The two main reasons not to run an overweight tow rig are it's illegal and unsafe

TOW-WEIGHT LEGAL

If you're wondering why we've focussed on towing weights over the last couple of issues, the answer is that it's a growing problem from two ends, with trouble in the middle. First, carmakers tend to overstate the ability of their cars to tow, promoting headline figures which are only possible in limited, unrealistic scenarios. Second, caravan makers are notorious for understating the weight of their trailers. So, if you have, say, an older Prado with a maximum braked towing capacity of 2500kg, and you buy a 2500kg caravan, you'll likely find that it's impossible to tow that caravan once you take your vehicle's payload into consideration. Then you run the risk of the caravan actually weighing more than it says, and before you know it, you're in the uncomfortable situation of being both dangerous and illegal, something the police are increasingly focusing on.

WORDS AND IMAGES ROBERT PEPPER

What you need to know to be

GUIDE Payloads

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HOW TO AVOID BEING OVER-WEIGHT
So let's look at what you need to do to avoid that nasty situation. First up, weigh your trailer which you can do at any public weighbridge, maybe even your local tip. Do so unloaded, and then loaded with fluids and gear. Then check those weights against the trailer's placarded Tare (unladen weight) and ATM (Aggregate Trailer Mass), which is the maximum permitted weight of the trailer. We'd be interested to hear the results, but we're guessing the two weights will be some way over the placard. If they are, well, you need to fix that which is another story for another time.

You then need to look up the maximum braked tow capacity of your vehicle, for example, 3000kg, and ensure the trailer when loaded is below that figure. Watch out for small print and caveats around the maximum braked tow figure too, because typically that means reducing towing capacity; for example maximum speeds, use of special towbars, restrictions on towball mass, and sometimes payload restrictions. And make sure the capacity is for your specific vehicle as it may vary with transmission and engine options.


Next, you need to measure your towball mass (TBM), which is also easily done at the weighbridge by measuring the jockey wheel, or at the towball itself with a special measuring tool available from auto accessory shops. The jockey wheel measure will be a little more than the actual TBM as the wheel is further towards the trailer wheels. That needs to be less than the lower of the maximum TBM on your vehicle or your vehicle's towbar and the two may not be the same.

BELOW Acronyms a go-go! But you need to understand them

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PAYLOAD IS MORE THAN JUST WHAT'S IN THE BOOT
Now you turn your attention to the tow vehicle, and weigh that, in its ready-to-tow configuration. The base unloaded weight is called the Tare, or kerb weight, and unfortunately, there are several different definitions. What you can be sure of though is that your 'unladen' car will weigh a bit more than the manufacturer's Tare weight, which probably won't include a towbar or full tank of fuel. Then you've got the GVM, or Gross Vehicle Mass, which is the maximum the vehicle can weigh. The difference between the Tare and GVM is the payload, which is how much the car can carry. Typically, that's between around 550kg to 800kg for 4WD wagons and 700kg to 950kg for 4WD utes.

And you may not think you've modified your tow vehicle very much, but virtually every modification you make adds weight, whether the part is a replacement or addition.


ABOVE Here’s a towbar placard saying it’s good to tow 3500… but only with a 250kg towball mass. Aussie trailers just aren’t built that way

ABOVE Here’s an example of the fine print in towing. Yes, the Pajero can tow 3000kg. Provided the towball mass is REDUCED to 180kg from 250kg

That heavy-duty suspension? Maybe 20kg heavier than stock. Light-truck tyres to replace standard passenger versions? Add another 2-3kg per wheel, more if you've increased tyre width or diameter. Driving lights? Another 5kg or so. Even the 'nothing' parts add up; your UHF radio system is 4kg or so, extra wiring for your electrics, brake controller, rubber interior mats and, before you know it, you've added another 100kg or more, and that's before we get to bullbars which could add 30 to 80kg, or second batteries, or rear storage systems which could be up to 100kg.  A long-range fuel tank will weigh more than standard even before you fill it. When you start looking at setting up a car for touring, payload disappears very quickly indeed.

There are also roof loads as well, which are typically quite low at 75 to 150kg, including the weight of the rack. That's one reason you should consider an alloy roof rack not steel as with some steel racks you've actually used all your roof load in the rack weight alone. And that roof limit is also for a static or bitumen load, not hours of rough corrugations (called dynamic load).

So payload might have reduced by a good 150 to 300kg before you even get into the car, or load it with your gear. The moral of this story is that cars, even 4WDs, cannot carry anything like as much as you think they can.

ABOVE Most 4X4s have a roof load of between 100 and 200kg including the roof rack, so it doesn’t make much to exceed it

PUTTING VEHICLE AND CARAVAN TOGETHER
Now we come to the combined weight of the trailer and tow vehicle; we'll call it, the 'combination'. The GCM (Gross Combination Mass) is how much the combination can weigh.

Here's your first likely problem; the GCM is likely to be less than the sum of the GVM, and the maximum braked towing capacity, so, for example, in the case of the Ford Ranger the GCM is 6000kg, the maximum braked towing capacity is 3500kg, and the GVM is 3200kg. That means that 6000 GCM - 3500kg max tow = 2500kg max weight for the Ranger, and take off the 2250kg tare weight that leaves us with a whole 250kg of payload. Take two adults out of that, and not a lot is leftover.


However, that's not quite the full story. If we assume the 3500kg trailer has a 350kg towball mass, then we're transferring 350kg from the trailer to the tow vehicle, so the load on the Ranger is 2500kg + 350kg = 2850kg. That's below the Ranger's GVM of 3200kg, so we're all good there. We can't use the extra payload of the Ranger (3200 - 2850kg = 350kg) because we're already at the 6000kg GCM limit, so we'd need to tow a lighter trailer than 3500kg. The rear axle load may also be a limiting factor, and that comes in to play because we're adding 350kg to it, and because the towball is a good 1.5m behind the rear axle, even more than 350kg is being transferred.

ABOVE Example caravan placard showing towball mass (ball loading at Tare, note Tare not loaded weight), GTM and ATM. Wise buyers will not hand over any more than a deposit until these numbers are proven on an independent weighbridge

BELOW Large caravan, relatively small tow vehicle. The Ranger isn’t highly modified – canopy, bullbar, UHF radio, towbar, aftermarket suspension, towing mirrors and driving lights but that alone is probably over 130kg, and out of a payload of 900kg that’s quite a bit before other gear, cargo and occupants are considered

A GVM UPGRADE WILL FIX IT, RIGHT?
Sadly, no. Some GVM upgrades merely total the front and rear axle loads and call that the GVM, which is kind of pointless as it's not much of an increase. Then you need to load the car precisely, so each axle load is balanced — the reason the axle loads typically total more than the GVM is so there can be front/rear load flexibility. There are other GVM upgrades which increase the axle loads and with it the GVM, but as the GCM isn't increased then you're able to carry more on the tow vehicle but tow less. In any case, there's a reason manufacturers set a GVM, and that's to do with stress on the vehicle such as the transmission, wheel bearings, driveline, cooling system ... everything. Go above that, and you're entering territory the vehicle wasn't designed to handle. For instance, if you increase the GVM by, say, 19 percent, then you're increasing wear and tear by 19 percent.

If all of this sounds confusing and complicated, and you'd rather not worry about an acronym soup along with reading the confusing fine print and doing calculations, well, you're not alone. Unfortunately, the reality is there's no way around it if you want to be legal and safe. But you can make your towing compliance a lot easier, although you're not going to like this solution. It's simple - tow no more than two-thirds of the maximum braked weight, or around 2350kg for a 3500kg rated vehicle. That way, you'll be reasonably certain your GVM, axle loads, towball mass, maximum braked towing weight and GCM are likely to be within limits. You can certainly go above that but the closer you get to the maximum braked towing capacity, the more carefully you need to pay attention to all those tediously boring limits. And our long-standing advice still stands —  if you want to tow a large, 3000kg-plus caravan then to do so safely you need something like a large American ute capable of towing 4500kg, or a small truck like a Fuso Canter.

This issue isn't going away anytime soon, so expect more articles on the subject in the coming months.

BELOW Look at the relative size differences between an LC200 and F-250, then tell us which one you think is least likely to be bossed around by three tonnes of caravan

If you're wondering why we've focussed on towing weights over the last couple of issues, the answer is that it's a growing problem from two ends, with trouble in the middle. First, carmakers tend to overstate the ability of their cars to tow, promoting headline figures which are only possible in limited, unrealistic scenarios. Second, caravan makers are notorious for understating the weight of their trailers. So, if you have, say, an older Prado with a maximum braked towing capacity of 2500kg, and you buy a 2500kg caravan, you'll likely find that it's impossible to tow that caravan once you take your vehicle's payload into consideration. Then you run the risk of the caravan actually weighing more than it says, and before you know it, you're in the uncomfortable situation of being both dangerous and illegal, something the police are increasingly focusing on.

The two main reasons not to run an overweight tow rig are it's illegal and unsafe.

GUIDE Payloads

TOW-WEIGHT LEGAL

What you need to know to be

WORDS AND IMAGES ROBERT PEPPER

HOW TO AVOID BEING OVER-WEIGHT
So let's look at what you need to do to avoid that nasty situation. First up, weigh your trailer which you can do at any public weighbridge, maybe even your local tip. Do so unloaded, and then loaded with fluids and gear. Then check those weights against the trailer's placarded Tare (unladen weight) and ATM (Aggregate Trailer Mass), which is the maximum permitted weight of the trailer. We'd be interested to hear the results, but we're guessing the two weights will be some way over the placard. If they are, well, you need to fix that which is another story for another time.

You then need to look up the maximum braked tow capacity of your vehicle, for example, 3000kg, and ensure the trailer when loaded is below that figure. Watch out for small print and caveats around the maximum braked tow figure too, because typically that means reducing towing capacity; for example maximum speeds, use of special towbars, restrictions on towball mass, and sometimes payload restrictions. And make sure the capacity is for your specific vehicle as it may vary with transmission and engine options.


Next, you need to measure your towball mass (TBM), which is also easily done at the weighbridge by measuring the jockey wheel, or at the towball itself with a special measuring tool available from auto accessory shops. The jockey wheel measure will be a little more than the actual TBM as the wheel is further towards the trailer wheels. That needs to be less than the lower of the maximum TBM on your vehicle or your vehicle's towbar and the two may not be the same.

BELOW Acronyms a go-go! But you need to understand them

TO TAKE A TOUR
CLICK HERE
OR SEE THE FULL RANGE AT
UNIVERSALCARAVANS.COM.AU

Australia’s most luxurious slide out caravans

ADVERTISEMENT
SCROLL TO CONTINUE

PAYLOAD IS MORE THAN JUST WHAT'S IN THE BOOT
Now you turn your attention to the tow vehicle, and weigh that, in its ready-to-tow configuration. The base unloaded weight is called the Tare, or kerb weight, and unfortunately, there are several different definitions. What you can be sure of though is that your 'unladen' car will weigh a bit more than the manufacturer's Tare weight, which probably won't include a towbar or full tank of fuel. Then you've got the GVM, or Gross Vehicle Mass, which is the maximum the vehicle can weigh. The difference between the Tare and GVM is the payload, which is how much the car can carry. Typically, that's between around 550kg to 800kg for 4WD wagons and 700kg to 950kg for 4WD utes.

And you may not think you've modified your tow vehicle very much, but virtually every modification you make adds weight, whether the part is a replacement or addition.


That heavy-duty suspension? Maybe 20kg heavier than stock. Light-truck tyres to replace standard passenger versions? Add another 2-3kg per wheel, more if you've increased tyre width or diameter. Driving lights? Another 5kg or so. Even the 'nothing' parts add up; your UHF radio system is 4kg or so, extra wiring for your electrics, brake controller, rubber interior mats and, before you know it, you've added another 100kg or more, and that's before we get to bullbars which could add 30 to 80kg, or second batteries, or rear storage systems which could be up to 100kg.  A long-range fuel tank will weigh more than standard even before you fill it. When you start looking at setting up a car for touring, payload disappears very quickly indeed.

There are also roof loads as well, which are typically quite low at 75 to 150kg, including the weight of the rack. That's one reason you should consider an alloy roof rack not steel as with some steel racks you've actually used all your roof load in the rack weight alone. And that roof limit is also for a static or bitumen load, not hours of rough corrugations (called dynamic load).

So payload might have reduced by a good 150 to 300kg before you even get into the car, or load it with your gear. The moral of this story is that cars, even 4WDs, cannot carry anything like as much as you think they can.

ABOVE Here’s a towbar placard saying it’s good to tow 3500… but only with a 250kg towball mass. Aussie trailers just aren’t built that way

ABOVE Here’s an example of the fine print in towing. Yes, the Pajero can tow 3000kg. Provided the towball mass is REDUCED to 180kg from 250kg

ABOVE Most 4X4s have a roof load of between 100 and 200kg including the roof rack, so it doesn’t make much to exceed it

PUTTING VEHICLE AND CARAVAN TOGETHER
Now we come to the combined weight of the trailer and tow vehicle; we'll call it, the 'combination'. The GCM (Gross Combination Mass) is how much the combination can weigh.

Here's your first likely problem; the GCM is likely to be less than the sum of the GVM, and the maximum braked towing capacity, so, for example, in the case of the Ford Ranger the GCM is 6000kg, the maximum braked towing capacity is 3500kg, and the GVM is 3200kg. That means that 6000 GCM - 3500kg max tow = 2500kg max weight for the Ranger, and take off the 2250kg tare weight that leaves us with a whole 250kg of payload. Take two adults out of that, and not a lot is leftover.


However, that's not quite the full story. If we assume the 3500kg trailer has a 350kg towball mass, then we're transferring 350kg from the trailer to the tow vehicle, so the load on the Ranger is 2500kg + 350kg = 2850kg. That's below the Ranger's GVM of 3200kg, so we're all good there. We can't use the extra payload of the Ranger (3200 - 2850kg = 350kg) because we're already at the 6000kg GCM limit, so we'd need to tow a lighter trailer than 3500kg. The rear axle load may also be a limiting factor, and that comes in to play because we're adding 350kg to it, and because the towball is a good 1.5m behind the rear axle, even more than 350kg is being transferred.

BELOW Large caravan, relatively small tow vehicle. The Ranger isn’t highly modified – canopy, bullbar, UHF radio, towbar, aftermarket suspension, towing mirrors and driving lights but that alone is probably over 130kg, and out of a payload of 900kg that’s quite a bit before other gear, cargo and occupants are considered

ABOVE Example caravan placard showing towball mass (ball loading at Tare, note Tare not loaded weight), GTM and ATM. Wise buyers will not hand over any more than a deposit until these numbers are proven on an independent weighbridge

A GVM UPGRADE WILL FIX IT, RIGHT?
Sadly, no. Some GVM upgrades merely total the front and rear axle loads and call that the GVM, which is kind of pointless as it's not much of an increase. Then you need to load the car precisely, so each axle load is balanced — the reason the axle loads typically total more than the GVM is so there can be front/rear load flexibility. There are other GVM upgrades which increase the axle loads and with it the GVM, but as the GCM isn't increased then you're able to carry more on the tow vehicle but tow less. In any case, there's a reason manufacturers set a GVM, and that's to do with stress on the vehicle such as the transmission, wheel bearings, driveline, cooling system ... everything. Go above that, and you're entering territory the vehicle wasn't designed to handle. For instance, if you increase the GVM by, say, 19 percent, then you're increasing wear and tear by 19 percent.

If all of this sounds confusing and complicated, and you'd rather not worry about an acronym soup along with reading the confusing fine print and doing calculations, well, you're not alone. Unfortunately, the reality is there's no way around it if you want to be legal and safe. But you can make your towing compliance a lot easier, although you're not going to like this solution. It's simple - tow no more than two-thirds of the maximum braked weight, or around 2350kg for a 3500kg rated vehicle. That way, you'll be reasonably certain your GVM, axle loads, towball mass, maximum braked towing weight and GCM are likely to be within limits. You can certainly go above that but the closer you get to the maximum braked towing capacity, the more carefully you need to pay attention to all those tediously boring limits. And our long-standing advice still stands —  if you want to tow a large, 3000kg-plus caravan then to do so safely you need something like a large American ute capable of towing 4500kg, or a small truck like a Fuso Canter.

This issue isn't going away anytime soon, so expect more articles on the subject in the coming months.

BELOW Look at the relative size differences between an LC200 and F-250, then tell us which one you think is least likely to be bossed around by three tonnes of caravan

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