The present invention provides an apparatus for and method of determining the traction conditions between engaged rotating members. A first roller member (12), in frictional engagement with a second roller member (14), is provided with means (32, 34) of support which facilitate adjustment of the position of member (12) so as to maintain a desired planal position of said member (12) relative to the second member (14). The accuracy of data obtained from this arrangement is greatly enhanced.
