This
Illustration shows OVER-ARCING, whereby the roller tip side of
the rocker arm is moving outward and "OVER" its path as it pushes
downward upon the valve. NOTE that "C" (ROLLER SWEEP), is .184".
The valve tip width is only .342" (11/32), so here more than 1/2 of the
tip's width sees roll across action, which induces side loads on the
valve tip, valve guide and valve stem. The roller does NOT "roll," much
to the argument of many. It merely provides a "pivotal" axis for
converting the linear motion into radial motion. In fact, anyone who's
ever seen high speed photography of the roller working atop the valve,
will confirm that is does more "skipping" than anything (kind of like an
airplane tire first hitting the runway).
Note the "Motion Lines"
(in YELLOW) angle between the trunnion and roller axis in
relation to the valve.
Of
the various points of information shown above, note "A"
illustrates that 30 degrees of rocker ROTATION is occurring to initiate
"B", a valve lift of .685". (See:
MID-LIFT-ARC) This is wasted motion, compared to the valve's
INSTALLED Geometry being set for MID-LIFT, which yields .709"
VALVE LIFT for the SAME 30 degrees of rocker rotation!
As with the
UNDER-ARCING illustration, which is purposely shown as the exact
opposite perspective, both extremes illustrate how the "symptom" of a
NET valve lift can be the same, while opposite circumstances prevail.
The main thing to understand from this, aside from the obvious increase
in roller SWEEP over the MID-LIFT example, is that the valve lift speed
is inverted between over-arcing and under-arcing. As stated in the
drawing, this example has the valve starting off the seat slower, then
increasing as the roller begins to leave the linear path pushing down
upon the valve, begins it is reaching a 90 degree relationship. When it
begins to follow over its axis, as shown above, the speed of the valve's
opening increases proportionately until reaching this 90 degree point.
This is measured at the crankshaft as additional crank degrees of
rotation to turn open the valve a specific amount, compared to MID-LIFT
geometry. Velocity will always be at its peak at 90 degrees to the
rotational axis.
OVER-ARCING
is the MOST COMMON symptom of rocker arm geometry found on most engines.
There are many reasons for this, but simply, it's because few people
understand how much and why the rocker needs to be RAISED to adjust its
pivot points. The junk plastic pushrod checkers that have been sold for
years, do NOTHING to set geometry accurately. In some cases, only making
it worse. At the very least, they give the engine builder (and his or
her customer) a very false sense of security.
