DESIGN and
INSTALLATION. These two simple perspectives of rocker arm function must be
understood separately. Because you can have one and not the other. If the "design geometry"
isn't correct, then your "installed geometry" can only be set for the
lesser of two evils, either setting the pivot points for a MID-LIFT
relationship on the "valve" side of the rocker arm, or the "push rod"
side of the rocker. Not both. On a stud mounted system, this is done by
changing the pushrod length. On a stand (shaft) system, this is done by
adjusting the stand's height.
From the above two
perspectives we establish that there are TWO kinds of geometry:
The
first is: "Design Geometry™."
The second is: "Installed
Geometry™."
Understanding the difference between
DESIGN and INSTALLED geometry is one good example; and it is
not only critical in choosing the right parts
for your engine, but also installing them correctly.
All Over-Head Valve (OHV)
engines have pushrods that link the cam and tappet information to the
rocker arm. The rocker arm pivots about an axis upon the cylinder head,
being pushed up by the pushrod, so that the opposite end of the rocker
arm pushes down upon the tip of the valve. This is really "engines 101"
-- but it's worth placing in perspective for our novice readers. As
stated elsewhere, there are two other perspectives that need to be
understood, in addition to what we call "design geometry" and "installed
geometry." These other two perspectives refer to the differences in
operating characteristics of the components we just spoke of. The "cam"
is a rotating device, with an eccentric lobe to it, that allows anything
sitting upon it to be pushed up as it rotates. This is an over
simplification that is really very precise and exact in its operating
characteristics. But this basic fact is true. The component that rides
upon it is known as a "tappet" or "lifter" or "cam follower," depending
on who's talking. Each term have their roots in the American language of
engine talk, but the part is the same. Riding upon the cam follower, is
the pushrod. Then the rocker arm, and finally the "valve" of our "valve
train." Everything, except the cam and the rocker arm, are linear in
their operating characteristics. They basically follow an in-line path.
But the cam and the rocker arm are "radial" and rotate about an axis.
Actually, the rocker arm doesn't rotate, it reciprocates (stops and
reverses itself); but none-the-less it follows around an axis, and this
has mechanical characteristics that must be understood to design and
install it properly on the engine with these linear components.
