Matching a trailer to your tow vehicle

A dual-cab on the Pacific Motorway can have a 3,500 kg braked rating on the window sticker, a 2,800 kg towbar under the bumper, and a 2,000 kg camper on a 50 mm ball. The driver will tell you the vehicle is “rated to three and a half”. The combination is rated to the towbar. Matching a trailer to a tow vehicle is a lowest-number problem. Everything else — the colour of the tub, the brand of the camper, the fact you have towed it before — is conversation.

This article is about that arithmetic for light Australian combinations: ATM, GTM, tare, GCM, towbar ratings and a towball download around 8–10 per cent. It is workshop practice, not a substitute for the vehicle’s handbook or a state agency. If the trailer itself is still a legal question, read ADR compliance first.

Collect the plates before you collect the brochure

Write seven numbers on a card and keep the card in the glovebox.

Vehicle braked tow rating. The maximum the maker will let you put behind the vehicle when the trailer has brakes. On many current dual-cabs this is 3,500 kg. On many wagons, utes and older 4WDs it is 2,000 kg or 2,500 kg. It is not a target.

Vehicle unbraked tow rating. Often 750 kg. This is the number that matters if you are looking at an unbraked box trailer. It is a vehicle limit, sitting beside the trailer-side GTM threshold in when your trailer needs brakes.

GVM. The maximum the laden vehicle may weigh, including people, fuel, canopy, tools and the download from the trailer.

GCM. The maximum the laden vehicle plus laden trailer may weigh. GCM is the number Australian combinations run out of first, and the one salespeople mention last.

Towbar and towball ratings. A genuine bar will state a towing mass and a ball download. Aftermarket bars sometimes state less than the vehicle. The bar wins. So does a worn 50 mm ball that was never rated for the camper you just bought.

Trailer ATM, GTM and tare. ATM is the trailer’s maximum laden mass, including download. GTM is the share on the trailer tyres. Tare is empty. Payload is ATM minus tare. If those stamps are a mystery, use payload, ATM and tare.

Coupling rating. A 50 mm head marked 2,000 kg does not become a 3,500 kg head because the vehicle is. Coupling types is the mechanical version of this paragraph.

The match is the lowest of those numbers. There is no average.

GCM is not a rumour

Take a dual-cab that weighs 2,350 kg unladen. Add a canopy, a steel tray, a second battery, 80 litres of diesel, two adults, a dog and a week of tools. You are looking at 2,900–3,200 kg on a public weighbridge before the trailer exists. If GCM is 6,000 kg, the trailer and its load cannot exceed 2,800–3,100 kg, even if the braked rating says 3,500 kg.

That arithmetic is why “the ute will pull it” is not a specification. Pulling is a hill question. GCM is a legal and thermal question. Gearboxes, rear axles and tyre load ratings were signed off as a combination, not as a party trick.

If the vehicle maker did not publish a tow rating or a GCM, do not invent one. Some states fall back on ratios of trailer mass to the vehicle’s unladen mass, and they treat override brakes more strictly than brakes the driver applies. South Australia publishes that fallback. It is a legal minimum for an old vehicle, not a reason to buy an unrated van and a 2-tonne trailer. Confirm with the handbook and the state agency.

Download: the 8–10 per cent that keeps the steer axle honest

Towball download is the vertical load the coupling puts on the vehicle. Australian practice for light trailers is about 8–10 per cent of the trailer’s laden mass, within the ball and bar limits. A 2,000 kg laden camper wants roughly 160–200 kg on the ball. A 1,400 kg plant trailer wants about 110–140 kg. A 750 kg box trailer wants about 60–75 kg.

Too little download — fridge on the tailgate, boat motor on the extreme rear, empty A-frame — and the trailer yaws. Too much, and you unload the front axle. Headlights point at the sky, steering goes light, and the rear tyres and towbar do a job they were not plated for.

Weigh it. A bathroom scale under the jockey wheel is better than a guess, and a public weighbridge is better than a bathroom scale. Weigh the vehicle’s front and rear axles with and without the trailer. If the front axle drops sharply when you couple up, the download is too high or the load is too far forward, and a weight-distribution hitch becomes a conversation — not a rating increase. That conversation is weight-distribution hitches and sway.

Height, brakes and the electrical match

A legal mass can still be a bad match.

Coupling height. AS 4177 wants the 50 mm ball between 350 mm and 460 mm from the ground on a laden vehicle. A trailer designed around a lifted 4WD will sit tail-high on a standard wagon and transfer the wrong download. Shank height and drop/rise receivers exist for this. A cushion hitch that leaves the trailer pointing at the horizon does not.

Brake type. An unbraked trailer cannot use the vehicle’s braked rating. An override trailer still needs a vehicle that is happy with override. Electric brakes need a controller and a plug with a working auxiliary. Air or power-hydraulic need the matching cab controls. The trailer’s ADR 38 band does not install those for you.

Electrics. Lamps must work with the vehicle’s lamps. A seven-pin flat on the vehicle and a seven-pin round on a used horse float is a Saturday adaptor, not a match. If you tow a breakaway trailer, the vehicle side of that circuit has to exist.

Tyres and cooling. The vehicle’s rear tyres must have the load index for GVM plus download. The trailer tyres must have the load index for GTM, per tyre, after you divide by the number of tyres and apply the maker’s dual-fitment rules. Long climbs in the ranges will tell you whether the gearbox oil and the radiator were specified for the GCM you are actually using.

Worked combinations, without the sales pitch

A 7×4 unbraked box behind a mid-size SUV. The trailer ATM is 750 kg, tare 220 kg. The vehicle’s unbraked rating is 750 kg, GCM is comfortable, download about 60 kg. This works until the owner puts a cubic metre of wet sand in a trailer whose payload is 530 kg. The match failed on payload, not on the vehicle.

A 2,000 kg ATM tandem plant trailer behind a dual-cab. Braked rating 3,500 kg, towbar 2,800 kg, GCM 6,100 kg, vehicle as-used 3,000 kg. The bar is the limit: 2,800 kg, not 3,500 kg. If the excavator plus trailer tare is 2,650 kg and download is 220 kg, you are inside the bar but hard on the rear axle. Weigh it.

A 2,800 kg hard-floor camper behind the same dual-cab. GCM leftover is 3,100 kg, so ATM is usable on paper. Download at 9 per cent is 250 kg, which may exceed the ball. The front axle will squat unless the load is right and, if the vehicle maker allows it, a WDH is set up properly. This is a camper match, not a box-trailer match. See camper trailers.

Specify the vehicle and the trailer as one job

If you are still buying, do not buy the trailer first and “get a bigger ute later”. The later ute may have the same GCM problem and a different towball height. Specify payload, download and the roads — Hume, corrugations, boat ramp — in the same brief. Specifying a custom trailer for Australian conditions is the trailer half of that brief. The towing hub is the rest.

This is not a substitute for the vehicle handbook or a state towing rule. Confirm ratings on the plates in front of you, and confirm any fallback rule with the state agency if the vehicle has no published GCM.

When the numbers are honest, the next handling question is whether you need springs in the hitch or just a better load. Start with weight-distribution hitches and sway control.