Tow Ball Mass
Tow ball mass is the mass placed on the tow ball (or other coupling device) of a tow vehicle by the tow hitch of a trailer.
The tow ball manufacturer, which may or may not be the same as the tow vehicle manufacturer, will set a maximum tow ball mass figure which may not be exceeded.
Maximum tow ball masses of some popular passenger vehicles today are surprisingly low. This is because the chassis, suspension, axles and tyres of such vehicles may not be designed to tow heavy loads, regardless of the vehicle's size or rugged outward appearance. See the maximum tow ball masses of some popular tow vehicles here.
Trailer manufacturers, on the other hand, will usually set a recommended tow ball mass which the trailer should place on the tow ball. This recommended mass is usually expressed as a percentage of the trailer's laden weight (e.g. 10%) and is important for trailer stability.
Tow ball mass is more informally referred to as nose weight.
This article explains the background to and logic of trailer tow ball mass recommendations.
The Need for a Heavier Nose
Optimising caravan nose weight is vital for safe towing. A billiard cue thrown light-end first rapidly changes ends. It becomes heavy-end first. Likewise, unless caravans are nose heavy, they’ll try to do the same.
More weight at the front end of a caravan helps the caravan to travel in a straight line. As a general rule, about 10% of a trailer's laden weight should be on the tow ball. Light caravans and camper trailers may require less, particularly if short and centrally laden.
There is a long-proven relationship between tow ball mass and the speed at which critical instability occurs: the lower the tow ball mass the lower the critical speed.
BUT caravan nose weight also pushes down on the rear of the tow vehicle and levers up its front. Too much trailer nose weight reduces weight on the tow vehicle's front wheels and negatively impacts tow vehicle steering and braking ability.
If nose weight is further increased to above 14% or so of laden weight, caravans are reluctant to move in any but a straight line, and equally to cease swaying if a strong sideways force is imposed on them. None of this is desirable, especially when needing to swerve to avoid a collision.
For safe towing therefore, tow ball mass must therefore be not too heavy but not too light.
Weight Distribution Across the Trailer
A caravan is designed by the manufacturer with its mass, A-frame and axles optimised for its recommended tow ball mass. However tow ball mass is also affected by the weight and location of payload (your personal effects). Manufacturers' assumptions regarding recommended tow ball mass for their trailers are only effective if a trailer is correctly loaded.
Trailer mass, including payload, should ideally be evenly distributed along the length of the trailer. Heavier loads should be close to the axle(s) and low down. Ideally, the A-frame should carry no load. Most importantly, nothing heavy should be at a caravan's rear - see here as to why and here on further information on correct trailer loading.
On no account should you add extra weight at the rear of a caravan to reduce tow ball mass. Instead, redistribute weight inside the trailer until correct tow ball mass is achieved.
The Right Percentage
For a typical, medium-sized ( 6 to 7 metres long) Australian built caravan, tow ball mass should be about 10% of the trailer's laden weight.
For a generally lighter UK or European built caravan, tow ball mass should be around 6% to 7% of the trailer's laden weight.
For even lighter camper trailers, a tow ball mass of 5% may be sufficient.
But in each case always follow the recommended tow ball weight of your trailer manufacturer and always make sure this is within the maximum tow ball mass allowance of the tow ball/tow vehicle manufacturer.
Measuring Tow Ball Mass
The easiest way to measure tow ball weight is to use a length of timber placed under the trailer hitch and on top of a set of bathroom scales. Note however that bathroom scales often weigh only up to 185 kg. Make sure the trailer is level and do not measure weight at other points on the A-frame (such as at the jockey wheel), since this will give a false weight reading. Raise the jockey wheel slowly as weight on the timber and bathroom scales increases, but keep the jockey wheel lowered (but just clear of the ground) as back-up support for the front of the caravan in case the timber collapses.
The best way to measure tow ball weight is to buy a proper set of tow ball scales. These are steel cylinders with an inner tube and spring and a readable scale down the side. They are not expensive (around A$75) and are safer to use due to their molded tow ball shape at the top and solid base. They are thus more stable. They can also typically measure tow ball mass up to 350 kg.
When to Measure
When you first buy your caravan or trailer, measure tow ball mass for a range of situations. This might be a light configuration (for weekends away), a medium configuration (for weeks away) and a heavy configuration (for months away).
Move items around within the trailer to see the impact on tow ball mass, but always do so either when the trailer is stationary or within the safe loading framework explained here. Also check tow ball mass with water tanks both empty and full; some caravans, in RV Books' opinion, dangerously achieve correct tow ball mass only with full water tanks.
Once you become familiar with the correct loading patterns for your caravan, continue to measure tow ball mass periodically to make sure no loads have shifted. Keep a set of tow ball scales handy and use them often, especially when you take on new or unusual loads such as bikes or other new accessories.
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