With a 5th wheeler the tow ball / hitch is located in the tray of the vehicle. Australian Design Rules dictate the type of hitch that must be used, though not the specific location of that hitch.
- If the hitch is behind the rear axle, the front of the towing vehicle lifts under towing conditions.
- If the hitch is over the rear axle, front axle loading stays generally the same.
- If the hitch is in front of the rear axle, front axle loading is increased. A slight front-end lifting effect may occur when speed increases, though this is only noticeable when accelerating hard from a standstill.
With a hitch in this position the tow vehicle can carry much more weight than it could if a caravan was connected to a towball as the fifth wheeler imposes (a great deal more of the mass) some 20% of the mass directly over or slightly in front of the towing vehicle’s rear axle. By so doing, pitching and snaking are all but eliminated.
When calculating the towing capacity of a vehicle for a 5th wheeler the manufacturer’s weight ratings of the tow vehicle must not be exceeded by the trailer, specifically the Gross Vehicle Mass (GVM) and the Gross Combined Mass (GCM).
For example if the GCM is 4.5 tonnes and the tow vehicle weighs 2.0 tonnes, then the maximum weight of the fully laden trailer must not exceed 2.5 tonnes.
Whilst it is normal practice to have about 20% of the fifth wheeler’s weight carried by the towing vehicle, that weight must not exceed the legal carrying capacity of the tow vehicle, particularly not exceeding the carrying capacity of the tow vehicles tyres nor the individual axle loading.