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Caravanning Now - What's the correct height for my Tow ball?

From the Caravan clubs handbook page 640 (1999 / 2000 edition ) it says the coupling ball is 50 mm diameter and when measured from ground to ball centre should be between 350 and 420 mm (vehicle fully laden - but not with caravan attached) in accordance with BS AU 113c or ISO 1103.

NOTE: If the car is actually at its fully laden weight (GVW) then you would exceed the GVW by the weight of the down force on the tow ball. (Thanks to Lutz Schelisch for pointing this out.)

NOTE 2: It's actually far more complicated.
You have to take max axle weight into account and also the overhang length.

For Example: A 75kg caravan nose weight on a 110 LandRover puts around 125kg on the back axle.
Since most cars carry most of their luggage behind the rear axle, it's pretty easy to exceed the rear axle max weight and remain in the gross vehicle weight.
75kg on a 110's tow bar actually takes ~50kg off the front axle.
Luggage behind the back axle also transfers weight from the front axle to the back axle, exacerbating the problem.

See also the page on what weight caravan you can tow.