Towball mass will directly affect the overall stability of your whole rig

Caravan towing safety has been a hot topic in recent days. A while back, caravan expert Dave Lewis from the Weightcheck Mobile Caravan Weighing business in Brisbane set out the importance of correct towball weight for grey nomads 

Towball Mass, or TBM, is a very important measurement that contributes to the overall weight of the caravan and also the tow vehicle.

The TBM directly affects the stability of the caravan and the whole rig, so it is important that you get the towball weight right.

Caravan manufacturers recommend a ball weight of between 9-11 % of the total fully laden caravan weight, or Aggregate Trailer Mass (ATM).

Caravan weight

The ATM of a caravan, is recorded on the Vehicle Compliance Plate, and is the maximum allowable weight of the fully loaded caravan. The ATM is measured with the caravan unhitched from the tow vehicle, and comprises the weight of the caravans’ wheels or axles, and the tow ball weight. Most caravan compliance plates will have a stamped weight called ‘Towball Mass at Tare’.

When a caravan is built, the compliance plate records the Tare Weight, which is the dry weight of the fully completed caravan. Tare weight is the sum of the wheel or axle weights, and the ball weight, and is measured with the caravan at level. Once the caravan takes on load, the breakup of axle weights and towball weight completely changes.

Using the 10% rule, your caravan needs to have a good distribution of weight in order to keep it stable. If the TBM is too light, the most common result is it will create dangerous sway in the van, which is a caravanner’s worst nightmare.

Alternatively, if the ball weight is too heavy it increases the stress on the tow vehicle, by adding heavy load on the rear axle whilst making the front axle lighter. This makes for a dangerous scenario by making the steering lighter and changes the braking dynamics of the vehicle. We recently conducted a series of tests on the effects the towball mass has on the weight dynamics of a caravan, and the results are very interesting.

One of the caravans we weighed was a tandem axle van with independent suspension, with a total loaded ATM of 2522 kgs, and a towball mass of 232 kgs at travel height. Travel height is measured with the hitch at the same height as when the caravan is attached to the tow vehicle.

We then measured the TBM 50mm lower than travel height, and the TBM decreased by 22kgs, the front axle got heavier by 129kgs, and the rear axle got lighter by 107kgs. We then measured the TBM at 50mm higher than travel height with the TBM increasing by 20kgs, the front axle getting lighter by 140kgs, and the rear axle heavier by 120kgs.

So, you can see that the whole dynamics of the caravan change with the towball weight measured at different heights. If you have your own towball scale you need to be taking the reading with the hitch at travel height.

Towball Mass is just as important for the tow vehicle as it is for the caravan, as it is included in the total fully loaded weight of the car, the Gross Vehicle Mass. The GVM of the tow vehicle, includes any after aftermarket accessories like bull bars, roof racks etc, fuel, passengers, cargo, and includes the TBM of the caravan when attached.

So using the 10% rule, some of the bigger caravans will use up to 350kgs of the available payload of the tow vehicle.

Towball weight is only one of six compliances that need to be checked to ensure your rig is within its limitations. With the authorities taking a harder stand on road safety, everyone needs to ensure their rigs are compliant, and weight compliance is undoubtedly the biggest issue.

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