What is Tire Uniformity?

• Actually “Non-Uniformity”
• A quantitative measure of variation within a tire.
• Usual variations are in forces and runouts.

Axis System

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 Variations

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

Force Variations

The ASTEC®Tire UniformityMachine 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

• 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.

Harmonic Analysis

• 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.

Harmonic Analysis

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

Harmonic Computation

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

Note: Generally, the 1stand 2ndharmonics of force variation influence the ride quality of a vehicle the most.

Lateral Force Variation

Once the tire is inflated, loaded and rotating, the lateral force variation becomes periodic.

Lateral Shift

• The average lateral force in one direction.
• Typically, a positive lateral shift in the clockwise direction (1stDIR), becomes a negative lateral shift in the counterclockwise direction (2ndDIR).

Conicity

• Is a lateral force defined as lateral shift clockwise plus lateral shift counterclockwise divided by 2. Conicity = [(LScw+ LSccw) / 2].
• Tire rolls like a cone.
• Used as an indicator of steering pull.
• Primary cause is off-center belt.
• Force is exerted in the same direction, clockwise or counterclockwise.

Geometry Variations

• Free Radius
• Loaded Radius
• Radial Runout
• Lateral Runout
• Bulge/Depression

Free Radius

Radius between the tire center and outside diameter when unloaded.

Loaded Radius

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.

Radial Runout

• Variation in the free radius
• 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.

Lateral Runout

Variation in sidewall geometry while inflated, loaded and rotating.

Bulge/Depression

• Sidewall geometry variation running in radial direction.
• Depressions are often caused by an overlap of the body ply splice.
• Bulges can be caused by open splices20

Imbalance Cause and Effect

All tires and wheels have some measurable imbalance.

Imbalanced tires and wheels cause the following ride disturbances:

• Vertical forces
• Fore-aft forces
• Steering moments
• Camber moments

Types of Imbalance

• Static Measurement
• Dynamic Measurement

Principles of Imbalance Forces and 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 repeatably measure F.

Measurement System

• Angular Velocity (?) is controlled by the drive system and measured by the instrumentation.
• Two measured forcesare combined andtranslated into theplane of the TWAto determine (F).

Types of Imbalance

Static Imbalance

• Illustrated as a concentrated mass at the TWA centerline.
• External forces are:
  • Top and bottom reaction forces, which are measured.
  • Centrifugal force which must be determined.
• Static Imbalance Simulation
• This imbalance may cause a bouncing sensation.

Couple Imbalance

• Two mass concentrations,equal and opposite in direction, and opposite sides of TWA centerline.
• Couple Imbalance Simulation
• This imbalance may cause steering wheel vibration sometimes referred to as ‘Nibble’.

Dynamic Imbalance

Combination of staticand couple.

Imbalance Correction

Units of Measure for Correction

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 thecorrection radius
• The ‘Static Scientific Units’are very commonly used for wheel specs.
• Example: 8 oz·instatic 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 thecorrection radius in thecorrection 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.

Knock-on Weights

Adhesive & Knock-on Weights