WARNING: This page is about a different car, the 2010 BMW M5, 2009 BMW M5, 2008 BMW M5, 2007 BMW M5, and 2006 BMW M5. However, it is still accessible from the selected car via links, so may be relevant.
TECHNICAL DATA - PARKING BRAKE E60
Handbrake drum diameter
mm
185
Surface roughness of braking surfaces (precision-turned)