Hayes Disc Brake today finally released the remaining details on its new Prime brake platform and as promised, the Poppet Cam pad contact adjustment mechanism is indeed quite clever.
Currently, the standard layout of all pad contact or 'dead stroke' adjustment systems is very similar in that there is some internal mechanism that varies the point when the master cylinder piston seals the chamber and actually begins to move fluid through the hose. While the traditional solution is to move the piston itself relative to a 'timing port' drilled transversely into the chamber (Avid's new Taperbore is similar in concept but with a necked-down cylinder wall that the piston seals against instead of a timing port), Hayes' system instead uses an additional poppet valve that runs right through the center of the piston itself.
The free stroke adjustment comes courtesy of the external 'Tophat' thumb-operated dial, which rotates an internal cam that then sets how far the master cylinder piston has to move before the poppet valve is sealed – unlike other systems, Poppet Valve doesn't alter the position of the master cylinder piston itself. Those familiar with RockShox's Motion Control damper might also find some similarities between its Floodgate mechanism and the Hayes setup, at least from a design standpoint.
Touted benefits of the Poppet Cam system include greater fluid flow to reduce heat buildup and lever pump and improved seal durability relative to traditional systems in that the master cylinder seal has no transverse port – and thus no sharp edges – to pass over. Based on the lever cutaways provided by Hayes, critical sealing surfaces on the poppet valve aren't subjected to wear during use, either, so that mechanism should be fairly robust as well.
More information (and some illustrative videos) can be found here (link to external website opens in new window).