The 1/3 Rule
as it has always been known is shown above. You'll also note that it
pertains to a "shoe" tip rocker arm, not a roller tip. The principle of
this geometry measures from the actual contact point of the rocker's pad
with the valve tip. It establishes that at 1/3 of the total valve lift,
a perpendicular (90° angle) is made between the valve and the rocker
arm's fulcrum. That's the history of this.
The reasoning for this
has never been accurately described in layman's terms, but has to do
with averaging out the shifting change in ratio. I've put together a
series of illustrations that show this in all three stages, with the
next few illustrations adding the radial circles created by these three
contact points. Understand that these radial circles are the path that
the rocker arm rotates at EACH of these three lengths, BUT... as you can
see from the red lines drawn between the tangent circles (green, red and
yellow), this dimension is constantly increasing from closed valve to
full lift, as set in this geometry.
Continue to click the
illustrations, and you'll see the next two positions (represented by the
little RED and YELLOW circles) shown. As explained in The History Of
MID-LIFT, it was the transposing of a roller tip directly in place
of the shoe tip that created a bad mistake, which the inefficiencies of
THIS concept only magnified, and was copied over and over for years,
clear up into the late 1990's by the most popular names in valve train
manufacturers; of both cam and rocker arms. In fact, the stubborn still
continue this twisted rhetoric to this day.
See:
HISTORY Of MID-LIFT
