How a Weight Distributing Hitch Works
Of all caravan mythology there is little that rivals how a weight distributing hitch works. It is mostly due to confusing ‘mass’ and ‘weight’. For most purpose, e.g. cooking, that difference matters not. Nevertheless, if weight is mistaken for mass when talking about WDHs, it results in possibly dangerous assumptions.
Mass is a measure of how much ’matter’ is contained in any object. Unless subject to an external force, mass does not change position or location.
Weight is a down force (caused by gravity) on the matter contained in any object. We mostly express that force (weight) in kilograms (or pounds). Engineers use newtons per kilogram. The Earth’s gravitational field strength is approximately ten newtons per kilogram (10 N/kg ).
As attempting to throw a billiard cue light end first shows, most things that move will attempt to do so heavy-end first. It's why tow ball mass is needed to keep your caravan stable. We refer to it as ‘mass’ because it is subject also to non-gravitational forces – such as wind-induced yawing.
Tow ball mass, however, assists one issue (stability) but introduces another. It exerts a down force on the tow hitch that is behind the rear axle of the tow vehicle. The local average distance between rear axle and tow hitch is about 1.25 metre. Like pushing down the handles of a wheelbarrow, tow ball mass pushes down the rear of the tow vehicle. This undesirably lifts its front, reducing the weight (down force) on the front tyres.
Such minor weight loss does not matter. Its effect is compensated by reducing front tyre pressure by about 20-35 kPa (3-5 psi). If that loss is greater, caravan owners may attempt to remedy it by using a Weight Distributing Hitch (WDH). This is fine for minor amounts of weight. Some owners, however, use a WDH to ‘correct’ situations where the tow ball mass exceeds manufacturers' recommendations. Or where the laden caravan is heavier than the laden tow vehicle. This can be non-effective and dangerous. Furthermore having overweight vehicles is illegal.
Here's How a Weight Distributing Hitch Works
A WDH is a down force distributing device. It cannot transfer mass. Scales thus indicate the WDH has ‘shifted’ part of the imposed tow ball weight from the tow vehicles’ rear tyres to restore weight on the front tyres. This however reduces the tow vehicle’s ‘cornering power’ by as much as 25%.
With or without a WDH, tow ball mass remains exactly the same. Because of this unchanged mass, if the caravan yaws (sways) when subject to a sudden strong side wind, those unchanged side forces are imposed on the tow vehicle’s rear tyres – that now have reduced ability to cope. This does not imply the WDH concept is flawed: but that it cannot overcome the laws of physics.
The SAE J2807 Recommendations
Recognising that towing issues were increasing, the Society of Automobile Engineers has derived a set of Recommended Standards (SAE J2807). All USA and the top three Japanese makers of vehicles likely to be used for towing now follow these.
The recommendations include limiting Front Axle Load Restoration (FALR) such that owners are advised to adjust the WDH such that the tow vehicle’s front end’s height (prior to the caravan being hitched up) is only partially restored. It is a compromise – but a necessary compromise.
Cequent, the world’s largest maker of WDHs, recommends to measure the height of the tow vehicle’s front wheel arch prior to and then after coupling up the caravan (with the WDH not in use). Then to adjust the WDH units tension such that about 50% of that height difference is restored. Do not attempt to recover much more than that.
Correctly balanced rigs – where the laden caravan is lighter that the laden tow vehicle - have no need for a WDH. RV Books strongly recommends this approach. Ongoing police roadside checks, however, show that almost 80% of all rigs are overweight. Many caravans are far heavier than whatever tows them. Here again, RV Books advises owners to remedy this – but, if not feasible, to use that WDH.
Owners of unbalanced rigs need to be aware that they may well seem stable in normal driving. However, it is a different matter entirely if needing to make a sudden strong swerve - or if hit by a strong side wind gust. In such an event, an unbalanced rig may be suddenly triggered into a literally non-controllable ‘chaotic’ state. This is more probable at speed. That chaotic state cannot be driver corrected – it almost invariably ends with a jack-knife and often a rollover.
This topic - of how a weight distributing hitch works - is covered also in our book The Caravan & Motorhome Book available in eBook form (pdf) via our Bookshop or in print form via most bookshops in Australia and New Zealand - or via booktopia.com.au.