Understanding trailer load capacity is critical in ensuring safe and reliable towing. In New Zealand, trailers should have a GVM (Gross Vehicle Mass) rating and in Australia, a GTM (Gross Trailer Mass) rating which is the trailer tare weight and its load combined.
Australia also requires an ATM (Aggregate Trailer Mass) rating which also includes coupling load.
Knowing the trailers loading limits and adhering to them, reduces the chance of the trailer failing in use as well as preventing a serious accident.
Unfortunately, having a trailer plate with the GVM noted, is not a legal requirement in NZ and there are thousands of trailers on the road that are consistently overloaded and towed illegally, primarily due to the lack of knowledge.
The GVM is the maximum loaded weight recommended by the trailer manufacturer and is based on a couple of factors -
Depending on the designated purpose of the trailer, a manufacturer will fabricate the trailer chassis and drawbar with material suitable for the end use. The manufacturer will consider material that is light and strong enough to accomplish the trailers end purpose without exceeding weight limits and hopefully without compromising strength.
For example, a boat trailer may need to be made to a GVM of 750kg for a boat weighing 600kg. To accomplish this, the manufacturer will need to design and build a trailer that has a tare weight of 150kg or less. To do this, all component weights (tyres/rims, suspension, axle, rollers, etc..) are taken into account and then a chassis designed around these.
A trailer is only as strong as its weakest or lowest rated component (including construction technique).
Trailer components need to function as a whole to make the trailer suitable to carry specific loads safely. From the design of the chassis and drawbar, the spring and axle rating, the hubs, rims and tyres right through to the safety chains and shackles - all need to be matched to be rated equal or exceed the desired gross trailer loading (GVM).
For example – building a single axle trailer for 2000kg GVM and fitting 1500kg rated hubs and stubs or 1000kg rated springs – its “Murphys Law” that the weakest part of your trailer will fail at the most inconvenient moment (think rainy night with a fully loaded trailer on the side of the road in the middle of nowhere with no cell phone coverage).
Always check the manufacturers recommended ratings for each component you fit to your trailer. The under-designed, un-rated or lowest rated component will always be the weakest link on your trailer - be wary of cheap or unbranded components.
If you are building your own trailer or replacing parts on your existing trailer, always fit components which are rated to or above the required or existing GVM. This includes even the smallest components like the nuts and bolts for mounting the coupling to the drawbar.
As with everything, there needs to be balance to the design. Overdesigning the trailer or fitting components way over the required rating can increase the trailers weight to an unpractical point.
In New Zealand, light trailers (up to 3500kg) are not legally required to have their tare weight, GVM or load rating identified by way of label or certificate and therefore there are thousands of trailers on the road whose owners have no idea what their trailers maximum load rating is. Trailers are legally required to have brakes if the GVM exceeds 2000kg - see legal braking requirements.
Three things need to be considered to determine the GVM on trailers where there is no identifying rating.
Firstly, get your trailer weighed. Call up your local tip or refuse station and ask nicely for permission to put your trailer over their truck scales. If this is not an option, find another local weighbridge and get on their good side.
Secondly, take your trailer into your nearest trailer manufacturer, engineering shop or garage and ask them to help determine the trailers GVM based on the trailers tare weight and its construction/components (including whether brakes are fitted). Normally this will only be an approximate (within 100kg or so) but will give you some idea.
Once you have the tare and GVM, make up a tag and fix it to your trailer for future reference.
Lastly and most importantly, check what your tow vehicle is rated to tow. No matter how high your trailer is rated or legally able to carry, the towbar rating is the determining factor in what the total GVM will be for that particular vehicle.
Most late model vehicles will have a maximum unbraked/braked towing capacity noted in the handbook and certified on the towbar.
If you are unable to determine the towing capacity of your tow vehicle, either check with your local towbar manufacturer, go online and check your vehicles specifications, or as a rough guide set out by the NZ Transport Authority use the following – “the laden weight of an unbraked trailer should not exceed three quarters of the unladen weight of the towing vehicle and then only if the towing vehicle's brakes and tyres are in excellent condition. A trailer heavier than this may prevent the vehicle combination from meeting the seven metre for 30km/hr brake performance requirement.”
If the tow vehicle/towbar is only certified or recommended to tow a trailer of 750kg GVM unbraked (check your vehicles handbook for specs) then it is unwise and potentially dangerous to tow an unbraked trailer exceeding 750kg GVM.
If your tow vehicle brakes or tyres are worn, seriously consider reducing the load towed in your trailer until the problems have been rectified. Where possible fit brakes to your trailer, both for safety sake and peace of mind.
Just remember that by overloading your trailer or tow vehicle, you could be charged with dangerous driving if you are caught or have an accident, incurring serious penalties as well as almost certainly voiding your insurance.
Just a note about boat trailers and their potential for overloading.
Boat owners have a habit of unintentionally overloading their boat trailers, particularly single axle trailers where slight variances in weight can make a day out on the water end on the side of the road with broken components or worse.
Adding extra fuel tanks, fishing tackle, bait, full chilly bins, ice, refreshments, dive/ski equipment, etc, can quickly add weight well above the capacity of the trailer.
Fitting a larger outboard, additional batteries, fancy rod holders or similar accessories, should only be undertaken after considering the capacity of the trailer and tow vehicle rating.
One of the most common faults with boat trailers is premature wheel bearing failure and normally occurs after a day on the water, on a hot summers day with an overloaded trailer.
Be aware of what your trailer load rating is, how much your fully fuelled boat and motor weighs and how much gear you are loading into your boat.