles rear tyres and partially restored it on its front tyres. Reducing the load on the tow vehicle’s rear tyres reduces their ‘cornering ability’. This is why a weight distributing hitch limits cornering.
That caravans roll-over yet most vehicles don’t show trailer dynamics is not understood or is ignored. Trailer dynamics simply explained tells why. The main cause is that hitch extending from the tow vehicle’s rear. It not only allows but causes both tow vehicle and caravan to sway (yaw). Worse – if one yaw’s clockwise it causes the other to sway anti-clockwise. The further the tow ball behind the tow vehicle’s rear axle, the greater that effect.
Do caravans need independent suspension is a marketing rather than an engineering issue. This article explains why caravan beam axles are generally better.
A conventional caravan must always be a compromise. This is because it is towed via hitch at some distance behind the tow vehicle’s rear wheels. If that vehicle sways clockwise, that hitch overhang causes (not just permits) the caravan to sway anti-clockwise. If the caravan sways clockwise, it causes the tow vehicle to sway anticlockwise. is essentially an unstable concept but safe within limits.
The maximum caravan weight safe to tow depends on what tows it. This article by Collyn Rivers explains why – and how to know what it is.
Long end-heavy caravans have a need for a Weight Distributing Hitch (WDH). For all, though, it inherently reduces tow vehicle stability. Here’s how and why.
Correct weight distribution hitch setting up compensates caravan tow vehicle front end lift but introduces instability if too tight. Here’s why.