Tire Uniformity Tester for Automotive Service Industry

Dan Parker

Hunter Engineering Co.

Dave Scribner

Hunter Engineering Co.

Copyright 2000 Hunter Engineering Company

ABSTRACT

An automotive wheel balancer that has the added capability of detecting and correcting vibration problems due to tire/wheel assembly non-uniformity has been developed. In addition to measuring the tire/wheel assembly imbalance this machine measures the rim radial and lateral runout and has a roller that contacts the tire with up to 700 pounds (318 kg) load to measure the radial force variation. With this information, the high spot of the tire is mounted at the low spot of the rim to minimize tire/wheel assembly vibrations. This vibration control system (GSP9700 by Hunter Engineering) is designed to be produced at a cost affordable to the automotive service industry. Measurements on this machine are compared to measurements made using a highly accurate industrial tire uniformity machine from Akron Standard.

INTRODUCTION

The automotive service industry now has a new tool that is being used to solve tire/wheel assembly related vibration problems. Although a tire/wheel assembly is perfectly balanced and aligned, an assembly can still cause vibration problems if it is not exactly round or if the tire's stiffness is not uniform around the circumference. In the past, if a tire/wheel assembly related vibration problem was not solved by balancing and aligning, the technicians would be forced to resort to costly and time consuming procedures involving road tests as well as tire and wheel replacements. The Hunter GSP9700 Vibration Control Center allows the tire technician to make a "Road Force MeasurementTM" which can quickly identify tire and wheel problems, and can direct the technician to match the high spot of the tire to the low spot of the rim to minimize tire/wheel assembly vibrations.

Customer complaints about ride quality have been increasing due to several factors:

A. Lighter weight suspension components don't absorb road force vibrations easily.

B. Strut suspensions and rack and pinion steering systems transfer road force vibrations more directly to the passenger compartments.

C. Tires are trending toward lower aspect ratios. Wheels are larger in diameter and are more likely to be damaged by pot holes, road debris, curbs and parking lot barriers.

D. Lower aspect tire ratios are more sensitive to road forces and dynamic imbalance.

E. Passenger cars, trucks and SUVs are very expensive investments. Customer expectations about ride quality are changing and getting higher.

F. Many customers buy a set of tires expecting them to restore their new car ride and handling.

G. Customers are holding on to their vehicles longer than ever. The average age of a vehicle on the road today is 8.5 years and the average vehicle has over 100,000 (160,935 km) miles on it.

A vibration in a tire and wheel assembly can be caused by:

· Imbalance in the Assembly

· Change in Sidewall Stiffness (tire uniformity)

· Rim Bent/Out-of-Round

· Tire Out-of-Round

· Wheel to Axle Mounting Error*

· Runout or Imbalance in Vehicle Hub*

· Brake Component Wear or Failure*

· Drive Train or Engine Component Wear or Failure*

· Vehicle Component Characteristics*

· Combination of Some or All Factors

*Factors not detected by the GSP9700 Vibration Control System.

ROAD FORCE MEASUREMENT™

Road Force Measurement™ evaluates the tire/wheel assembly by simulating an actual road test. The GSP9700 Vibration Control System is equipped with a load roller to take the Road Force Measurement. The load roller places up to 700 pounds (318 kg) of force on the rotating tire, then automatically withdraws from the tire/wheel assembly. The GSP9700 Vibration Control System reduces diagnostic time by identifying vibrations that would not be detected by the normal balancing procedure such as runout, non-uniformity of the tire, and balancer mounting error. It verifies if the assembly is round when rolling automatically with no additional procedural steps, while performing a high precision balance measurement..

Various manufactures have published limits on tire/wheel assembly free (unloaded) radial runout measured at the center of the tire tread. While this is a measurement that a technician can make with a relatively inexpensive gage, it is not a measurement that has close correlation to vehicle vibration problems. SAE recommended practice J332, which is widely used in the tire industry, describes tire testing equipment to measure tire uniformity. This practice stresses the importance of measuring force variation while the tire is under load and does not acknowledge unloaded free runout measurement. The uniformity of most tires manufactured today is measured with a machine, which conforms, to SAE J332. Tires which do not meet uniformity specifications may be brought into specification by grinding small patches of rubber off the sides and footprint of the tread. This grinding procedure is done to improve the radial force variation - but may not make any improvement to the free runout measurement. A tire with large free radial runout may roll vibration free under load, and a tire with low free radial runout may vibrate when rolling under load.

To understand the effects of radial force variation, we use a model of the tire as a collection of springs between the rim and the tire tread. If the "springs" are not of uniform stiffness, a varied force is exerted on the axle as the tire rotates and flexes. This creates a vibration in the vehicle. Refer to Figure 1 on page 7.

The GSP9700 Vibration Control System takes a measurement of the tire/wheel assembly's radial force variation and displays the results. If the GSP9700 perceives a problem with the force variation, it will prompt you to measure rim runout. When the rim runout data is entered, the GSP9700 extracts and displays the first through fourth harmonic force variation contributions of the tire and of the wheel.

The GSP9700 allows the first harmonic of the tire to be matched opposite of the first harmonic of the wheel to decrease the first harmonic vibration. The tire and/or wheel may need to be replaced or OE-Matched prior to balancing.

HARMONIC VIBRATIONS

A vibration that occurs one time for every revolution is defined as a first harmonic vibration: Refer to Figure 2 on page 7.

During OE-Matching the first harmonic vibration of the tire is aligned opposite of the first harmonic vibration of the wheel to decrease the first harmonic vibration of the assembly. This reduces the vibration felt inside of the vehicle.

A vibration that occurs twice for every revolution is defined as a second harmonic vibration. Because there are two vibrations per revolution the second harmonic vibration occurs at twice the frequency of a first harmonic vibration: Refer to Figure 3 on page 7.

Multiple harmonic vibrations may also occur. For instance, the third order vibration causes a three times per revolution vibration: Refer to Figure 4 on page 8. Multiple harmonics vibrations per revolution are not able to be cancelled by OE-Matching.

Multiple harmonic vibrations may occur at any harmonic level, dependant upon the number of vibration components in each revolution. Refer to Figure 5 and 6 on page 8and 9.

THE GSP9700 FORCE VARIATION MEASUREMENT:

1. The tire and wheel assembly is accurately mounted to the vibration control system's spindle. A variety of mounting cones and adapters are available for the GSP9700.

2. A 6.00 inch (152.4 mm) diameter roller pushes against the tire using a double convolute bellows air spring. An air pressure sensor is used to determine the load on the roller and an angle sensor measures the position of the roller.

3. The tire stiffness is determined by forcing the roller against the tire at two different loads while measuring the change in roller position.
(Tire Stiffness) = (Change in Load) / (Change in Roller Position)

4. A constant load of up to 700 pounds (318 kg) is applied to the tire while the sensor measures the roller's movement in the radial direction as the tire/wheel assembly is rotated at 90 RPM by a DC motor. An optical encoder with 512 resolution positions per revolution measures the tire/wheel assembly angular location. 128 roller position and tire/wheel assembly angle measurements are taken per revolution, for four revolutions.

5. The tire/wheel assembly is removed from the vibration control system , the tire is removed from the wheel, then the wheel is remounted on the vibration control system's spindle in the original angular position. A separate measuring arm equipped with two angle sensors measures the radial and lateral runout of the rim's bead seating surfaces as the DC motor turns the rim at 25 RPM for two revolutions.

6. The GSP9700 examines all the data points taken and calculates the first harmonic using a discrete Fourier Transform of the loaded runout of the tire/wheel assembly, and the first harmonic of wheel runout. The peak-to-peak value (Total Indicated Runout) and second, third and fourth harmonics of force variation are also calculated and displayed on other screens.

7. Tire radial runout is computed by subtracting the rim runout from the tire/rim assembly loaded runout.

8. The measurements of loaded radial runout are then converted to force variation using the equation:
(Loaded Radial Runout) X (Tire Stiffness) = Force Variation

EVALUATION OF FORCE VARIATION MEASUREMENTS

To evaluate the force variation measurements made by the GSP9700 a comparison is made to the force variations measurements made by the Akron Standard Model D-70. This is a highly precise machine designed for high production tire manufacturers.

SAE recommended practice J332 describes testing machines for measuring the uniformity of passenger car and light truck tires. Tire uniformity testing machines used by tire manufacturers and car manufacturers have followed this practice for over 20 years. The Akron Standard Model D-70 conforms to this practice.

The main features of the Akron tire testing machine are:

1. A spindle assembly that supports accurately machined rim halves. Conveyors and automation is provided to allow quick mounting inflation and de-mounting of tires.

2. A 33.625 inch (854 mm) diameter drum which can be loaded against the tire at a specified radial load up to 4000 pounds (1815 kg) and means for holding a fixed tire to drum center distance during measurements is provided.

3. The tire/wheel assembly is rotated against the drum at user specified speeds in both clockwise and counter clockwise directions.

4. Strain gage force sensors measure the forces between the tire and drum in the radial and lateral directions as the tire/wheel assembly is rotated. An optical encoder measures the angular location of the wheel.

5. Instrumentation for calculating and displaying peak-to-peak, time averages and the amplitude and phase of the first ten harmonic of components of force variation is included. Refer to Figure 7 on page 9.

TESTING PERFORMED

A GSP9700 was installed at the Akron Standard plant next to an Akron Standard D-70 machine, measurements were made using Akron Standard personnel.

A total of 23 tires were used for the testing: fifteen 14 inch diameter passenger car tires with approximately 1400 pound (635 kg) maximum load rating from four different manufacturers and eight 15 inch diameter sport utility vehicle tires with 2028 pound (920 kg) maximum load rating.

A total of 131 force variation measurements were made on the GSP9700. The first two 14 inch tires and the first two 15 inch tires were each measured 10 times without removing the wheels from the balancer. The first seven 14 inch tires were each measured 5 times and the last eight 14 inch tires were each measured 3 times removing the tire from the rim and remounting the tire at a different angular location on the rim after each measurement. The first four 15 inch tires were each measured 5 times and the last four 15 inch" tires were each measured 3 times removing the tire from the rim and remounting the tire at a different angular location on the rim after each measurement. 500 pound (227 kg) roller load was used for all 14 inch tire measurements, and 700 pound (318 kg) load was used for all 15 inch tire measurements.

A total of 380 force variation measurements were made on the Akron Standard D-70. All 23 tires were measured 10 times removing the tire from the rim and remounting the tire at a different angular location on the rim after each measurement with a 500 pound (227 kg) load on the 14 inch tires and a 700 pound (318 kg) load on the 15 inch tires. The 14 inch tires were then measured exactly as before except with a 1200 pound (544 kg) load. This was the load recommended by the SAE standard for these tires.

REPEATABLIITY

The standard deviation is a measure of repeatability. If a population of measurement has a `normal' distribution then 95.4% of the measurements will lie within +/- 2 standard deviations of the mean. Refer to Table 1 and 2 on page 10.

ACCURACY

The accuracy of a measuring device is often expressed by how well the device correlates to a know standard. In this case the averaged Akron Standard measurements are used as the standard to which the GSP9700 measurements are compared. Refer to Graph 1 through 8 starting on page 11. This chart shows the correlation of GSP9700 individual tire measurements to the averaged Akron Standard measurement.

The R² statistic is often used in the tire industry to describe correlation.

Perfect correlation would have a R² value of 1.00 and all points would lie on a 45 degree line.

EFFECT OF ROLLER LOAD

The GSP9700 has a roller load limit of 700 pound (318 kg) To investigate how serious an effect this has on the measurements of passenger car tires the 14 inch tires were tested at the SAE practice recommended load of between 1200 and 1280 pound (544 and 581 kg) for each of these tires. Graph 8 on page 19 shows the effect of this increased load.

TIRE/RIM MATCHING EFFECTIVENESS

The GSP9700 is useful to the tire technician because it enables the high spot of radial force variation to be matched to the low spot of rim runout. This results in a tire/wheel assembly with low radial force variation. The amount of improvement by matching which can be achieved depends on the amount of tire force variation and rim runout which is present. Data has been collected on 6771 wheels that have been measured by 26 different GSP9700 Vibration Control Centers in retail tire shops. Refer to Graph 9 on page 20.

The median value of rim runout is in the 0.006 to 0.010 inch (0.15 to 0.25 mm) bracket while the median value for tire force variation is in the 16-20 pound (7.26-9.07 kg) bracket. On typical passenger car and light truck tire/wheel assemblies, 0.001 inch (0.02 mm) rim runout will result in 1 pound (0.45 kg) assembly force variation.

Based on the data shown above, correctly OE-Match mounting wheel and tires will reduce the assembly force variation by nearly 50%.

Graph 10 on page 21 shows what the average force variation for the 23 test tires would be when mounted on a rim with .007 inch (0.18 mm) Total Indicated Runout (T.I.R.). The Akron Standard force variation amount and angle are assumed the true values. Any angle measurement difference between the Akron and the GSP9700 will result in a higher value for the GSP9700 average force variation.

ISSUES CONCERNING USE OF GSP9700

Below are some reasons why the GSP9700 may not OE-Match or quantify exactly the value of the tire or the assembly.

· INCORRECT MECHANICAL WHEEL MOUNTING ON THE SPINDLE

· EXTERNAL RIM MEASUREMENT VS. ACTUAL BEAD SEAT MEASUREMENT

· TIRE AIR PRESSURE READINGS BEFORE AND AFTER MATCHING ARE DIFFERENT

· INCORRECT TIRE BEAD SEATING PROCEDURES

· INSUFFICIENT USE OF TIRE MOUNTING LUBE DURING MOUNTING

· RIM SAFETY HUMP DESIGN WHEELS INHIBIT TIRE BEAD DURING BEAD SEATING

· TEMPORARY TIRE FLAT SPOTS FROM PARKING THE VEHICLE

· EXCESSIVE LATERAL RUN-OUT OF TIRE AND/OR RIM

CONCLUSION

The GSP9700 Vibration Control Center is a tool that is priced for the auto service industry and it is capable of solving tire/wheel assembly vibration problems, which cannot be solved by a standard wheel balancer.

The comparison testing of 15 passenger car tires and 8 S.U.V. tires showed strong correlation with the industrial tire uniformity machine from Akron Standard in measurements of radial force variations.

Matching the high point of loaded tire force to the low point of rim radial runout can significantly reduce tire/wheel assembly radial force variation only if the rim has some radial runout in it. The measurements taken on 6771 wheels from 26 different GSP9700 Vibration Control systems located in retail tire shops (fig. 17) indicated that on average matching will result in a radial force variation reduction of nearly 50%.

ACKNOWLEDGMENTS

We wish to thank Jim Chase and others at ITW Akron Standard for their technical advice, for providing access to a D-70 machine for comparison testing and for their assistance in conducting these tests.

CONTACT

Information on the Hunter GSP9700 may be obtained from the Hunter web site at www.GSP9700.com or by calling 1-800-448-6848 or 314-731-3020.

DEFINITIONS, ACRONYMS, ABBREVIATIONS

FORCE VARIATION

A change in the force between a tire/wheel assembly and a roller while rotating under a load. Force variation can cause a vibration although the tire and rim may be perfectly round and the tire is balanced.

HARMONIC

A vibration which is identified by its' number of occurrences per revolution. Measured by analyzing data points and calculated using a discrete Fourier Transform. For example, a 1st Harmonic Vibration has a once-per-rev vibration component.

OE-MATCHING / FORCE MATCHING

Original Equipment-Matching is a method of aligning the highest/stiffest spot of radial tire uniformity with the lowest average point (harmonic) of rim runout to decrease rolling vibration in the tire/wheel assembly.

MATCH MOUNTING

See "OE-MATCHING / FORCE MATCHING." May be a term that is used incorrectly by some wheel balancer manufacturers to describe their OE-Matching procedure that is actually an Optimizing procedure.

OPTIMIZING

Optimizing positions the rim with respect to the tire, minimizing the amount of correction weight required.

P, P/SUV, LT

"P Tires" refers to passenger tires, "LT Tires" refers to light truck tires and "P/SUV Tires" refers to P-Rated sport utility vehicle tires

TOTAL INDICATED READING (T.I.R.)

Data measurements taken by the load roller (measured in pounds or kilograms) or Dataset Arms (measured in inches or millimeters) represent the actual runout measured. The T.I.R. data represents the difference in value between the highest and lowest value measured.

APPENDIX

Figure 1. Force variation creates vibration

Figure 2. Types of First Harmonic Vibrations

Figure 3. Types of Second Harmonic Vibrations

Figure 4. Types of Third Harmonic Vibrations

Figure 5. Rim Measurement

Figure 6. Load Roller Measurement

Figure 7. Applied Force

STANDARD DEVIATION OF GSP9700 TIRE RADIAL FORCE VARIATION MEASUREMENTS
NOT TAKEN OFF BALANCER BETWEEN MEASUREMENTS

TIRE

NUMBER OF MEASUREMENTS

PEAK-TO PEAK (lbs.)

1ST HARMONIC (lbs.)

2nd HARMONIC (lbs.)

3rd HARMONIC (lbs.)

14" 1

10

1.0

0.6

0.2

0.2

2

10

1.2

0.8

0.5

0.4

15" 1

10

0.9

0.2

0.5

0.3

2

10

0.4

0.3

0.1

0.4

Avg.

 

.88

.48

.33

.33

Table 1. Repeatability Table

STANDARD DEVIATION OF GSP9700 TIRE RADIAL FORCE VARIATION MEASUREMENTS
TIRE REMOUNTED AT NEW ANGLE BETWEEN EACH MEASUREMENT

TIRE

NUMBER OF MEASUREMENTS

PEAK-TO PEAK (lbs.)

1ST HARMONIC (lbs.)

2nd HARMONIC (lbs.)

3rd HARMONIC (lbs.)

14" 1

5

1.3

1.4

0.5

0.4

2

5

1.1

1.6

0.6

0.4

3

5

1.5

1.4

0.7

0.4

4

5

1.4

1.6

0.6

0.7

5

5

1.1

1.6

0.7

0.4

6

5

1.5

1.5

0.4

0.9

7

5

2.2

1.9

0.4

0.5

8

3

1.2

1.4

1.8

1.0

9

3

0.9

1.8

0.4

0.3

10

3

0.9

1.8

0.4

0.3

11

3

2.3

2.0

0.4

0.4

12

3

1.5

1.5

0.9

0.4

13

3

0.5

0.5

0.6

0.5

14

3

0.8

1.2

0.8

0.8

15

3

0.5

1.1

0.5

0.4

15" 1

5

1.8

0.5

1.3

0.2

2

5

1.2

1.1

0.3

0.4

3

5

1.4

0.8

0.5

0.6

4

5

0.8

1.2

0.4

0.6

5

3

1.4

1.0

0.5

0.3

6

3

2.1

1.0

0.2

0.4

7

3

1.4

1.6

0.3

0.7

8

3

0.8

1.8

0.9

0.4

avg.

 

1.3

1.4

0.6

0.5

Table 2. Repeatability Table

Graph 1. GSP9700 vs Akron Standard Avg. 1st Harmonic Radial Force Variation For Passenger Car Tires

Graph 2. GSP9700 vs Akron Standard Avg. 1st Harmonic Radial Force Variation For SUV Tires

Graph 3. GSP9700 vs Akron Standard Avg. Peak-To-Peak Radial Force Variation For Passenger Car Tires

Graph 4. GSP9700 vs Akron Standard Avg. Peak-To-Peak Radial Force Variation For SUV Tires

Graph 5. GSP9700 vs Akron Standard Avg. 2nd Harmonic Radial Force Variation For Passenger Car Tires

Graph 5. GSP9700 vs Akron Standard Avg. 2nd Harmonic Radial Force Variation For SUV Tires

Graph 6. GSP9700 vs Akron Standard Avg. 3rd Harmonic Radial Force Variation For Passenger Car Tires

Graph 7. GSP9700 vs Akron Standard Avg. 3rd Harmonic Radial Force Variation For SUV Tires

Graph 8. GSP9700 vs Akron Standard Avg. 1st Harmonic Radial Force Variation

Graph 9. Radial Force Variation / Rim Runout of 6771 Wheels.

Graph 10. Projected Results from Matching

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