A device for pulling a trailer and/or retaining a load carrying unit, which device is mountable on the rear end of a motor vehicle body , comprising a supporting arm , in particular a ball neck, which is connected to the motor vehicle body by a first end region and which is provided at a second end region with an element , in particular a coupling ball, for attaching the trailer and/or for fixing the load carrying unit, and furthermore comprising sensors for sensing reversible deformations of the supporting arm caused by loads on the supporting arm , wherein there are associated with a supporting arm portion of the supporting arm strain sensors which are influenced by the reversible deformations of the supporting arm, and characterised in that , for sensing at least one selected bending load, which is caused by a force on the element in the vertical direction, at least one strain sensor is arranged on one side and at least one strain sensor is arranged on an opposite side of a plane region (FB) of a vertical neutral reference plane (NRFV) associated with the selected bending load, wherein the vertical neutral reference plane (NRFV) runs through a vertical neutral axis (NFV) and perpendicularly to a bending movement plane which is formed under the vertical bending load (BV), in that the strain sensors are each arranged at a spacing from this plane region (FB), in that the strain sensors are oriented approximately parallel to the plane region (FB) of the neutral reference plane (NRFV, NRFQ), and in that the strain sensors are arranged and oriented in such a way that they sense a horizontal tensile loading on the element by a horizontal longitudinal force (FX). comprising a supporting arm , in particular a ball neck, which is connected to the motor vehicle body by a first end region and which is provided at a second end region with an element , in particular a coupling ball, for attaching the trailer and/or for fixing the load carrying unit, and furthermore comprising sensors for sensing reversible deformations of the supporting arm caused by loads on the supporting arm , wherein there are associated with a supporting arm portion of the supporting arm strain sensors which are influenced by the reversible deformations of the supporting arm, and characterised in that , for sensing at least one selected bending load, which is caused by a force on the element in the vertical direction, at least one strain sensor is arranged on one side and at least one strain sensor is arranged on an opposite side of a plane region (FB) of a vertical neutral reference plane (NRFV) associated with the selected bending load, wherein the vertical neutral reference plane (NRFV) runs through a vertical neutral axis (NFV) and perpendicularly to a bending movement plane which is formed under the vertical bending load (BV), in that the strain sensors are each arranged at a spacing from this plane region (FB), in that the strain sensors are oriented approximately parallel to the plane region (FB) of the neutral reference plane (NRFV, NRFQ), and in that the strain sensors are arranged and oriented in such a way that they sense a horizontal tensile loading on the element by a horizontal longitudinal force (FX). <EOS>