Supporting load measuring device and retrofitting method for a vehicle trailer device

The device (1) has a force sensor (5) e.g. metal film-strain gauge, arranged in a force path between a towing device and a reference location for measuring a supporting load transferred by the towing device. An evaluation electronics unit (7) is in electrical contact to the sensor for evaluating a supporting load measuring value. A display unit displays the load. A power supply unit supply electrical energy to the electronics unit. The sensor, the electronics unit and the supply unit are provided in a weather-proof sensor housing. An independent claim is also included for a method for retrofitting a towing device with a supporting load measuring device.