As the warmer weather comes around, school holidays approach and annual leave is booked - we’re all thinking of loading up the bus and heading out somewhere with the family, trailer or caravan in tow.
We’re all conscious of loading extra weight into our cars but when it comes to overloading your vehicle, ‘She’ll be right, Mate’ isn’t the right attitude and could potentially have dangerous or expensive consequences.
So what do you mean? How easily can I overload my 4x4?
Before we begin, let’s get a few terms out of the way first:
- Tare Mass - Empty weight of the vehicle with ~10 litres of fuel
- GVM - Gross Vehicle Mass - Maximum allowable weight of the vehicle, including tow ball weight
- Payload - Difference between Tare Mass and GVM, essentially how much you can load into your vehicle.
- GTM - Gross Trailer Mass - Total weight of trailer and trailer payload.
- Towing capacity - Maximum allowed weight of towed trailer/caravan including trailer payload
- GCM - Gross Combination Mass - Maximum allowed weight of vehicle, towed weight and payloads combined
- Tow ball weight - The kilogram force pushing down on the tow ball from whatever trailer you are towing
Let’s take some specifications right off the Ford Australia website for the 2019 Ford Ranger. Arguably one of the most popular vehicles for weekend warriors, tradies, and businesses purchasing fleet vehicles. The 3.2TD 4x4 XL Double Cab Chassis in particular. This vehicle has a tare mass of 2012kg and a GVM of 3200kg. Doing the math shows us that we have a 1188kg payload. Nice one, Gary!
That’s heaps of payload capacity, right?!
Yeah, it looks that way at first glance, but let’s add some modifications to our ute and load up for a week away with the family.
- Family of 4 - 260kg
- Steel bullbar and winch with steel cable - 80kg
- Steel undertray/gearbox bash plate - 20kg
- Dual battery setup, catch can, snorkel, other mods such as driving lights, etc - 30kg
- Side steps - 30kg
- ‘Service body’ style lockable canopy - 150-200kg
- 80 litres of drinking water in container - 80kg
- Full length aluminium roof rack - 20kg
- Clothes, games, food for 7 days - 30kg
- Tent, camp chairs, cooking stove - 60kg
- Two gas bottles - 9kg ea.
- Two full size spare tyres mounted on alloy rims - 35kg ea.
- Miscellaneous (small compressor, maxtrax, tools, first aid kit, jack, so on - 60kg)
Alright, so if your maths is as good as mine you’ve just added 860, 1000.. Uh, 958kg to your ute. Spot on Bevan! But wait, we still need to count the up-to 350kg of tow ball weight from a heavy caravan. Now we’re overweight and towing (hah!) a dangerous line when it comes to legality and insurance coverage.
Another way of looking at this is, you’re in the same Ford Ranger, you’re towing a car trailer with a stout 4x4 on it (your mates busted Nissan, I bet.) And your GTM is 3500kg, the maximum allowed on this vehicle. The Ranger has a GCM of 6000kg.
- GCM - GTM - Tare Mass = Payload allowable in vehicle.
- 6000 - 3500 - 2012 = 488kg.
488kg of payload remaining and we haven’t even taken into consideration the tow bar weight, fuel, driver and passengers, or potential recovery tools/gear.
In practice, this exact situation can rarely happen but it shows how quickly you can overload even a pretty sturdy modern 4x4.
The generally accepted rule across Australia is 10% of the trailer weight is allowed to be placed on the tow ball. That being said, a whole lot of other countries with similar laws impose strict limits of 100kg ball weight regardless of trailer size.
So who cares? What can go wrong?
When adding more weight to a vehicle, there are a lot of components affected. Items like springs can be permanently damaged and start sagging. As suspension load is increased you can increase the risk of torn or damaged bushings as well as flogging out the shock absorbers and bump stops quicker. Don’t forget about your poor wheel bearings too, as they’ll cop the worst of it.
A flow on effect of damaged, sagging or torn suspension components is your alignment will get knocked out of spec easily, causing premature tyre wear.
Another component affected by overloading (and improper load balancing) is the chassis. This is the infamous “dual cab banana”, which can occur on vehicles that see off-road abuse. The primary cause? Generally too much weight far back and excessive tow ball weight. You’ve all seen the photos of bent utes on the internet. Maybe that’s why your mate is on the back of the car trailer we mentioned earlier? Who knows.
Photo credit: 4x4earth.com forums (link)
A lot of the reason for the phenomenon of bent chassis has to do with where the weight is loaded in the tray. We got one of our MS Paint artists to draw up a nifty diagram showing you what we mean. Apart from the chassis, the tub of the ute (or tray/toolbox so on) is not connected to the cab. The chassis is the only component holding the two in place, so you can think of it as two seperate vehicles sharing a frame.
We drew this, are you proud, Mum?
In this diagram, the ‘force’ is the suspension pushing back up on the chassis, whether it’s coil suspension, aftermarket bag kits or spread across multiple mounting points of a leaf spring, the effect is the same. The load is whatever you have in your tray, imagine a ‘service body’ style lockable canopy with two spare tyres on the back, that puts a lot of weight far back and high up, adding more weight further away from the part of the chassis under the most stress.
Now combine that with a few hundred kilograms of tow ball weight and you can see how the forces add up. The ‘fulcrum’ is the weakest point in the chassis, where it has no extra bracing from a sturdy cab or the multiple mounting points from the tub reinforcing the structure. Part of the reason why wagon style bodies don’t have this issue is the way the body reinforces and mounts along the entire length of the chassis.
As well as suspension and mechanical, tyres can cop a lot of abuse as the weights rack up. Referred to as the ‘load index’, this measurement is what the tyre is rated to safely perform under. There is a stamping on the sidewall of the tyre that you can check out here to tell what your tyre will cope with.
So what can I do about it?
Whether you’re looking to load up the work ute with tools or looking to take the family away for a week with a well stocked caravan, you have several options: taking some of the weight out of your vehicle and moving it into your trailer will help, as long as you don’t exceed the axle weights of your trailer.
Most 4x4s and SUVs will have aftermarket GVM upgrades available however often these do not upgrade your GCM rating. Some state and territory governments are moving to allow this as we start to see larger American ‘trucks’ become more common on our roads.
Items commonly forgotten about when modifying a car to safely handle heavy loads are the brake system, engine and gearbox cooling as well an uprated towbar/hitch setup. None of these will raise your maximum legal limits but it will make the vehicle perform better and more reliably under extreme towing.
How can I weigh my ute and trailer?
The good news is that a quick trip to a local weighbridge can take the guesswork out of your weights. You will need to weigh your combination in a few ways to get all the figures you’ll need.
- Weigh the whole combination
- Weigh just the tow vehicle while trailer is hitched up (trailer wheels not on weighbridge)
- Weigh just the trailer wheels while hitched up
- Weigh the tow vehicle without trailer attached
This will give you a pretty good idea how much weight is on each component on your setup as well as how much weight is on the towball of your vehicle.
Armed with this information, you can make an accurate decision on where to shift your payload and cut weight where you have to. Taking the time to do it once and do it properly, it could be the difference between a fun holiday away and an insurer who won’t pay out should the worst happen.
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