How a Weight Distributing Hitch Works
Updated December 2018
How a weight distributing hitch works is often misunderstood. 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 can result in 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 ).
How a Weight Distributing Hitch Works: caravan stability
Throw a billiard cue light end first and it will switch to move heavy-end first.The same applies to caravans. To be stable, they must be front heavy (i.e. to have tow ball mass).
Tow ball mass, however, assists stability but in doing so it creates a problem. It exerts a down force on the tow hitch that typically 1.25 metre behind the tow vehicle's rear axle. 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 weight (down force) on its front tyres.
Minor weight loss does not matter - as long as its effect is compensated by reducing tow vehicle 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 where 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 distributes a down force (weight). It cannot transfer mass. Scales 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. What those scales are measuring however is a down force. It's like pushing down hard on a set of scales whilst standing behind them.
This however reduces the tow vehicle’s ‘cornering power’ by as much as 25%.
With or without a WDH, tow ball mass remains 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. Those tyres, however, now have reduced ability to cope. This does not imply the WDH concept is flawed: but it cannot overcome fundamental laws of physics.
How a Weight Distributing Hitch Works: the SAE J2807 Recommendations
Recognising that towing issues were increasing, the Society of Automobile Engineers(led initially by Toyota) 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 the tow vehicle’s front end height (prior to the caravan being hitched up) is only partially restored when the caravan is attached. 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.
How a Weight Distributing Hitch Works: but do you really need one?
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.
A stability trap
Owners of unbalanced rigs need to be aware that they may 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. It is available in digital (pdf) form via our Bookshop or in print form from most bookshops in Australia and New Zealand - or via booktopia.com.au.