Antanaitis, D., Ciechoski, C., and Riefe, M., "Further Research into the Role of the Caliper Piston in Brake Roughness," SAE Int. J. Commer. Veh. 8(2):258-270, 2015, doi:10.4271/2015-01-2667.
Previously published research  covering the role of piston material properties in brake torque variation sensitivity and roughness concluded that phenolic pistons have significantly higher low-pressure range compliance than steel pistons, which promotes lower roughness propensity. It also determined that this property could be successfully characterized using a modern generation of direct-acting servo hydraulically actuated brake component compression test stands.This paper covers a subsequent block of research into the role of the caliper piston in brake torque variation sensitivity (BTV sensitivity) and thermal roughness of a brake corner. It includes measurements of hydraulic stiffness of pistons in a “wet” fixture, both with and without a brake pad and multi-layer bonded noise shim. Inertia dynamometer measurements of BTV sensitivity against a machined 20-micron thickness variation rotor with multiple (prototype) phenolic piston geometries, as well as steel pistons with two different piston to bore clearance levels followed. Finally, higher-temperature thermal roughness testing of multiple phenolic piston geometries and steel pistons with tighter and looser piston to bore clearances were run.The results of these studies give some insight into the role of the caliper piston - its material properties, geometry, and clearance to the bore - in brake roughness and performance.