Updated May 2017
Typical WDH (weight distributing hitch) Pic: Jayco.
Any trailer suspended on a hitch that overhangs a tow vehicle’s rear axle tends to yaw (sway). Such yaw is reduced by having the trailer’s front end heavier than its rear. This, however, pushes down on that lever-like overhung hitch. As with pushing on the handles of a wheelbarrow, this levers up the front wheels of the tow vehicle. This lessens the grip of the (steering) front tyres.
The effect of insufficient front end weight. Pic: original source unknown.
To counteract this, a WDH (a springy semi-flexible beam) is attached between caravan and tow vehicle. Via the tow vehicle’s rear wheels, the WDH levers the front wheels back down. This wholly or partly restores the weight.
Adding a WDH to a rig that is unsafe without one however causes problems. This is particularly so in emergency avoidance swerves. Like using a truss to fix a hernia it assists one problem but causes another.
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.
Need for a WDH – tow ball mass
The WDH problem is associated with tow ball mass. Within limits, increasing such mass assists stability. For caravans with (desirable) centralised mass and loading, ‘the improvement becomes less when nose mass rises above 6-7 per cent of the total weight’ ∗. But, for long end-heavy caravans it needs to be at least 10%.
Nose mass increases the load on the tow vehicle’s rear tyres. Doing so, however, increases their ability (up to a point) 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. Locally made caravans, however, have become longer and heavier (some exceed 4.0 tonne). They need a high nose mass. That, however this levers up the front of the tow vehicle. This, in turn, reduces the grip of the front tyres.
Via that lever effect, a WDH assists to restore the load on the front wheels. The WDH, however, cannot reduce lateral forces. Because of this, a WDH reduces the ability of a rig to stay in a straight line. (Technically speaking, it reduces the margin of understeer.)
Adjusting the WDH
Historical recommendations have been to adjust a WDH to fully counteract the sag caused by caravan 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, distorts 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. It does not follow the normal (i.e. Coulomb) linear law of friction. Instead it is typically 0.8 of that.
When a heavy caravan sways, the WDH consequently increases the side forces on the tow vehicle tyres’ rear footprints. This, because their behaviour is not linear, disturbs their required ratio of grip and action front/rear. If that ratio exceeds a critical level the (rear) tyres lose all grip. The rig is likely to irreversibly jack-knife.
The beginning of a jack-knife. Pic: copyright 2006 caravanandmotorhomebooks.com
It has been known since 1978 or so that using a WDH at any adjustment reduces the tow vehicles desired margin of the understeer necessary for straight line stability.
Fully correcting the effect of tow ball mass is seriously counterproductive. Never over-tension a WDH. The world’s major maker of WDHs (Cequent) advises to adjust 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 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.
It also assists stability (of dual axle caravans) if the rear tyres are inflated by about 35 kPa (5 psi) higher than the front pair.
The EU approach – no need for a WDH
The above issue is well understood. It is also covered in associated articles on this website.
It has long since been addressed in the EU and also followed by EU caravan firms now building caravans in Australia.
This issue is very much speed-related. The EU has 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.
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/
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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.