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Old 10-11-2011, 12:39 PM
oldbogie oldbogie is online now
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Quote:
Originally Posted by cobalt327
I'm hoping you can clear this up for me. Comp Cams has long had the diagram below posted on their site on valve train geometry.

Do they depict the correct action? It would seem to me that the roller would move towards the intake side of the valve tip the more lift applied, not towards the exhaust side the more lift was applied. Disregard their depiction of where the base circle is in comparison to the mid- and full-lift positions. What I'm concerned with is whether the roller moves towards the intake side or the exhaust side as lift increases.

Thanks in advance.

This is accurate, there is no perfect solution to this short of a direct acting overhead cam. The length of the push-rod controls the start, mid, and end points of the sweep, which is why its so important got this right assuming that the rocker dimensions are not changeable.

What's happening is the rocker is making a rotation around its fulcrum. At the valve stem on the tip of the rocker it is describing a secant of a circle that moves to a tangent then back to a secant as the rocker presses on the valve stem. A secant is a line that passes inside the perimeter of circle cutting that perimeter in two places. A tangent occurs when the two secant points meet at the same location so the line only touches the perimeter at one place.

The rocker tip forms the perimeter of the circle. The valve stem is the line that can be a secant or tangent depending upon where it intersects the circle described by the rocker. The rocker actually is making two different circles as it moves those being the valve-stem side and the push-rod side. The difference between their radiuses is equal to the lift ratio.

Jim Millers argument is that the side loading on the stem and resulting harmonics is at its minimum when a line that connects the center of rotation of the fulcrum makes a 90 degree angle with the valve stem when that line passes through the roller's axle and that axle is centered on the stem at mid lift. This is not really inconsistent with what the Comp tech site is driving on.

However, conventional design of roller tip rockers Mr. Miller argues passes that line through the contact point which makes an inaccuracy of the roller's movement on the stem equal to the radius of the roller. This is to say that there are two lines of secant and tangent; 1) that of the tip contact point and 2) that of the roller's rotation center. They are not describing the same secant line through the perimeter of the circle of rotation. This difference is a side load on the stem and a rocking motion along the roller's axis if the stem and roller are not meeting perfectly flat to each other, which I rather doubt happens on the best of race engine builds, especially after the first head service.

When I say this is a "tempest in a tea pot"; I mean that with the typical engine with 1.4 to 1.6 ratio rockers and lifts under .5 inch the difference is so small as to have no practical effect. However, with extreme lift cams and/or rocker ratios where the valve is being opened .6 to .8 inch this is something you want to consider as the motions are becoming quite extreme and there gets to be a real risk not only of extreme side loads on the stem but of not being able to keep the contact point on the stem if you?re not very careful.

Bogie
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