Abstract: A wear determining arrangement (30, FIG. 2(a)) for determining wear of a rubbing member of a friction couple, such as an iron brake disc, which is subject to slow wear by frictional engagement by brake pads, comprises a carrier (31) dimensional to overlie the rubbing surface of the disc and be releasably attached thereto by electromagnet (51). The carrier supports a transducer (33), such as magnetic displacement sensor having probe (43), that is mounted on a carriage (36) for scanning the probe across the surface. After an initial scan to map the surface the carrier is removed and re-attached after a period of brake usage, whereupon the surface is scanned again and wear assessed. The rate of wear is normally small, and to ensure that the amount which occurs in a reasonable period can be measured, the carrier is provided with datum projections (46 and 47) and the electromagnet is mounted by way of resilient coupling means (52) to bias the carrier into a stable “tripod” relationship on the surface.