People are increasingly using caravans that, when laden, well exceed the laden weight of whatever tows them. It is, for example, not uncommon to see 2500 kg (laden) dual-cab utes towing 3500 kg caravans.
For on-road stability, a conventional caravan needs to be nose-heavy by 8-10% of its laden weight. When hitched to its tow vehicle that (typically 200-350 kg) pushes down on the rear of that vehicle. As with pushing down on the handles of a wheelbarrow, that levers up the front of the tow vehicle , thereby reducing the weight on the tow vehicle’s front tyres.
Where that tow ball weight is comfortably within the laden tow vehicle’s payload, that front weight reduction is unlikely to cause any but a slight decrease in the understeer necessary for stability. Furthermore, that weight decrease is compensated by reducing tow vehicle front tyre air pressures (by as little as 14 kPa (2 psi).
A so-called Weight Distributing Hitch (WDH) may then be added to resolve the weight problem. If doing so, however, it fixes that weight problem - but introduces unwanted effects.
How a WDH works
A WDH is, in effect, a springy beam that, by levering up the rear of the tow vehicle, restores weight (down-force) from the tow vehicle’s rear tyres to its front tyres.
While this seems a good concept, a WDH can only counteract the caravan’s tow ball downforce. Side forces (when the caravan yaws), however, are still imposed unchanged on the tow vehicle’s rear tyres that now (due to that WDH) are less able to counteract them.
The overall effect is to reduce the rig’s ‘cornering power’ by an appreciable amount. In extreme (but nevertheless encountered) circumstances, these side forces may cause that tow vehicle to oversteer. If that happens, a jack-knife is virtually inevitable. In all cases, the effect is to lower that rig’s stable top speed and often to below 100 km/h. That does not imply that the rig will inevitably misbehave at speed – but is more likely to if ‘hit’ by a strong enough side force.
Reducing a WDH’s unwanted effects
The above effect is inherent, but its extent can be reduced by adjusting the WDH to correct only 50% or so of the weight transferred. Check this by (before coupling up the caravan) measuring the distance between the top of the tow vehicle’s front wheel and the wheel arch. Then, with the WDH not engaged, coupling-up the laden caravan. Then adjust the WDH such that the distance is halved.