Abstract: For indicating relative rotational displacement between a fixed first member, such as a stud (20) on a wheel hub, and a second member, such as a wheel nut (22), which is rotatably engaged with the first member, a safety device (24) comprises a body (26) having a bore formed with equispaced grooves enabling the device to be releasably secured to the second member (22) in one position of a plurality of positions and having indicating means, such as a pointer (28), to indicate the position of the device (24) relative to a reference, such as a mark (32) on the rim (16) covering the wheel hub (14). The bore within the body (26) is closed at one end, e.g. by bevelled top wall (36), so that the body additionally serves as a dust cap for the conjoined first and second members (20, 22).