Towing Without a Distributing Hitch (WDH)
Often abbreviated to WDH, a weight distributing hitch is not needed if a tow vehicle’s laden weight is equal to or exceeds that of the laden caravan. Nor is a WDH hitch needed for any trailer under 4 metres. This article explains how and why a WDH causes instability problems. Furthermore, it explains why towing without a weight distributing hitch is feasible.
A weight distributing hitch attempts to compensate for an issue that should not exist. That issue is towing a laden caravan that exceeds 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 that happens, it levers up the front of the tow vehicle. This reduces the weight on the tow vehicle’s front tyres.
Where, however, that tow ball weight is comfortably within the laden tow vehicle’s payload, that front tyre weight reduction is too small to be an issue. Reducing the tow vehicle’s front tyre air pressures by about 14 kPa (2 psi) compensates for that minor effect.
Nevertheless, such caravan owners may fit one because a sales-person advised it. Or by following misleading advise on Australian caravaneers forums. Fitting one when not needed, may introduce unnecessary and potential issues.
How a WDH works – and its unwanted effects
A WDH is, in effect, a springy light 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 that may seem a good concept, a WDH can only counteract the caravan’s tow ball downforce. When the caravan yaws, the side forces imposed on the tow vehicle’s rear tyres that now carry less weight, are less able to counteract them.
The overall effect of a WDH is to reduce the rig’s ‘cornering power’. It does so by an appreciable amount. Strong side forces on the tow vehicle’s rear tyres may cause those tyres to steer the tow vehicle. If that happens, a jack-knife is virtually inevitable.
The above does not imply that the rig inevitably misbehaved at speed – but it is more likely to if ‘hit’ by a strong enough side force – such as wind gust. Or by cornering too fast.
Fitting a WDH to a rig that does not need one is not only pointless. It introduces issues that do not exist without one. A WDH is only needed when a laden caravan weighs more than the laden tow vehicle. If the weight issue is only minor, towing without a weight distributing hitch is feasible by carrying the caravan’s spare wheel in the tow vehicle.
A full explanation of Towing without a weight distributing hitch is in our Why Caravans Roll Over – and how to prevent it.