Camshafts and Compression

Bewy said

A shoe rocker works exactly the same as a roller rocker. The shoe can be likened to a large diameter roller

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My understanding, the tip of the rocker, roller or sliding/ radius, travels through an arc so there’s always going to be some angularity between the contact point and the pivot point / axle of the rocker through its sweep. Like Smokey says there. The clue is in the name, rocker!

Of course it can be improved by having good valvetrain geometry.