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Table of Contents
Tire Uniformity
What is Tire Uniformity?
Tire Uniformity is:
D
Actually “Non-Uniformity”
D
A quantitative measure of variation within a tire
D
Usual variations are in forces and runouts
Axis System
To measure tires, the tire industry uses an axis system which bisects the tire center.
Forces are measured along these axes:
D
Fz = Radial Force
D
Fy = Lateral Force
D
Fx = Tangential Force
Z
Center of
Tire
X
Y
X
Y
Z
Figure 3-1 Axis Measurement System
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Introduction
January 2001
Model B, Rev 1
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Tire Uniformity
Radial Force (Fz)
D
Z
The vertical force between the tire and
the road (or loadwheel on the
ASTEC Machine).
The Radial Axis is perpendicular to the road. This
is the axis where radial force (Fz) is applied to
the tire.
Z
Fz
(Load)
Figure 3-2 Radial Axis
Lateral Force (Fy)
D
The side-to-side force along the rotation
axis between the tire and the road (or
loadwheel of the ASTEC Machine).
Y
The Lateral Axis is where side-to-side forces (Fy)
are applied to steer the vehicle.
Y
Fy
(Steering Forces)
Figure 3-3 Lateral Axis
Introduction
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Tire Uniformity
Tangential Force (Fx)
D
The driving force between the tire
and the road (or loadwheel on the
ASTEC Machine.)
X
The Tangential Axis is parallel to the road, in the
direction of travel. This is the axis where the
driving force (Fx) is applied to the tire.
X
Fx (Driving Force)
Figure 3-4 Tangential Axis
Moments
Steering Moment (Mz)
Z
D
The moment that induces steering pull
D
Also called aligning torque
Motion of Tire
About the Z Axis
This is the torque that aligns the steering wheel
back to the center after a turn.
Z
Figure 3-5 Steering Moment (Mz)
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Tire Uniformity
Rolling Resistance Moment (My)
D
The moment that resists tire rotation
Y
Y
Motion of Tire
About the Y Axis
Figure 3-6 Rolling Resistance
Moment (My)
Overturning Moment (Mx)
D
The moment that induces camber
This moment would tend to make a tire tip over on
its side.
X
X
Motion of Tire
About the X Axis
Figure 3-7 Overturning Moment (Mx)
Introduction
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Tire Uniformity
Force Variations
Force Variation is the change in the forces as the tire rotates. The change in force is due to
inconsistencies in tire manufacturing.
Radial
Force
The ASTEC Machine measures two types of
force variation:
D
Radial Force Variation
D
Lateral Force Variation
Note: There must be a load on the tire
to generate any force variation. Radial
and lateral force variations are
measured in both directions (clockwise
and counterclockwise).
Lateral
Force
Figure 3-8 Force Parameters
Measured
Radial Force Variation
D
Once the tire is inflated, loaded and
rotating, the radial force becomes periodic.
See Figure 3-9.
D
There is only a slight difference in radial
force variation when the tire is rotated in
either direction.
One Rotation
+10 Volts
(+2500 lbs.)
0 Volts
(0 lbs.)
1st Dir.
2nd Dir.
Spindle Direction Change
Figure 3-9 Radial Force Variation
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Tire Uniformity
Lateral Force Variation
D
Once the tire is inflated, loaded and
rotating, the lateral force variation
becomes periodic. See Figure 3-10.
One Rotation
+10 Volts
(+1250 lbs.)
1st Dir.
0
2nd Dir.
-10 Volts
(-1250 lbs.)
Spindle Direction Change
Figure 3-10 Lateral Force Variation
Lateral Shift Variation
D
Typically, a positive lateral shift in the
clockwise direction (1ST DIR),
becomes a negative lateral shift in the
counterclockwise direction (2ND DIR).
This statement is true if the absolute value
of plysteer is more than the absolute value
of conicity.
Introduction
+10 Volts
(+1250 lbs.)
1st DIR
LSCCW
The average lateral force in one direction.
(See above paragraphs.)
LSCW
D
0
2nd DIR
-10 Volts
(-1250 lbs.)
Figure 3-11 Lateral Shift Variation
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Tire Uniformity
Conicity
D
Is a lateral force defined as lateral shift
clockwise plus lateral shift
counterclockwise divided by 2.
D
Tire rolls like a cone.
D
Used as an indicator of steering pull.
D
Caused by off-center belt (about 30 N/mm
of offset).
D
Force is exerted in the same direction,
clockwise or counterclockwise.
Counterclockwise
Rotation
Lateral Force
Clockwise
Rotation
Figure 3-12 Conicity
Plysteer
D
3-8
Belt Ply
Cords
Is a lateral force defined as lateral shift
clockwise minus lateral shift
counterclockwise divided by 2.
D
Plysteer force depends on direction
of rotation.
D
“Crab Walk” effect.
D
Does not affect ride performance.
D
Result of belt ply angles.
D
Outer-most ply has the dominant effect.
Counterclockwise
Rotation
Plysteer
Force
Introduction
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Clockwise
Rotation
Plysteer
Force
Figure 3-13 Plysteer
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Tire Uniformity
Harmonic Analysis
Force variation is a complex waveform. See
example in Figure 3-14.
Fourier analysis expresses the complex
waveform as the sum of multiple sine waves,
or harmonics. Each harmonic is defined by an
amplitude and phase angle.
Figure 3-15 is a Fourier analysis of the
complex waveform in Figure 3-14, broken
into four harmonics. The sum of these
four harmonics approximate the
original waveform.
Force Variation
Waveform
Tire Rotation
0_
Figure 3-14 Force Variation Waveform
First
Harmonic
A force variation waveform can be broken
down into an infinite number of harmonics.
Second
Harmonic
Waveform
Fourth
Harmonic
Harmonic Computation
360_
Third
Harmonic
Figure 3-15 Harmonic Analysis
The ASTEC Computer can compute the first
through the tenth harmonic. Both the
harmonic amplitudes and angles are available
in the TIGRE Program (see Chapter 9).
Note: Generally, the first and second
harmonics of force variation influence
the ride quality of a vehicle the most.
Introduction
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Tire Uniformity
Geometry Variations
Free Radius
D
Radius between the tire center and outside
diameter when unloaded.
Free Radius
Figure 3-16 Free Radius
Loaded Radius
D
Radius between tire center and road (or
Loadwheel) when loaded.
Note: A typical loaded radius is an
inch and a quarter less than an
unloaded radius.
Loaded
Radius
Figure 3-17 Loaded Radius
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Tire Uniformity
Runout Probe
Radial Runout
D
Variation in the free radius
Runout refers to the variation in
roundness, or change in distance from
the center of the tire outward to the
tread, as the tire is rotated.
Runout High
(Point A)
Runout Low
(Point B)
Out Of Round Tire
Runout Graph
Distance
A
+20
-20
B
0
Degrees
360
Figure 3-18 Radial Runout
Lateral Runout
D
Variation in sidewall geometry while
inflated, loaded and rotating.
Figure 3-19 Lateral Runout
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Tire Uniformity
Bulge/Depression
Measurement
D
Sidewall geometry variation running in
radial direction.
D
Weak spots in the tire due to a lack of
cord material.
D
Result of the sidewall splice overlap, gap
overlap or misalignment.
D
Also caused by defects in calendared fabric
in sidewall ply.
Figure 3-20 Bulge
Quality Control
D
Goal of every tire manufacturer is to make
as many tires as possible with acceptable
force variations.
D
Process variations are so large that most
tire makers verify force variation by
100% inspection.
Bivariate Normal Distribution Curve
Force variation exhibits a bivariate normal
distribution curve, bounded by zero.
See Figure 3-21.
D
Much longer and higher tail.
D
Significant number of tires do not meet
specifications because of force variations
or other uniformity problems.
100
Percent of tires with
specified force variation
D
Good Tires
Cutoff Point for
Acceptable Tires
Bad Tires
0
0
4
9
13
daN of force variation
18
Figure 3-21 Bivariate Normal
Distribution Curve
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Tire Uniformity
Grinding
Rubber Removed to
Reduce Footprint
Why grind?
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Grinding increases production yield.
D
Grinding reduces scrap.
D
A 5% gross profit means you must sell 20
good tires to pay for one scrapped tire.
D
Force variation may be corrected by
changing the footprint.
Force Correction using
Shoulder Grinding
D
Reduces the footprint area by reducing
tread width, at the point of high force
variation. See Figure 3-22.
Figure 3-22 Shoulder Grinding
Force Correction using
Center Grinding
D
Reduces the footprint area by reducing
tread length, at the point of high
force variation. See Figure 3-23.
No Rubber
Removed
Rubber Removed to
Reduce Footprint
ÁÁÁÁÁ
ÁÁÁÁÁ
Figure 3-23 Center Grinding
Introduction
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