Running clearance is referred to by the Brake Industry (Vehicle OEM's, Foundation brake suppliers, Fiction material suppliers, etc.) during discussion relating to the performance and acceptance criteria of air disc brakes. There is no universally accepted definition of running clearance even though the limits are regularly specified in Customer requirement specifications, brake suppliers product capability specifications and brake test procedures. As a result there can be misconceptions, unrealistic demands and expectations related to this subject. All are interested in the "Vehicle running clearance" when measured in a vehicle installation, this however depends on many variables related to the foundation brake, friction material, rotor, actuator, wheel end and temperature of the rotor and pads at any point in time. Brake suppliers are also interested in the above but will focus heavily on the "Designed running clearance" (based on drawing tolerances), "Predicted running clearance" (a calculation of the brakes running clearance based on a number of measured parameters, "Natural running clearance" (for a particular brake and defined test method) and the "Test running clearance" (during development testing and validation) of the brake as the variables controlling these are within their control. The other variables are often specified and controlled by the Customer or other component suppliers. This paper describes the numerous definitions of "running clearance" and looks in detail into the variables that affects it's specified and measured values.
Paul Thomas - Meritor Heavy Vehicle Braking Systems UK Ltd.