The use of reinforced phenolic composite material in application to hydraulic pistons for brake calipers has been well established in the industry - for sliding calipers (and certain fixed calipers with high piston length to diameter ratios). For decades, customers have enjoyed lower brake fluid temperatures, mass savings, improved corrosion resistance, and smoother brake operation (less judder). However, some persistent concerns remain about the use of phenolic materials for opposed piston calipers. The present work explores two key questions about phenolic piston application in opposed piston calipers. Firstly, do opposed piston calipers see similar benefits? Do high performance aluminum bodied calipers, where the piston may no longer be a dominant heat flow path into the fluid (due to a large amount of conduction and cooling enabled by the housing), still enjoy fluid temperature reductions? Are there still benefits for judder with the much shorter length to diameter ratio the pistons have in these applications? Secondly - it is clear that the much shorter length to diameter ratio of the piston in opposed piston calipers will result in significant increases in contact stress on the piston material at its contact points to the bore, when it is pressurized against pads with significant taper wear - will the phenolic material have adequate durability to withstand this? Can a simple “application guideline” for phenolic pistons be defined, potentially based on piston diameter (governing peak clamp load) and length to diameter ratio (which determines the correlation between clamp load and contact stress in the piston material at the bore contact points)? To address the first question, a battery of comparative tests was run on a high performance 6 piston aluminum-bodied brake calipers with high performance low-metallic brake pads and a large 18” wheel envelope two piece, cast iron plate and aluminum hub rotor. Fluid consumption, drag, brake torque variation, and fluid temperatures were measured through tests designed to exercise these behaviors, with both the production aluminum pistons and prototype phenolic piston calipers. The second question was explored through lab-based durability testing, abusive inertia dyno testing, and analysis of parts failed during testing. Pistons of the same phenolic material (Durez 29504B) were prototyped in opposed-piston caliper configurations in two sizes (51mm and 34mm) and tested to failure. The analysis of the data changed the authors’ initial thinking substantially about the failure mechanics of the piston in severe use, but still resulted in a simple, free body diagram based application guideline and a clear path for future work.
