On an artic - where is the wheel base caluculated ?.. some say kingpin to centre of the axle - but which axle if the trailer has 3 axles ?
Wheelbase is calculated as follows,
On a 4x2, 6x2 or 8x2 tractor/rigid, it is from the centre line of the front axle to the centre line of the drive axle, if it has a tag axle the distance from the centre line of the front axle to the centre line of the rearmost axle is know as the OAS, Outer Axle Spread. The OAS and the wheelbase are the same on a mid lift.
On a double drive vehicle the wheelbase is calculated from the centre line of the front axle to the centreline of the rear bogie.
On a trailer the wheelbase is calculated from the king pin, to the centre line of the axle on a single axle, to the centre line of the rear bogie on a tandem and the centre line of the middle axle on a triaxle.
On a four or more axle trailer I have no idea whatsoever 
newmercman:
Wheelbase is calculated as follows,On a 4x2, 6x2 or 8x2 tractor/rigid, it is from the centre line of the front axle to the centre line of the drive axle, if it has a tag axle the distance from the centre line of the front axle to the centre line of the rearmost axle is know as the OAS, Outer Axle Spread. The OAS and the wheelbase are the same on a mid lift.
On a double drive vehicle the wheelbase is calculated from the centre line of the front axle to the centreline of the rear bogie.
On a trailer the wheelbase is calculated from the king pin, to the centre line of the axle on a single axle, to the centre line of the rear bogie on a tandem and the centre line of the middle axle on a triaxle.
On a four or more axle trailer I have no idea whatsoever
Just bang it on and give us a ring when you are empty 