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In this paper, the measured mode shape of a squealing drum brake is presented. The squealing noise is the result of a self-excited vibration that can occur on most types of brakes. In the literature, there are several suggestions for the mechanisms behind the squeal instabilities. Therefore, there is a need for measurements to verify the models. The vibration signals are measured with accelerometers mounted on a drum brake of a heavy duty vehicle. The mode shape is measured in operation, i.e. while the vehicle is running on a test ground. The result from the measurements is the mode shape of a squealing drum brake. The radial and tangential vibration of the drum and the shoes are measured and the accelerometers are distributed both axially and tangentially. The measured shape is a complex mode containing a wave. This wave moves in the same direction as the rotation of the drum. As the drum rotates during the measurement, a Doppler effect influences the measured frequency.