Rust accumulated on disk rotor surfaces causes brake judder. In this paper, the relation between the frictional properties of the rusted disk rotor and brake judder was examined by taking two approaches.
First, the conditions reproducing the rusted disk rotor were considered. It was found that the rusted disk rotor equivalent to that collected from area where problems of rust judder are reported frequently could be reproduced by dipping in NaCl solution and the rust of this disk rotor formed two layers.
Second, a hypothetical model and a theoretical formula of the torque variation caused by the rusted disk rotor were devised. Then, the influence of the coefficient of friction (hereinafter COF) differences between a disk pad and the rusted area / rust-free area with the disk rotor, half of which was covered with a rusted layer, for torque variation was verified. It was found that the COF of the rusted area / rust-free area of the disk rotor were greatly different. In this model, if the difference was large, torque variation increased in spite of decreased rust thickness by brake application. This phenomenon was observed in the rust removal test. In order to reduce the difference, a disk pad was produced by reducing the contents of inorganic substances in the disk pad with a Mohs hardness of 4.0 to 6.0, which are the typical hardness values of cast iron and rust, respectively, and the test was conducted using the disk pad. As a result, this phenomenon did not occur.