Updated 12 March 2016

Weight distribution hitch setting up corrects caravan tow vehicle front end lift but can introduce instability if it is set too tight.

Weight distributing hitch ezy lift Jayco

Typical WDH (weight distributing hitch) Pic: Jayco.

Conventional caravans are towed via a hitch that overhangs the tow vehicle’s rear axle. If that caravan sways, it causes the tow vehicle also to sway (and vice versa). This effect is reduced by having the caravan nose heavy. For caravans with centralised mass and loading, ‘the improvement becomes less significant when the nose mass rises above 6-7 per cent of the total weight’, says University of Bath’s Professor Josh Darling1.  For long end-heavy caravans it really needs to be 10% plus.

Imposing such heavy tow ball mass on that overhung hitch increases the load on the tow vehicle’s rear tyres, up to a point increasing their ability to counteract and dampen sway. That mass however causes the rear of the tow vehicle to drop as the rear springs compress. It also levers up the front of the tow vehicle, reducing the weight on its front wheels and hence tyre grip. As tyre grip is essential for steering, reducing it is (to put it mildly) not desirable.

To address this physically inherent issue, a separate industry then developed. Its resultant weight distribution hitch assists – but much as a truss used to support a hernia is better addressed by treating that hernia.

 

Donkey better

The effect of insufficient front end weight. Pic: original source unknown 

 

Weight distribution hitch setting up

A WDH (in effect) is a springy semi-flexible beam between the caravan and its tow vehicle. It levers the front wheels back down and also slightly increases the weight on the caravan’s tyres. A common recommendation is to adjust the WDH to fully counteract the static sag caused by the caravan’s nose ball weight.

Doing so, however, with high caravans of high nose weight, introduces instability when that rig pitches. If/when it does the WDH causes weight changes front/rear to hugely increase. This in turn causes the front/rear tyre loadings to do likewise – resulting in their required ratio of grip and action varying front to rear.  If that exceeds a critical level the (rear) tyres lose all grip. The rig is then likely to irreversibly jack-knife. 

 

Caravan sway out of phase web

The beginning of a jack-knife. Pic: copyright 2006caravanandmotorhomebooks.com

Weight distribution hitch setting up

Excess weight distribution hitch setting up (tightness) to fully correct the effect of tow ball mass can be counterproductive. The Society of Automotive Engineers (SAE) standard, ‘Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating’, (J2807) brings a hint of clarity to this. The standard, significantly uses only 50% correction in its main stability testing when using a WDH.

Whilst Hayman Reese in Australia still recommends 100% correction, Chief Engineer (Rick McCoy) of its parent company in the USA (Cequent) is reliably reported as having long since suggested that only 50% correction be used. The Ford motor company suggests likewise. (Reference 1 explains just why in detail).

Correcting tow ball mass by 50% only typically results in the caravan’s nose being lower by about 50 mm. This is desirable anyway as airflow under the front of a caravan tends to cause (undesirably) to lift.

To achieve this, adjust the lowered jockey wheel until the caravan is level – then measure (and note) the height of the front of the caravan chassis from the ground. Then, with the tow vehicle laden and the equivalent weight of driver and passengers on board, lower the caravan onto the tow ball. Again measure the front chassis height from the ground. Adjust WDH bar spring tension so as to correct only 50% of that height difference. This is likely to cause the front of the caravan to be about 50 mm lower than when level. 

caravan jack-knife source unknown

Caravan jack-knife in the UK – Pic: original source unknown

Tyre pressures when towing

When towing, increasing the tow vehicle’s rear tyre pressure reduces the tendency to dangerous oversteer when the caravan sways and pitches. One well-known paper2 states that: ‘Reduced front tyre pressures on the tow vehicle can return the combination vehicle to car-alone [desirable] understeer, thereby completely negating the understeer lost to hitch load, with or without load leveling.’  I can only legally suggestnot recommend, this be considered. It has however resulted in many caravan owners finding it successful. (Never increase a tow vehicle’s front tyre pressure).

I have on a few occasions suggested (with known success) increasing rear pressures by 35-45 kPa (5-6.5 psi) and decreasing front tyre pressure by 20-35 kPa (3-5 psi) whilst staying within the tyre makers’ maximum and minimum limits. Caravan tyres should be inflated to whatever tyre makers advise for the weight borne (many are hugely under-inflated). Stability is assisted by using Light Truck Tyres. These cannot necessarily carry more weight but have stiffer side walls.

 

The EU approach

The above issue is well understood (and covered also in associated articles on this website). It has long been recognised in the EU. There, few caravans have or need a WDH. With most it is not possible to fit one. This approach is now also followed by a few local caravan builders. They build far lighter and often shorter caravans. This issue is very much speed-related and (in the EU) there is a general and well observed tow limit of 80 km/h/.

 

References 

  • 1. Klein, R, Johnston, D, Szostak, H.   ‘Effects of Trailer Hookup Practices on Passenger Car Handling and Braking.’  Society of Automotive Engineers Inc. SAE 780012 March 1978.
  • 2. Darling J., Tilley D., and Gao B.,  2008.   ‘An experimental investigation of car-trailer high-speed stability’,  Dept of Mechanical Engineering, University of Bath, UK.

The general topic is covered (more technically and in depth) in my recently updated Caravan and tow vehicle dynamics.

See also the excellent: caravanchronicles.com/guides/understanding-the-dynamics-of-towing/ 

If you found this article of value, my books will prove even more so. They include Caravan & Motorhome ElectricsSolar That Really Works (for RVs), Solar Success (for home and property systems), and The Camper Trailer Book. The author’s all new Caravan & Motorhome Book covers every aspect of the subject matter.

The author is an ex General Motors research engineer with a particular, and now 60 year, interest and writing in this area. See About the author

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