Updated November 2017
Long end-heavy caravans have a need for a Weight Distributing Hitch (WDH). For all, though, it inherently reduces tow vehicle stability. Here’s how and why.
Typical WDH (weight distributing hitch) Pic: Jayco.
Need for a WDH – tow ball mass
Within limits, caravans must be nose heavy. For light European-style caravans with (desirable) centralised mass and loading, ‘the improvement becomes less when nose mass rises above 6-7 per cent of the total weight’ ∗. These caravans neither have a WDH nor even provision for one.
Locally made caravans, however, have become longer and heavier (some exceed 4.0 tonne). They need a high nose mass of at least 10% at all times. Such nose weight (up to 350 kg) however is imposed on a hitch that overhangs the tow vehicle’s rear axle. The effect is like pushing down on the handles of a wheelbarrow. It levers up the front wheels of the tow vehicle. This (in effect) lessens the grip of the (steering) front tyres.
To counteract this, a WDH (a springy semi-flexible beam) attached between caravan and tow vehicle, levers the front wheels back down. This wholly or partly restores the weight. But whilst partially fixing that frontal weight problem it inherently introduces another.
The WDH issue
Only too often overlooked, whilst a WDH assists the front end weight issue, it cannot compensate for yaw (snaking) forces.
The beginning of a jack-knife. Side forces on a tow ball cannot be corrected by a WDH. Pic: copyright caravanandmotorhomebooks.com
Worse – by reducing the imposed nose weight on the tow vehicle’s rear tyres – it reduces their ability to counteract those yaw forces. That in turn causes them to run wide – in effect introducing the unstable result shown above.
Need for a WDH – how tyres behave
The area of a tyre in contact with the road (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. For the same reason, any side force will cause a tyre to have a steering effect – thus including rear tyres when subjected to yaw forces.
Here’s how a tyre behaves when steered – or subjected to a side force.
A correctly cornering rig follows the green dotted line shown below – i.e. it runs very slightly wide. That running-wide effect, known as understeer, automatically reduces the risk of jack-knifing. The red dotted line shows what happens if understeer is too great.
Pic: original source unknown.
When a heavy caravan yaws, the WDH’s reduction of rear tyre loading reduces their ability to resist the imposed yaw forces. This disturbs their required ratio of grip and slip angle action front/rear. If the rear slip angle exceeds a critical level the result may build up to that shown below – dotted red line – where those (rear) tyres may even lose all grip
If the rear tyre’s increasingly cause the vehicle to self-tighten the turn (oversteer), the result may result in final jack-knifing. Pic: original source unknown.
Caravan jack-knife in the UK – Pic: original source unknown.
Need for a WDH -adjusting a WDH
It has been known since the late 1970s that using a WDH to compensate for loss of tow vehicle front end weight prejudices the tow vehicle’s desired handling. If the WDH is adjusted to fully compensate it introduces a loss of cornering ability of 25-30%. (In physics terms it reduces it from about 0.4 g down to about 0.3 g).
Despite this, until recently recommendations have been to adjust a WDH to fully counteract caravan nose ball weight. Now, however, following recommendations (in a major study – SAE J2807), 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 undesirably lift.
Need for a WDH – tyre pressures when towing
Regardless of using a WDH or not, when towing increase the tow vehicle’s rear tyre pressure by about 35 kPa (5 psi). Decrease front tyre pressure by 20-35 kPa (3-5 psi). This virtually restores the previous steering characteric. 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.
Need for a WDH – summary
The above issues have long been well understood.
They have been substantially addressed in the EU, and also followed by EU caravan firms now building caravans in Australia. They are also covered in associated articles on this website.
Need for a WDH – further information
See also the excellent UK/EU related: 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.