Getting your Tow Ball Weight (TBW) right is one of the most critical factors for a safe, sway-free towing experience. When you are standing in your driveway looking at a fully packed caravan, the numbers on the compliance plate don't tell you how the van will actually behave on the highway. That behavior is almost entirely dictated by how much downward pressure is sitting on your vehicle's tow hitch.

If your TBW is too light, the back of your caravan acts like a pendulum. Wind from passing trucks or a slight dip in the road can trigger a violent sway that is incredibly hard to recover from. Conversely, if it is too heavy, the massive downward force pushes the back of your car down. This acts like a seesaw, lifting your front wheels off the tarmac, severely reducing your braking efficiency and steering control.

What Exactly is the 10% Rule?

If you have spent any time around caravan parks in Australia, you have likely heard the golden "10% rule." In Australia, the general rule of thumb is that your Tow Ball Mass should be roughly 10% of your fully loaded caravan's weight, otherwise known as its Aggregate Trailer Mass (ATM).

For example, if your loaded caravan weighs 2,500kg ready for a trip, your ideal TBW should be hovering right around the 250kg mark. However, treating this rule as absolute law can sometimes get you into trouble.

European caravans, for instance, are designed differently. They often have axles mounted further forward and rely on advanced friction hitches, meaning they might only require a 5% to 7% tow ball weight. Australian off-road vans are built for harsh, corrugated tracks and tend to prefer a slightly heavier nose—sometimes up to 12%—to ensure the connection remains totally solid over deep ruts. The 10% figure is your starting baseline, not your absolute finish line. You must always cross-reference this baseline against the maximum tow ball rating of your specific vehicle and the tow bar itself (these can be two different numbers, and you must adhere to the lowest one).

The Mechanics of Weight Distribution

To understand why TBW fluctuates, you need to picture your caravan's axle group as the pivot point on a seesaw. Everything you place in the front boot—gas bottles, spare tires, heavy toolboxes, or a generator—adds directly to your tow ball weight.

Everything you pack behind the axle—like jerry cans on the rear bumper, a heavy outboard motor under the rear bed, or rear-mounted water tanks—subtracts from your tow ball weight by acting as a counterweight. While putting heavy items at the extreme rear of the van might technically reduce your TBW to a legal limit, it is the worst possible thing you can do for stability. Heavy items at the extreme ends of the caravan create a dumbbell effect, massively increasing the likelihood of high-speed sway.

How to Measure TBW at Home

The compliance plate on your caravan states the "Tare Tow Ball Weight." Ignore it. That is the weight of the caravan empty as it rolled out of the factory. The moment you fill the water tanks, add two 9kg gas bottles, load the fridge, and throw in your camping chairs, that factory number becomes entirely irrelevant. You need to measure your loaded weight.

Method 1: The Portable Tow Ball Scale (The Safest Way)

The most accurate and stress-free way to measure this before every trip is by using a specialized portable Tow Ball weight scale. These are inexpensive, spring-loaded vertical scales designed specifically for this job.

• Pack your caravan exactly as it will be on the day of travel. Ensure water tanks are full if you intend to travel with them full.
• Park the caravan on level ground.
• Chock the caravan wheels securely.
• Place the portable scale directly under the hitch coupling.
• Slowly wind down the jockey wheel until the entire weight of the hitch is resting on the scale and the jockey wheel is completely off the ground.

The number indicated on the gauge is your actual travel TBW.

Method 2: The Bathroom Scale and Plank Trick

If your tow ball weight is relatively light (under 130kg) and you don't have a specialized scale, you can use a standard set of bathroom scales. However, most modern caravans easily exceed the maximum limit of household scales, which will break them instantly. For heavier vans, you can use the "bridge method" to distribute the weight.

You will need a strong piece of timber, two pieces of pipe, a brick, and your bathroom scales. Place the brick and the bathroom scale exactly 90cm apart. Place the piece of timber across them, resting on the small pipes to act as pivot points. Zero the scale (or note the starting weight of the timber). Lower your caravan hitch exactly in the center of the timber (45cm from the brick, 45cm from the scale).

Because the hitch is exactly in the middle, the scale is only taking half the weight. Take the number shown on the bathroom scale, multiply it by two, and you have your estimated tow ball weight. While this works in a pinch, we highly recommend investing in a proper hitch scale for long-term safety.

The Weighbridge Method: The Ultimate Truth

While home methods are excellent for pre-trip checks, nothing beats a certified public weighbridge to get your exact baseline figures. Doing a full weighbridge run gives you your vehicle weight, your caravan weight, and your tow ball weight with absolute legal accuracy.

To calculate your TBW at a weighbridge, you need to take two specific measurements:

1. Weigh the fully hitched rig: Drive onto the weighbridge so that only your vehicle's wheels (front and rear) are on the scale. Leave the caravan wheels entirely off the scale pad. Write this number down. This is your loaded vehicle weight including the downforce of the caravan.
2. Weigh the unhitched vehicle: Drive off the weighbridge, disconnect the caravan in a safe area, and drive just the car back onto the scale pad. Write this number down. This is your standalone vehicle weight.

Subtract the second number from the first number. The difference is exactly how much downward force the caravan was applying to the car. That is your certified Tow Ball Weight.

How to Fix a Bad Tow Ball Weight

If you have crunched the numbers and discovered your TBW is sitting at 350kg when your vehicle is only rated for 250kg, you have a serious legal and safety issue. You need to adjust your load.

The most effective way to change your TBW is to manage your fluids. A single 95-liter water tank positioned at the extreme front of the chassis adds nearly 95kg of weight. If you empty the front tank and fill a rear tank instead, you can drastically alter the balance.

Next, look at the front boot or A-frame. Do you have heavy toolboxes, jerry cans, or a generator sitting right behind the hitch? Relocate the heavy items directly over the caravan axles. Packing heavy items low and directly over the axles is the golden rule of caravan packing. It maintains a healthy TBW without creating the dangerous pendulum effect at the extreme ends of the trailer.

Remember, safety on the road starts in the driveway. A properly balanced rig handles predictably, brakes efficiently, and ensures you arrive at your campsite ready to relax, rather than exhausted from fighting a swaying steering wheel for hundreds of kilometers. Take the time to measure, adjust, and measure again.