Glossary

Axis Migration
Also known as Precession, Axis migration is the phenomenon that creates track flare. With each rotation of the ball,
the axis of rotation reorients itself. It is possible to plot several points on the surface of the ball that marks
the axis of rotation at various points down the lane.
Axis Rotation
Axis Rotation tells us how the PAP is oriented relative to the initial path of the ball, measured in degrees.
Eg. If the ball is traveling exactly parallel to the lane, if the PAP is parallel to the lane, the ball has 90°
axis rotation. If the PAP is parallel to the foul line, it has 0° axis rotation.
Axis Tilt
Axis Tilt tells us how the ball is oriented relative to vertical plane, perpendicular to the lane bed, measured in degrees.
Eg. If the PAP is exactly parallel to the lane bed, the ball has 0° axis tilt. If the PAP is exactly on the top of the ball
(ie. the ball is spinning on its top), the ball has 90° axis tilt. Incidentally, the can never have less
axis rotation than axis tilt.
Center of Gravity (CG)
The Center of Gravity is point of balance of a bowling. This is often marked on the surface of the ball anywhere
from 0″ to 6-3/4″ from the Pin. However, the actual Center of Gravity will be located within 3/16″ from the geometric
center of the ball.
Center of Grip
The exact center of all the gripping holes in the ball.
Differential Ratio
The differential ratio, usually referred as simply diff. ratio, is quotient of the
int. diff., divided by the
total diff. and gives us an indication of how
quickly the ball will respond to friction. The differential ratio is expressed as a decimal fraction.
Eg. If the total diff. is 0.038″ and the int. diff. is 0.022″, then the diff ratio is 0.58.
Grip Midline
Also known as the midline, the Grip Midline is a line perpendicular to the Grip Centerline.
If the ball has only two gripping holes, the midline is a line that bisects both of those gripping holes.
Grip Centerline
Also known as the centerline, the grip centerline is a line that bisects the thumb hole and passes exactly halfway between
the finger holes. In the case of a ball drilled with no thumb hole, the centerline bisects the finger holes and is perpendicular
to the midline.
High RG
The point on the ball with the highest RG value. This is the point the ball will spin to on a deTerminator.
This is also that marks the shortest axis in asymmetrical balls and is also the place where the RG Contour is at its flattest.
Intermediate RG
The point on the ball 6-3/4″ and mutually perpendicular from both the High RG axis and Low RG axis.
Intermediate RG Differential
Usually referred to as simply the int. diff., the Intermediate RG Differential is the difference between the value
Intermediate RG axis (PSA) and the value Low RG axis. Eg. If the High RG is 2.450″ and the Int. RG is 2.428″,
the int. diff. is 0.022″.
Low RG
The point on the ball with the lowest RG value. It is marked by the Pin on all balls.
Mass Bias
See
PAL
See Positive Axis Line
PAP
See Positive Axis Point
Pin
A colored marker on the surface of the ball that locates the Low RG.
Positive Axis Line
Another name for the Vertical Axis Line
Positive Axis Point
Also known as the PAP, Axis Point, or simply Axis, the Positive Axis Point is the end point of the axis about
which the ball rotates straight of the bowler’s hand. The PAP is used as the point of origin when laying out a ball.
The PAP is usually expressed as two measurements, one horizontal, one vertical (e.g. 4-1/2″ right, 1 1/4″ up),
relative to the bowler’s Center of Grip and Grip Midline. Right hand bowlers will always have a PAP to the
right of the Center of Grip, while left hand bowlers will be the opposite.
PSA
See Preferred Spin Axis
Preferred Spin Axis
See High RG
NAP
See Negative Axis Point
Negative Axis Point (NAP)
This is the point exactly opposite the PAP and is therefore the other end point to the same axis of rotation.
To find the NAP, simply measure 13-1/2″ from the PAP.
Radius of Gyration
Usually referred to as just the RG, the Radius of Gyration is a measure of how concentrated the mass of a given
object is about a given axis.
RG
See Radius of Gyration
Total Diff
See Total RG Differential
Total Differential
See Total RG Differential
Total RG Differential
Usually referred to as simply the diff., or total diff., the Total RG Differential is the difference between the value
High RG axis (PSA) and the value Low RG axis. Eg. If the High RG is 2.450″ and the Int. RG is 2.412″,
the int. diff. is 0.038″. The total diff. gives us an indicator of how much the ball will flare.
The bigger the value, the more the ball will flare.
Track Flare
The phenomenon seen as a result of axis migration. The result is several oil lines on the surface of the ball which
marks the point of contact of the ball in the oil. Track flare allows more fresh surface of the cover to touch the lane,
thus allowing more friction to be generated between the ball and lane as it travels toward the pins.
VAL
See Vertical Axis Line
Vertical Axis Line
A line perpendicular to the Grip Midline that passes through the PAP.

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