Updated March 2016

Long end-heavy caravans need a weight distributing hitch. Light ones rarely do. Whilst essential for the former, such hitches are not a cure for bad design. For many rigs, a weight distributing hitch (WDH) is an undesirable way of solving an issue better avoided by intelligent design.

 Weight distributing hitch ezy lift Jayco

Typical WDH (weight distributing hitch) Pic: Jayco.

Any trailer suspended on a hitch that overhangs a tow vehicle’s rear axle has an inherent tendency to sway. This is substantially dampened by having the trailer’s front end heavier than its rear. An unwanted result however is that such added weight pushes down on that overhung hitch. Like pushing down on the handles of a wheelbarrow, this levers up the front of the tow vehicle – lessening the grip of the (steering) front tyres.

 

Donkey better

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

To counteract this, a WDH (in effect a springy semi-flexible beam) is attached between the caravan and its tow vehicle. Via the tow vehicle’s rear wheels, the WDH levers the front wheels back down, wholly or partly restoring the weight.

Adding a WDH to a rig that is unsafe without one however introduce problems – particularly in emergency avoidance swerves. (It is like using a truss to fix a hernia that should be addressed by surgery). This has long been understood. In Europe, caravans are typically 40% lighter than local product. They have very low nose mass (typically 5%). They rarely exceed 5.5 metres (about 18 ft). These ‘vans neither have WDHs nor provision for them.

The WDH problem, generally understood since the 1970s, is associated with the desirable tow ball mass. As a generalisation, increasing tow ball mass assists caravan stability. For caravans with the desirable 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 Darling *. But for long end-heavy caravans it really needs to be 10%-14%.

Nose mass works because it increases the load on the tow vehicle’s rear tyres. This in turn increases their ability to counteract and dampen sway (yaw). This works well enough with caravans up to a couple of tonne (and about 5-6 metres) when towed by a vehicle heavier than the caravan.

US and locally made caravans, however have become longer and heavier (some exceed 4.0 tonne). They need a high nose mass. As with pushing down on the handles of a wheel barrow, this mass levers up the front of the tow vehicle, reducing the grip of the steering front tyres. And that’s when the need for a WDH (to keep those tow vehicle’s front wheels on the road) introduces a problem.

Adjusting the WDH

The general recommendation is to adjust the WDH such it fully counteracts the sag caused by the caravan’s nose ball weight. Doing so, however, particularly with high nose weight, can introduce major instability. The cause is to do with how tyres behave.

The area of a tyre in contact with the ground (called its ‘footprint’) is about the size of a human hand. When the steering wheel is turned, the front wheel rims exert a side force on the tyre walls. That, in turn, distort their footprints causing the vehicle to take up the desired direction. The steering force exerted (or reacted) is related to the weight on the tyre but is less than linear – i.e. it does not follow the normal (Coulomb) linear law of friction. It is typically 0.8.

When a heavy caravan sways (even more so if it pitches) the forces are very much higher (than at rest). The WDH alternately increases and decreases the forces on the tow vehicle tyres’ footprints. Because their behaviour in not linear, this disturbs their required ratio of grip and action front to rear. If that exceeds a critical level the (rear) tyres lose all grip and the rig is likely to irreversibly jack-knife. 

 

Caravan sway out of phase web

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

There is increasing evidence that using a WDH to fully correct the effect of tow ball mass is counterproductive. Never over-tension a WDH. Many authorities now advise adjusting hitch spring bars to correct tow ball mass by 50% only. This 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 it to lift.

caravan jack-knife source unknown

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

Tyre pressures when towing

When towing increase the tow vehicle’s rear tyre pressure by about 35 kPa (5 psi) and decreasing front tyre pressure by 20-35 kPa (3-5 psi). This reduces the tendency to dangerous oversteer when the caravan sways and pitches. Never increase the tow vehicle’s front tyre pressure.

The caravan tyres must never be inflated to less that the tyre maker’s advice for the weight they bear. 

The EU approach

The above issue is well understood (and covered in associated articles on this website). It has long since been addressed in the EU. It is now also being followed by a few local caravan builders. They build far lighter and generally 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/. Very few EU-design caravans have or need a WDH. With most it is not possible to fit one.

See also http://caravanandmotorhomebooks.com/reducing-caravan-sway/ also http://caravanandmotorhomebooks.com/making-caravans-stable/

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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* 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.