Updated 3 May 2017

Correct weight distribution hitch setting up compensates caravan tow vehicle front end lift but introduces instability if too tight. Here’s why.

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 causes the rear of the tow vehicle to drop as the rear springs compress. It also levers up the front of the tow vehicle. This reduces the weight on its front wheels and hence tyre grip. As tyre grip is essential for steering, reducing it is not a good idea.

To address this issue, a separate industry then developed weight distribution hitches. But these act much as a truss to support a hernia. It is better, however, to eliminate the need for such a device.

 

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. It also slightly increases the weight on the caravan’s tyres. A common recommendation is to adjust the WDH to fully counteract the caravan’s nose ball weight.

Doing so, however, with caravans of high nose weight, causes instability when that rig yaws or pitches. If/when it does the WDH causes weight changes front/rear to hugely increase. This results 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 – understeer

Any WDH reduces the tow vehicle’s desirable margin of under-steer – i.e. it causes it to be inherently less stable. It also reduces its cornering ability by 25% plus. 

Because of this, excess weight distribution hitch tightness to fully correct the effect of tow ball mass is seriously counterproductive.

The SAE standard, ‘Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating’, (J2807) quantifies this. The standard used only 50% correction in its main stability testing when using a WDH. It nevertheless found even that amount precludes achieving the desirable (0.4 g) cornering ability.

Hayman Reese in Australia still recommends 100% correction. Chief Engineer (Rick McCoy) of its US parent (Cequent), however,  recommends only 50% correction be used. The Ford motor company suggests likewise. (Reference 1 explains why.)

Correcting tow ball mass by 50%  typically results in the caravan’s nose being lower by about 50 mm. This is desirable as airflow under the front of a caravan otherwise generates lift.

To do this, adjust the lowered jockey wheel until the caravan is level. Now measure (and note) the front of the caravan chassis’s height 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 no more than 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

Weight distribution hitch setting up – tyre pressures when towing

When towing, increase the tow vehicle’s rear tyre pressure and reduce the tow vehicle’s front tyre pressure to retain understeer.

One well-known paper2 states: ‘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.’ (The italics are mine.)

I can only legally suggestnot recommend, this be considered. Many caravan owners find it successful. (Never increase tow vehicle front tyre pressure).

I suggest to increase rear pressures by 35-45 kPa (5-6.5 psi). Decrease front tyre pressure by 20-35 kPa (3-5 psi) whilst staying within the tyre makers’ maximum and minimum limits.

Inflate caravan tyres to whatever tyre makers advise for the weight borne (many are hugely under-inflated). Stability is assisted by using Light Truck Tyres. These do necessarily carry more weight but have stiffer side walls. Here too (for dual axle ‘vans) to increase the rear pair of tyres by 35-45 kPa).

The EU approach

The above issue is well understood (and covered 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 even to fit one. This approach is now also followed by EU caravan makers who also build their products in Australia. They are far lighter.

This issue is also very much speed-related and (in the EU) there is a general and well observed tow limit of 80 km/h/.

Weight distribution hitch setting up – about 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 plus 60 year, interest and writing in this area. See About the author