Towing Without a Weight Distributing Hitch
Usually abbreviated to WDH, a weight distributing hitch is not needed if the laden weight of the tow vehicle equals or exceeds that of the laden caravan. Nor is a weight distributing hitch needed for any trailer under about 4 metres. Towing without a weight distributing hitch is safer. It enables the rig to retain stability at a higher speed.
Nevertheless, such caravan owners may fit one because a sales-person advised it via misleading advise on Australian caravanners forums. Fitting one when not needed introduces unnecessary and potential issues. This article explains how and why.
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. The resultant force levers up the front of the tow vehicle. This reduces the weight on the tow vehicle’s front tyres. That weight reduction makes a minor change to that vehicle’s handling. 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.
That vital, however, when towing (and even more if using a WDH) is to increase tow vehicle rear tyre pressures by 50-70 kPa (7-10 psi). That alone makes a major improvement in on-road handling and safety.
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 down-force. When the caravan yaws, the side forces imposed unchanged on the tow vehicle’s rear tyres that now carry less weight, are now less able to counteract them.
The overall effect of that unnecessary WDH is to reduce the rig’s ‘cornering power’. It does by an appreciable amount. In the extreme, but encountered circumstances, the 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 misbehaves 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, that is readily fixed (for example) by carrying the caravan’s spare wheel in the tow vehicle.
A full explanation of the above is in our Why Caravans Roll Over – and how to prevent it.