How a Weight Distributing Hitch Works

Hayman Reese Weight Distributing Hitch (WDH) Pic: Hayman Reese.

Hayman Reese Weight Distributing Hitch (WDH) Pic: Hayman Reese.

Major updated - June 2019 

A Weight Distributing Hitch (WDH) is required for rigs where the laden caravan weighs more than the laden tow vehicle. In RV Books opinion, a WDH should not be used for rigs other than that.

As this article shows a WDH assists in one respect but inherently reduces the 'cornering power'  of rigs that use one. In some cases a WDH can contribute to jack-knifing. This article explains how a WDH works - and why its use should avoided if possible.

How a Weight Distributing Hitch Works

Throw a billiard cue light end first and it will flick over to heavy-end first. The same applies to caravans. To be stable they must be front end heavy (i.e. to have tow ball mass).

Tow ball mass exerts a down force on the tow hitch. Like pushing on 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. This reduces the tow vehicle's straight line stability.

Where the caravan/tow vehicle weight ratio is more or less equal, that minor front tyre weight loss does not matter. The loss in stability is compensated by reducing the tow vehicle's front tyre pressure by about 20-35 kPa (3-5 psi).

Problems set in when the laden caravan weighs more than the laden tow vehicle. The greater tow ball mass causes the front wheels' down-force to be too low. This must somehow be corrected.

 

The need for a WDH - Pic: caravansplus.com.au

The need for a WDH - Pic: caravansplus.com.au

How a Weight Distributing Hitch Works

A WDH is a semi-flexible sprung beam between the caravan and tow vehicle that in effect lifts the rear of that vehicle. This partially or fully restores the down force on the tow vehicle's front tyres.

At first, this seems a 100% solution and many users quite wrongly assume it is. But what is overlooked is critical.

The WDH addresses and largely resolves the tow ball mass effects. What it does not and cannot reduce is the lateral yaw forces on the rear of the tow vehicle. Worse, those forces are imposed on the tow vehicle's rear tyres that - because the WDH has 'lifted' weight from them -  are rendered less able to cope. 

If the caravan now yaws (sways) when subject to a sudden strong side wind, those unchanged lateral yaw forces are imposed on the tow vehicle’s rear tyres. Those tyres, however, because of their reduced down load, have reduced ability to resist those lateral yaw forces.

The extent by which the WDH shifts that weight down force is directly related to how tightly the WDH is adjusted. Until recently, industry and owner advice was to 're-level the rig'. That, however, causes too much weight to be lost from the tow vehicle's rear tyres (and their ability to resist yaw forces). 

Adjusting a WDH - main text explains how

Adjusting a WDH - main text explains how

The amount by which the WDH is adjusted to restore that front tyre loading is known as Front Axle Load Restoration (FALR).

Recognising that too great an FALR can seriously reduce stability, the (US) Society of Automobile Engineers (led initially by Toyota) included this issue in its set of Standards (SAE J2807). The recommendations include limiting 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 very necessary compromise.

That stressed in J2807 is that a WDH inherently reduces a rig's ultimate cornering power (typically by 25%).

The J2807 Standard has been adopted by all US makers of vehicles likely to be used for towing and by Japan's three major such makers.

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 unit's tension such that about 50% of that height difference is restored. Do not attempt to recover much more than that.

In this issue, having correct tow vehicle's tyre pressures is essential. As the front (when towing) carry less weight, they must never be increased. Ideally reduce them by 30-50 kPa. The rear tyres need to be increased by 50-70 kPa.

Be aware that a correctly set-up tow-vehicle's steering will not feel 'sports car like'. It will not (and should not) feel instantly responsive. This can fool some drivers who are unaware that an instantly responsive vehicle whilst towing is likely to be far more close to a jack-knifing situation than one that is slower to respond.


Weight Distributing Hitch - 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 but never over adjust it.

Stability trap 

Owners of unbalanced rigs need to be aware that they may seem ultra-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 far 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.

Do also buy our all-new digital mini-book Why Caravans Rollover - and how to prevent it . Just released it, for the very first time just why caravan rollover is explained in plain English. It explains the many ways by which you can prevent that happening. The book also has a Question and Answer section that enables you to assess the current stability of your own rig - and how to improve it.

why not buy a book?

This article is based on content from our featured RV books. These books contain extensive information on a range of topics of interest to RV users and potential buyers. By purchasing a book, you are not only educating yourself but also supporting the work of independent RV writers. If you have found this article useful, please also visit the RV Bookshop.

 

This product has been added to your cart

CHECKOUT