Towball weight should be measured with the Caravan in the fully laden condition. The caravan should be levelled on a flat firm surface. The hand brake should be held on, with the wheels chocked both sides and ends. The towball weight can be measured either directly or by ratio.
Household bathroom scales are used for both methods. To safeguard bending the top face of your bathroom scales, place a broad piece of at least 20mm thick hardwood timber directly on top. If you are confident your towball weight is under the maximum of your scales, use the direct method – otherwise the ratio method.
In the unladen state, when your caravan was first manufactured, a vehicle compliance plate should have been fixed to the side of the “A” frame. If the relevant information has been permanently marked in; the Gross trailer Mass (GTM)1 can be subtracted from the Aggregate trailer Mass (ATM)2. This is the original towball weight, caravan fully laden at manufacture.
We want to measure the towball weight of the fully laden caravan.
Equipment needed: Bathroom scales, 20mm thick broad hardwood timber to go on top. Three pieces of 100x50mm hardwood timber, two being approximately 300mm long and one piece 1m long. One short piece of water pipe or broom handle approximately 25mm diameter. One piece of hardwood timber or steel 50mm or 25mm square approximately 200mm long.
Direct weighing method.
Place scales (with 20mm thick broad hardwood timber on top) directly beneath van ball coupling level on firm ground.
Position 200mm long hardwood timber vertical about centre of scales under van ball coupling.
Keeping an eye on the bathroom scales measuring window; carefully raising the caravan jockey wheel, to lower the coupling until the full weight is just off the wheel.
Ensure you don’t wind too far off the maximum limit of your bathroom scales. – Otherwise use the ratio method.
If your direct towball weight reading is within your scales limit, establish whether the weight is within the recommended 10 to 15% of the laden weight (GTM) of the caravan.
Ratio method of weighing – as per sketch.
Standing in front and looking at the caravan:Place one of the short pieces of 100x50mm hardwood timber on the ground, 300mm to the right of centre of the caravan coupling. Place the bathroom scales to the left of the caravan coupling with 20mm thick broad hardwood timber on top, as its centre is 600mm from the centre of the coupling. Place the 25mm square piece of hardwood timber or steel on the 100x50mm piece of hardwood timber on the ground and the piece of water pipe or broom handle on the centre of the bathroom scales. Place the long piece of timber across and resting on both the piece of broom handle or water pipe and the 25mm square hardwood timber or steel. Using the other short piece of hardwood timber place it vertically under the caravan coupling and on top of the long piece of hardwood timber and raising the caravan jockey wheel,
to lower the coupling until the full weight is just off the wheel.
Check positioning dimensions are to sketch.
Observe the weight indicated on the bathroom scales and multiply this by three. This is your true laden towball weight. Ideally this weight should be within 10 to 15% of the laden weight of your caravan.
If the weight observed on the scales exceeds the scales capacity, use an appropriate longer hardwood horizontal beam. Extend the 600mm dimension by 300mm to make 900mm. (3 units of 300) Multiply the scale reading by the unit distance between the scales and square timber rest at opposite end of beam. Eg Unit distance 900mm (3 units of 300) + 300 (1 unit of 300). Total 3+1=4, this is the scale multiplying factor. Eg Scale reading this example 120kg x 4 = 480kg towball weight.
NOTE: Viscount aerolite caravans were often below 10% and towed well, as the lightweight chassis/caravan cabin was unconventional construction, although care should be taken not to exceed the GTM.
Even on 4WD vehicles with greater towing capacity than passenger cars, their vertical ballweight stamped on the compliance plate of the hitch type quality towbar seldom exceeds 120kg.
Now compare the calculated towball weight with the recommended towball weight as given by the manufacturer of the towing vehicle. If there is a drastic difference between the weights you have calculated and that recommended by the vehicle manufacturer, it may be necessary to either transfer some items within the caravan towards the rear of the unit. This will lighten the towball weight: or towards the front to increase the towball weight. The load equalizing bars on the tow hitch should not have to redistribute excessive weight if fundamental loading is imbalanced.
If internal or external loading of the caravan is to be altered ensure that side to side balance is maintained.
Eg. Internal loading – Battery, annex and pegs, poles etc, watertank, generator, spare wheels, tools, gas bottles and portable fridge etc.
Note. Gas bottles and storage batteries should be secured internally only if isolated from the main caravan interior, in their own regulation vented compartments.
Eg. External loading – rear bumper bar, spare wheels, Jerry cans, battery, generator, gas bottles, tools/toolbox, annex poles/pegs etc, underslung water tanks etc.
If you are not certain of the weight of your caravan you should take it to a public weighbridge and have it weighed, preferably in the laden state.
If the towball weight cannot be achieved within the desired range, more drastic methods may need to be undertaken, in relation to axle positioning or the transfer of heavy rigidly attached items. A reputable caravan chassis repairer or caravan maintenance/repair specialist should be consulted.
Towball weight is an important requirement for safe and stable operation of a vehicle and caravan combination.
Remember it is wiser to remove little used items until required, if practical. Extra ballast is inefficient, costly and can lead to caravan instability if overloading and imbalance occurs over time.
1. ATM. The total mass of the laden trailer when carrying the maximum load recommended by the manufacturer. This will include any mass imposed onto the towing vehicle when the combination vehicle is resting on a horizontal supporting plane.
2. GTM. The mass transmitted to the ground by the axle or axles of the trailer when coupled to a towing vehicle and carrying its maximum load approximately uniformly distributed over the load bearing area.
NOTE: These definitions are in accordance with the ADRs and may differ from other definitions in use.