Industry Terms

It is a quantitative measure of force and runout variation within a tire.

The primary parameter measured is Radial Force Variation.  Radial Force Variation is a property of a tire that characterizes the dynamic behavior of forces generated (such as steering, traction, braking, and load support) between a vehicle and the road surface.  As the tire rotates, the spring elements of the tire make contact with the road surface and are compressed.  As each individual spring element rotates out of the contact area it recovers to its original length.  Variations in the effective stiffness of each of these spring elements result in radial force variation.  The change in effective stiffness and therefore forces generated is due to variation in the thickness of the tire and variation in the elastomeric properties of the tire.

Under the same conditions, if the same load is applied at a constant radius to a rotating tire, it will generate a lateral force and a Lateral Force Variation.  As the tire turns, it undergoes repeated deformation and recovery as it enters and exits the contact area.  If the lateral force is measured between the tire and the road surface the lateral force will vary as the tire turns.  The average value of this generated lateral force is called Lateral Shift.  Lateral Force Variation is the small amount of variation in lateral force around the lateral shift.  The change in these forces is due to inconsistencies in the tire tread and sidewall area.

Force variation refers to the change in the force as the tire rotates along the surface of the road, providing the center of the tire remains at a constant height above the surface of the road.  There must be a load on the tire to generate any force variation.

The Free Radius of a tire is defined as the average radius of the inflated tire from the rotational center of the tire to the tread surface.  Tire Runout variation is the variation measured around the free radius.

To measure tire uniformity, the tire industry uses an axis system which bisects the tire center.

Forces are measured along these axes:

• Fz = Radial Force
• Fy = Lateral Force
• Fx = Tangential Force4

Force Variation is the change in the forces as the tire rotates.  The change in force is due to inconsistencies in tire manufacturing.

The ASTEC® Tire Uniformity Machine measures two types of force variation:

• Radial Force Variation
• 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 of rotation (clockwise and counterclockwise).

Radial Force Variation is a property of a tire that characterizes the dynamic behavior of forces generated (such as steering, traction, braking, and load support) between a vehicle and the road surface.   As the tire rotates, the spring elements of the tire make contact with the road surface and are compressed.  As each individual spring element rotates out of the contact area it recovers to its original length.  Variations in the effective stiffness of each of these spring elements result in radial force variation.  The change in effective stiffness and therefore forces generated is due to variation in the thickness of the tire and variation in the elastomeric properties of the tire.

Once the tire is inflated, loaded, and rotating, the radial force becomes periodic.  There is only a slight difference in radial force variation when the tire is rotated in either direction.

The measured force variation waveform is called the Composite Waveform.

Fourier analysis expresses the original waveform as the sum of multiple sine waves, or harmonics.  Each harmonic is defined by an amplitude and phase angle.

Example: Fourier analysis of the composite waveform, broken into four harmonics.  The sum of these four harmonics approximate the original waveform.

A force variation waveform can be broken down into an infinite number of harmonics.  The ASTEC® Computer computes the 1st through the 10th harmonic.  Both the harmonic amplitudes and angles are available in the TIGRE™ program for display and recording.

Note: Generally, the 1st and 2nd harmonics of force variation influence the ride quality of a vehicle the most.

If a load is applied at a constant radius to a rotating tire, it will generate a lateral force and a Lateral Force Variation.  As the tire turns, it undergoes repeated deformation and recovery as it enters and exits the contact area.  If the lateral force is measured between the tire and the road surface the lateral force will vary as the tire turns.  The average value of this generated lateral force is called Lateral Shift.  Lateral Force Variation is the small amount of variation in lateral force around the lateral shift.  The change in these forces is due to inconsistencies in the tire tread and sidewall area.

A tire must be rotating to generate a lateral force.  As in the radial force diagram, once equilibrium is established for the inflated, loaded, and rotating tire, the lateral force variation becomes periodic.

The average value of this force is called Lateral Shift; the variation is called Lateral Force Variation.

Lateral Shift Variation will change significantly when the direction of rotation is changed.  A positive lateral shift in the clockwise direction, (1st DIR.), will become a negative lateral shift in the counterclockwise direction (2nd DIR.).

Today tires could wind up in service in either direction of rotation; therefore they must meet the specifications in both directions of travel.

Conicity is directly related to steering pull.  A tire that has high conicity will give a vehicle high steering pull, a strong pull in either direction.  The ASTEC® machine will measure the degree, or amount of steering pull.

The word conicity is derived from the word cone.  Conicity refers to when a tire physically behaves as if it were shaped like a cone.  If a tire has conicity, it would mean that a lateral force is generated in the same direction no matter which way it was rotated.

Conicity is defined as lateral shift clockwise plus lateral shift counterclockwise divided by 2.

Conicity = [(LScw+LSccw) / 2].

The Primary cause is an off-center belt.

The Free Radius of a tire is the average radius of the inflated tire from the center of the tire to the tread surface.

The Loaded Radius is the radius of the inflated tire under its rated load. A typical loaded radius is an inch and a quarter less than an unloaded radius.

Radius between tire center and road (or Loadwheel) when loaded.

• Radial 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.

Variation in the free radius

Variation in sidewall geometry while inflated, loaded and rotating.

During tire building, things can happen that affect the outside appearance of the finished sidewall of the tire.  During the tire manufacturing process an overlap, or gap, in the cord material will show up in the finished tire as Bulges or Depressions.

Bulges and depressions are appearance problems, which may become serious enough to prevent the tire from being sold.

All tires and wheels have some measurable imbalance, typically caused by inaccuracies in tire components, construction, and curing; and inaccuracies in wheel manufacturing. Imbalanced tires and wheels cause the following ride disturbances:

• vertical forces
• fore-aft forces
• steering moments
• camber moments

Centrifugal Force

• When a mass (m) is rotated around an axis at radius ( r) and angular velocity (w), it exerts a centrifugal force ( F) in the radial direction.
• Newton’s Law:
   
• Units are: Imbalance is (mr), with units of mass times distance
  In the case of a TWA, imbalance (mr) is unknown, so the centrifugal force (F) and angular velocity (w) must first be measured to determine (mr).
  The machine must accurately and repeatedly measure (F).

Measurement System

• Angular Velocity (?) is controlled by the drive system and measured by the instrumentation.
• Two measured forces are combined and translated into the plane of the TWA to determine (F).

Static Imbalance

This Figure illustrates the static imbalance concentrated mass at the TWA centerline. The centrifugal forces are shown as continuous waveforms, which will be analyzed to determine mr.

Static imbalance can be simulated as a weight that is split evenly onto the top and bottom planes.

As a vehicle travels at a high rate of speed, the passengers may experience a bouncing sensation which is typical of static imbalance.

Couple Imbalance

This illustration shows a TWA with two mass concentrations, equal and opposite in direction, and on opposite sides of the TWA centerline.
The centrifugal forces are shown as continuous waveforms.

Couple imbalance can be simulated as two equal weights placed on opposite planes in opposing directions.

As a vehicle travels at a high rate of speed, the passengers may experience a wobbling sensation which is typical of couple imbalance.

Dynamic Imbalance

Dynamic imbalance is the combination of static and couple imbalance. Dynamic imbalance is simulated using weights positioned in typical static and couple scenarios. As a vehicle travels at a high rate of speed, the passengers may experience a combined bouncing and wobbling sensation.

Vehicle manufacturers and tire retailers compensate for imbalance by adding weights to the tire and wheel assembly. A simplified example of an imbalance correction is illustrated. Weights are applied in matching locations on the top and bottom plane, thus correcting any imbalance and eliminating ride disturbance.

Static Imbalance (or Unbalance!) Units

Scientific Units:

• cgs: gram centimeter (g•cm)
• SAE, AIAG: kilogram millimeter (kg•mm)
• English: ounce inch (oz•in)

Auto Industry Units:

• Grams (or Ounces) at the correction radius

The ‘Static Scientific Units’ are very commonly used for wheel specs.

Example: 8 oz•in static imbalance, If r = 8 in The imbalance would be expressed as “1 oz imbalance” applied at 8 in, and divided in half on each side of wheel.

Couple Imbalance (or Unbalance!) Units

Scientific Units:

• cgs: gram centimeter²(g•cm ²)
• SAE, AIAG: kilogram millimeter²(kg•mm²)
• English: ounce inch²(oz•in²)

Auto Industry Units:

• Grams (or Ounces) at the correction radius in the correction plane37

Example: 56 oz•in² couple imbalance If r = 8 in and w = 7 in The imbalance would be expressed as “2 oz imbalance” applied at correction radius, in the correction plane, with one half the weight (1 oz) in each plane.

Per-Plane or Dynamic Imbalance Units

Scientific Units:

• cgs: gram centimeter (g•cm) in the correction plane
• SAE, AIAG: kilogram millimeter (kg•mm) in the correction plane
• English: ounce inch (oz•in) in the correction plane

Auto Industry Units:

• Grams (or Ounces) at the correction radius in the correction plane

The ‘Per-Plane Scientific Units’are very commonly used for tire specs.

Wheel with two standard knock-on-Weights

Flangeless Wheel with One Adhesive Weight and One Standard Knock-on Weight.