Tow Ball Mass - updated December 2018
Tow Ball Mass Definitions
Tow ball mass is the mass placed on the tow ball (or other coupling device) of a tow vehicle via the tow hitch of a trailer.
Both vehicle manufacturer and tow hitch manufacturer will set a maximum tow ball mass. The legal maximum is whichever is the least.
Maximum tow ball masses of some popular passenger vehicles today continue to be lowered. This is because emission regulations continue to be tightened - and vehicle emissions relate to weight. As a consequence (regardless of the vehicle's size or rugged outward appearance) the chassis, suspension, axles and tyres of such vehicles may not be designed to tow heavy loads. See the maximum tow ball masses of some popular tow vehicles here.
Trailer manufacturers, however, usually recommend the tow ball mass which the trailer should place on the tow vehicle. This usually expressed as a percentage of the trailer's laden weight (e.g. 10%) and is important for trailer stability. In reality, however, trailer length is also involved - and long heavy trailer needs more than a short trailer of the same weight.
(Tow ball mass is informally referred to as nose weight.)
This article explains the background to and logic of all this.
The Need for Tow Ball Mass
Optimising caravan nose mass is vital for safe towing. A billiard cue thrown light-end first rapidly changes ends: it becomes heavy-end first. Unless caravans are nose heavy they’ll do the same.
As a general rule, about 10% of a trailer's laden weight should be on the tow ball. 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 major instability occurs: the lower the tow ball mass the lower the critical speed. That imposed tow ball mass, however, must usually have to be a compromise. This is because that, as the nose weight pushes down on the rear of the tow vehicle it levers up its front. This reduces the weight on the tow vehicle's front wheels and degrades the rig's cornering and braking ability.
If nose weight is too high, the caravan seems ultra-stable - but also reluctant to move in any but a straight line. This can result in an emergency swerve being impossible, ans equally to cease swaying if a strong sideways force is imposed. 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.
Tow Ball Mass - weight distribution along the trailer
A caravan should be 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). In a few recent examples, manufacturers have relied on front-located water tanks to be full whilst towing to obtain that required mass! Thus assumptions regarding recommended tow ball mass are only valid if a trailer is correctly loaded.
Trailer mass, including payload, should ideally be centralised with the heavier loads 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.
Never add extra weight at the rear of a caravan to reduce tow ball mass. Instead, redistribute weight inside the trailer as centrally as possible until correct tow ball mass is achieved.
Tow Ball Mass - 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.
Tow Ball Mass - how to measure it
A simple way to measure tow ball mass is via a length of timber placed under the trailer hitch and on top of a set of bathroom scales. Do not attempt to measure weight at other points on the A-frame (such as at the jockey wheel), since this will give a false reading. Raise the jockey wheel slowly as weight on the timber and bathroom scales increases, but keep the jockey wheel just clear of the ground in case the timber collapses. Note that most bathroom scales only measure up to 185 kg.
A better way is to buy a proper set of tow ball scales. They are not expensive (around A$75) and are safer to use due to their moulded tow ball shape at the top and solid base. They are thus more stable - and typically measure tow ball mass up to 350 kg.
Tow Ball Mass - 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 within the trailer to see the impact on tow ball mass. here. Also check tow ball mass with water tanks both empty and full. Some recently-made caravans (in RV Books' opinion dangerously) achieve correct tow ball mass only when the water tanks are full.
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 been shifted and overlooked. Keep a set of tow ball scales handy and use them often, especially when you take on new or unusual loads such as bicycles or other new accessories. You may find others would truly welcome you measure their tow ball mass!
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