George P posted:"The goal of any valve train, as far as I'm concerned, is to perfectly follow the motion imparted by the cam lobes."
Agreed.
"The rocker arm geometry as I have drawn it makes no attempt to optimize geometry at the push rod cup; but it optimizes geometry at the valve tip."
Yes, let's ignore the pushrod side of the rocker arm for now, and focus on the valve side for the moment...
"By optimum geometry I mean: (1) motion at the rocker arm tip shall be most linear, (2) side thrust between the valve stem and valve guide is minimized, (3) valve guide wear is therefore minimized too, (4) the width of the rocker arm tip's wear pattern across the tip of the valve is also minimized, and (5) valve motion imparted by rocker tip motion is maximized."
So, to simplify just a bit....can we agree that the minimal sweep pattern (and therefore the most 'efficient' linear motion, etc.) at the valve stem tip is an end result of optimum geometry?
"There are several "theories" regarding rocker arm geometry. Frankly the rocker arm instructions provided by some of the cam companies is the worst! Jim Miller's is OK. I've seen the "fulcrum line or motion line" explained as per the pictures you posted many times over the years, but with due respect to the author it is geometrically wrong. What I've drawn is correct."
I will agree that most cam companies are barking up the wrong tree when it comes to proper rocker arm geometry, However, I believe that Jim Miller has got it right. I have been meaning to analyze this relationship in SolidWorks for a while now, this may be the kick in the pants I've needed to get it done...
"There are two aspects this rocker arm geometry doesn't accomplish: (1) geometry between the rocker arm and push rod is not optimized, it relies on the design of the rocker arm to be reasonably "accurate". (2) Nothing has been done to center the rocker arm's contact patch on the valve tip."
Duly noted.
"The "ball end" design of push rods makes geometry between the rocker arm's push rod cup and the push rod less critical. Less than perfect geometry at that end will not result in the accelerated wear that poor geometry at the other end does."
Agreed.