Hayes Disc Brake today finally released the remaining details on their new Prime brake platform and as promised, the pad contact adjustment mechanism is quite clever.
The new Poppet Cam system uses an additional poppet valve that runs right through the centre of the piston itself.
Free stroke adjustment is achieved via an 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 with other systems, the Poppet Cam doesn't alter the position of the master cylinder piston itself.
Touted benefits include greater fluid flow to reduce heat buildup and lever pump, and improved seal durability relative to traditional systems because 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.
Those familiar with RockShox's Motion Control damping might also find some similarities between their Floodgate mechanism and the Hayes setup, at least from a design standpoint. Most of the brakes currently on the market use a similar design for their pad contact or 'dead stroke' adjustment system.
All use some sort of internal mechanism that varies the point when the master cylinder piston seals the chamber and actually begins to move fluid through the hose. The traditional solution is to move the piston relative to a 'timing port' drilled transversely into the chamber. Avid's new Taperbore uses a similar concept but with a necked-down cylinder wall that the piston seals against instead of a timing port.
More information (and some illustrative videos) can be found at www.hayesdiscbrake.com/prime/index.shtml.