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The required fifth-wheel coupling size is determined by the D value.

D value for fifth-wheel coupling equation:

D = ( 0.6 · G · T · U ) / ( T + R - U )

Permissible gross weight of the semi trailer equation:

R = ( D · (T · U ) / ( ( 0.6 · G · T ) · D )

If the permissible gross weight of the semi trailer and the D value of the fifth-wheel coupling are specified, the permissible gross weight of the trailer unit can be calculated with the following formula:

Permissible gross weight of the tractor unit equation:

T = ( ( D · ( R - U ) ) / ( ( 0.6 · G · R ) - D )

If the fifth-wheel load is sought and all other loads are known, the following formula can be used:

U = T + R - ( 0.6 · G · T · R ) / D

D = D value, in kN
R = Permissible gross weight of the semi trailer unit in metric tons, including the fifth-wheel load
T = Permissible gross weight of the tractor unit in metric tons, including the fifth-wheel load
U = Fifth-wheel load, in metric tons
G = Gravity, 9.81 m/s2  