As you are maybe aware if you are a motorhome owner, many motorhomes sold in New Zealand have a Gross Vehicle Mass (GVM) of 3500kg or less, so that they meet the requirements for a WOF. However with tare weights often around 3200 – 3300 kg, this doesn’t leave much payload (ie the difference between GVM and tare weight), which could be as little as 200kg! Add an extra occupant, food, drink, and clothing means you quickly run the risk of running overloaded. Just exceeding the axle weight limit on one axle while still being within the GVM is still dangerous as you could be exceeding the design mass for the axle, suspension, rims or tyres of that axle. Running overloaded is also likely to invalidate your insurance if you were involved in an accident.
Overseas research has found that around half of all motorhomes are overloaded. It would not be unreasonable to assume similar results in New Zealand. Operating overweight has a direct effect on safety & comfort, and can result in an expensive police ticket as well. Lack of knowledge of the load capacity is often the cause for this overloading. In many instances the vehicle is packed without realising the consequences.
By re-rating your vehicle, it will be safer at higher loads as additional suspension components will need to be fitted to support the extra weight.
As an example, my Motorhome. It is a Fiat Ducato that is 7.5m long, so has a large rear overhang. On the brake rollers at VTNZ when getting a COF, the rear axle weight in the past has been 2000 kg (the maximum allowed).
So you can imagine what happens once I add water, fill the fridge, people, bikes and anything else in the garage. I was weighing in just under GVM, but 170kg over on the rear axle, exceeding the tyre and rim ratings on the rear axle. Not Good.
My Fiat Ducato which has the “light” chassis now has a GVM of 4000kg, and the rear axle rating has increased from 2000 to 2240kg with the fitting of an approved supplementary air-assisted suspension kit to the existing mechanical suspension, from VB Air Suspension.
The benefits with using our approved air kit are, greater loading allowance on the rear axle, as well as better handling and ride performance through the full weight range from unladen up to fully laden weight. The driver controls included with the Comfort Kit allow for partial raising / lowering of the rear suspension to give better access for loading and for leveling up.
Below is the increase in ground clearance at the rear of my motorhome that I can achieve by varying the pressure in the airbags.
|Pressure (Bar)||Increase (mm)|
Note: exceeding 3.5 bar air pressure is only for slow speed maneuvering below 5km/hr. Note: Comfort kits have a pressure relief valve set to 3.5bar on the compressor.
If you have a motorhome based on the Fiat Ducato, Citroen Jumper / Relay or Peugeot Boxer with only a small gap (15mm or smaller) between the rear axle and suspension bump stop like below (ie between the red buffer and the top of the leaf spring), then its quite possible that you too are exceeding the rear axle weight limit of your vehicle.
A technical report written by Fred Hollows, the NZTA adviser to the New Zealand Motor Caravan Association, can be found here further discussing the problem with GVM’s of 3500 kg.
Motorhome Tyre Pressures
Here is a link to a UK website with a calculator for suggested motorhome tyre pressures. https://www.tyresafe.org/check-your-pressures/motorhomes/
Also see their technical information guide on tyres https://loadsafe.nz/wp-content/uploads/2020/08/motorhome-tyre-pressures.pdf