The fifth wheel (3) of an articulated vehicle comprising the coupling between a semi-trailer (2) and a tractor (1) is provided with measuring apparatus for measuring forces imparted from the semi-trailer. The apparatus comprises a sensing device (5) for measuring horizontal loads (AD), and a sensing device (6) for measuring vertical loads (W). The signals from the sensing devices (5) and (6) are fed into an electronic controller (26), in turn to cause the brakes (27) on the trailer (2) to be applied to achieve balanced braking, anti-lock control, and traction control of the wheels. We can therefore use a tractor fitted with a braking system of the brake-by-wire type with a conventional semi-trailer, without modifications to semi-trailer or to the braking components of the semi-trailer.