The fifth wheel of an articulated vehicle comprising the coupling between a semi-trailer and a tractor is provided with measuring apparatus for measuring forces imparted from the semi-trailer. The apparatus comprises a sensing device for measuring horizontal loads, and a sensing device for measuring vertical loads. The signals from the sensing devices and are fed into an electronic controller, in turn to cause the brakes on the trailer to be applied to achieve balanced braking, anti-lock control, and traction control of the wheels. 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 can therefore be used.