The number a manufacturer puts on its “maximum tow rating” is the upper limit of what _that vehicle_ could conceivably pull, if the stars aligned: the driver of the tow vehicle was the only person in it; there were no optional equipment on the tow vehicle; the hitch and accessories were special, and somehow moved the weight of the trailer exactly over the rear axle of the tow vehicle and no more; the liability lawyers worked for the trailer manufacturer and were deeply in debt to the tow vehicle maker; the passengers in the tow vehicle were all in complete agreement and motionless about that Mack Truck bearing down on you at 80mph; the tow vehicle was pickled in aspic and would labor over mountains in searing heat until the engine melted; … the list goes on.
The weight numbers that actually matter
Before you connect anything, you must understand three numbers, and they’re not Grades 3, 5, and 7. Gross Combination Mass (GCM) is the maximum weight of the whole shooting match – the tow vehicle plus the laden trailer. The manufacturer determines it, and crossing that line is illegal. Aggregate Trailer Mass (ATM) is the weight displayed on the trailer’s paperwork when it’s fully loaded, including the portion of that weight being carried by the tow ball. And Gross Vehicle Mass (GVM) is the max weight your tow vehicle can legally be. That includes fuel, you, old mate, the kids, and their gear, and also the push down from the trailer coupling.
Here’s how people screw the pooch. Let’s say your vehicle has a GVM of 3,000kg, and a GCM of 6,000kg. The back of the envelope suggests the vehicle can tow 3,000kg. But who can’t load four grown-ups, fuel, camping gear, and a toolbox into the car and not use 500kg – 700kg of your GVM? And if you do the same math on the GCM side, 500kg – 700kg of your GVM also comes off the tow weight because you’re up against the limit on both sides of the equation.
Do these calculations in advance of every trip. Do them when you fill up the gas tank after you’ve just stopped for fuel. Do them while you drive to your buddy’s house to pick him up because you’re allowed to tow him in the van. Because illegal is illegal.
How tow ball weight controls whether you arrive safely
The downward force exerted by the trailer’s coupling onto the tow ball of the towing vehicle is known as the tow ball weight (TBW). The rule of thumb is that TBW should be 10% of the total loaded weight of the trailer. So on a 2,500kg trailer, you apply about 250kg of pressure to the ball.
This isn’t a fluke number. It derives from the impact that weight has on the distribution of yaw inertia, which is the resistance to a trailer rotating around its length. Too little weight on the tow ball, and the tail wags the dog. A lateral impulse – a sudden gust of wind, a lane changing manoeuvre, the slip of a passing semi – can cause the trailer to fishtail, something exacerbated by the instability created with each swing. Eventually, the frequency is so high that the driver cannot steer quickly enough to correct it. The tow vehicle is dragged in.
Too much tow ball weight, and the tug loses its teeth. You move loading off the tug’s front wheels and onto its rears. This reduces your steering authority and the effectiveness of your front brakes. You’ll see your headlights pointing towards space and drive a porpoising pig that no longer wants to turn corners.
A weight distribution hitch can help spread tow ball download over all axles so that it doesn’t overload the poor old inadequately-sprung rear suspension of your wagon. Similarly, sway control can eliminate the swings and roundabouts by electronically or mechanically sniffing oscillation at its source and applying a counteracting force (usually brakes on one side of the rig). But sway control is not a band-aid for a fundamentally poor match of weights.
The legal and insurance exposure most people don’t think about
The figures quoted by the transport authorities of some Australian states and road safety organisations are sobering: of caravans and heavy trailers weighed at police checkpoints on the open road, between 50 and 75 per cent are found to be operating over their legal weight limits, or are outside the limits of their legal weight-bias setting. In real terms, two-thirds to three-quarters of other towing rigs on your average thoroughfare will have some legal exposure.
Operating over GCM or GVM, though, puts you south of manufacturer’s certification. This is an important difference, because most vehicle underwriters’ policies contain a caveat that absolves the insurer from liability if the vehicle is being operated beyond the manufacturer’s certified limits in the event of a claim. A single-vehicle rollover as a result of caravan-sway, where the towing vehicle was 200kg over GCM, could sound the death-knell for any insurance claim – it doesn’t get reduced, it gets voided.
Defect notices can be issued, the vehicle can be required to be unloaded by the side of the road, and fine-infringement notices can be (and, in commercial operations, routinely are) issued to the driver and operator by enforcement authorities. The road vehicle rules in Australia are incorporated under the moral and useful-sounding banner title of Australian Design Rules (ADRs), and they are, by and large, black-letter law. There’s very little wriggle room.
What the mechanical toll looks like over time
Towing with a vehicle that exceeds its capacity isn’t just a legal problem. It also wears components in ways that often go unnoticed until one of them gives up the ghost when you can least afford it.
The most common victim of high-load towing is the transmission, which depends on fluid to keep the clutch packs and planetary gear sets cool. When demand is too high for too long, fluid temperatures climb and the fluid loses its lubricating and cooling properties. Result: transmission damage. Most transmission coolers are marginal for extended high-load towing in high ambient temperatures. They’re usually just big enough for the transmission’s needs during normal driving, not when it’s working at maximum capacity and beyond up a long grade on a hot day.
The rear suspension takes a bashing when the tow ball weight is at the upper end of the permitted scale. Coil springs or leaf springs compress under load, which can change the vehicle’s attitude, the angle of the attached trailer, and lead to excessive wear in the shock absorbers and bushings. A drooping rear end not only rides poorly and looks unattractive, it can change the tow ball height sufficiently to induce dangerous levels of sway in the trailer because it’s no longer running level.
Brakes wear faster when you’re using them to scrub off the trailer’s speed as well as the vehicle’s. If the trailer’s braking system is poorly adjusted or is simply too puny for the load, the vehicle brakes take most of the strain with every stop. Regular inspection of suspension components, couplings, and braking hardware is non-negotiable for anyone towing regularly. Sourcing reliable trailer supplies perth ensures worn components get replaced with ADR-compliant hardware before a failure happens at highway speed, not after.
Brake system requirements and what they actually mean
Braking requirements for trailers in Australia are based upon two weight factors: the Aggregate Trailer Mass (ATM) and the Gross Trailer Mass (GTM). The ATM is the total mass exerted on the ground by the trailer wheels and the GTM is the trailer mass supported by the towbar and coupling (the downward force onto the tow ball).
The first regulation specifies that trailers with an ATM of 750kg or greater or trailers with an ATM less than 750kg where the GTM is 10% or more of the trailer’s ATM are required by law to be fitted with brakes.
The second requirement dictates that if the trailer’s GTM is more than the unladen mass of the trailer divided by the number of wheels on the ground, the trailer must be fitted with brakes.
The trailer type defines the brake system that can be fitted: override (mechanical) or electric.
Axle configuration, aerodynamics, and fuel consumption
Single-axle trailers are advantageous in the sense that they are lighter, easier to steer, and have less resistance when rolling. Meanwhile, tandem axle trailers provide more capacity for carrying loads, better stability, and if one tire fails you probably won’t even notice. The downside is that tandem axles must have the trailer perfectly level when hitched. If the trailer nose is low, the front axle will carry too much weight, while the rear carries too little. This causes uneven tire wear and negates the lateral stability that the tandem setup should provide.
Also, the aerodynamics of the trailer make a big difference in fuel consumption. A tall, squared front creates a huge low-pressure area behind the tow vehicle at highway speed, and the engine works much harder to keep the vehicle moving. Height, width, and length of both the trailer and the gap in-between the back of the tow vehicle and the front of the trailer all contribute. Some of the caravan designs with sloped fronts or fairings cut this a lot, and the fuel savings are very real on a long trip.
How to use a weighbridge to confirm you’re legal
The only sure way to obtain your correct weights is to visit a public weighbridge. Three separate weighings will give you all the numbers you need.
First weigh your tow vehicle only, with all passengers, all fuel and all load on board, but without the trailer attached. From this, you’ll know the actual operating weight of the vehicle and will be able to check it against the manufacturer’s recommended maximum (GVM).
Then weigh the vehicle with the loaded-for-camping trailer hitched (i.e. the coupled combination weight). This number must be lower than the manufacturer’s recommended maximum for gross combined mass (GCM).
If you then drive so far forward that the rear of the trailer’s wheels is on the weighbridge while your tow vehicle remains off it, the weighbridge operator will also be able to record the weight on the trailer’s wheels. Subtract this number from the coupled combination weight and you will know how many kilograms your towball is pressing down. This is your towball weight and it should be 8% to 12% of the total weight of the trailer. If any of those three numbers are too high, you have a problem to fix before you hit the road.
The pre-tow checklist that actually covers the risk
Before you couple up and start towing, you should give everything a physical once-over. Are your safety chains being used in a way that lets your trailer pivot without pulling themselves tight or going to ground? Are the D-shackles attaching your chains to the towing vehicle moused or otherwise secured so they can’t vibrate apart? Can you fit the point of a pen between the ball hitch and your trailer’s coupler? If you can, your coupler’s worn and you’re looking at potentially dangerous levels of play on the road.
The breakaway switch cable needs to be attached to the towing vehicle separately from the safety chains so that the trailer brakes can activate if the trailer separates. This switch is required by law on electrically braked trailers, but it’s a good idea to fit one if your trailer weight exceeds a certain threshold.
If your coupler has wear indicators, check them against the coupler’s maximum service wear limit.
Check and reinflate your trailer tires (which sit idle for long periods and can develop flat spots, cracks or slow leaks on the sidewall) before each trip.
Now, those items above might seem a bit nitpicky, but the thing is you’re only as safe as your smallest, weakest component. Getting the right configuration from the outset, knowing your real weights, keeping good hardware on your brakes and hitch, and running a consistent pre-departure check each time is what keeps your setup legal and your truck under your control.