They are also subjected to severe shock loads and high temperatures during braking. Like other rolling element bearings, all of the usual causes of bearing failure also apply to wheel bearings. Such conditions exist in wheel bearings of automobiles that are shipped over long distances by rail or motor truck. Or, fretting might occur when a vehicle is stored for a long time without moving. In this case, vibration of the ground or building can be sufficient to cause fretting.
Grease often turns reddish-brown due to the formation of a red iron oxide as a result of fretting.
Iron oxide is an abrasive and may rapidly wear depressions in the rolling elements. Fretting corrosion appears to depend on a combination of bearing load, speed of vibration, and angle of oscillation. Bearing metallurgy seems to have little influence on fretting. Removing the load on wheel bearings by jacking up the vehicle during long-distance haulage or prolonged, stationary storage is the only cure for the problem. The type of grease used may also affect the fretting phenomenon.