Overhung Hitches - updated December 2018

When a side force hits a trailer attached by a hitch that is NOT overhung (top), there is NO consequential side force on the tow vehicle. When a side force affects a trailer with an overhung hitch (bottom), the resultant side forces on the tow vehicle are material
When a side force hits a trailer attached by a hitch that is NOT overhung (top), there is NO consequential side force on the tow vehicle. When a side force affects a trailer with an overhung hitch (bottom), the resultant side forces on the tow vehicle are material

An overhung hitch is a connection between a tow vehicle and trailer that is located behind the rearmost point of the tow vehicle. These are the hitches used to tow caravans and camper trailers.

Overhung hitches are not used in fifth wheelers. For this type of RV as well as 50-plus tonne heavy transport semi-trailers or B-doubles, the tow hitch is located above a rear axle or centre-line of multiple rear axles.

An overhung hitch acts as a lever that not only transfers side forces from the trailer to the tow vehicle, but causes the forces to act in opposite directions. A caravan that swings clockwise is forced, via that overhung hitch, to push the tow vehicle such that it swings anticlockwise - and vice versa. This so-called 'phase reversal' results in inherent instability. The greater that overhang, the worse the effect. The effect is undesirable at low levels and very dangerous at high levels. 

Physicists reading this will realise this is the seriously unwanted Double Pendulum effect that may result in the 'chaos effect'). In essence it triggers a so-called positive feedback loop that escalates the effect of each force on the other within only a few cycles. Here, the effect is 'fuelled' by the rig's critical momentum.

We do not want the tow vehicle to be manipulated by anything other than its steering wheel, but the overhung hitch does precisely that - to the extent of potentially overwhelming the rig. Never use a tow hitch that extends that overhang (often to ease coupling and uncoupling). Safety is surely more important. 

So why are overhung hitches used for caravans and camper trailers?

 It is because a tow vehicle is normally used for purposes other than towing. Having a permanent tow hitch inside the rear luggage compartment of a tow vehicle is not practical for everyday use.

The adverse effects of an overhung hitch can be reduced, but never eliminated. A full explanation is in our shortly forthcoming digital mini-book devoted to explaining all aspects of caravan and tow vehicle dynamics, how to assess the stability of your own rig and how to improve its margin of stability.

The only total solution is to use a fifth-wheeler. It's over the tow vehicle's rear-axle hitch has no overhang: the trailer is able to pivot around its hitch free of all side force. It is inherently stable. This was recognised by the road transport industry (globally) almost 100 years ago.


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