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.
Overhung hitches introduce inherent instability that causes (not just allows) yawing (swaying) or pitching of either the tow vehicle or the trailer, which in turn causes the other to also yaw and/or pitch.
The overhung hitch effectively acts as a lever that transfers side forces from the trailer to the tow vehicle but in opposite directions: a caravan that swings clockwise will, through its overhung hitch, push the tow vehicle such that it swings anticlockwise - and vice versa. This so-called 'phase reversal' causes a seriously unwanted Double Pendulum effect.
If the Double Pendulum effect happens above a certain critical speed, it introduces a so-called positive feedback loop that escalates the effect of each force on the other within only a few cycles.
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.
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 forthcoming Australia & New Zealand RV Handbook.
Because a fifth-wheeler's over-the-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. Now, heavy transport, with the huge weights it has to carry, rarely uses overhung hitches.
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