Tire testing and finite element modeling. (Cover Story).

The exciting stories about tires are the innovations - all the wonderful developments in chemistry, materials and design. What isn't told is the story about the support that is given by testing to bring about acceptance of the changes.

The term tire testing conjures different images for different people. For some, it means that their work is delayed as they wait for their project to get to the top of the priority list so that it can be tested. Others may think of the drudgery of collecting all the data needed so their new design can get into production. Suppliers hope that their new products will get incorporated into test tires to document improvements. Managers are worded about the time it takes and the budget dollars that have to be spent on testing. Whatever the feelings are toward testing, it is recognized that testing is a vital part of the design, development and production of tires.

Testing started as simple experiments such as the rolling resistance testing of the first pneumatic tire designs. The next move was to running vehicles with tires on them to determine basic characteristics. Further testing was on controlled routes or special test tracks that would allow for higher speed testing under controlled conditions. Technology in testing brought about special machines that can quickly and accurately characterize certain aspects of a tire's performance. Then came modeling to better understand why a tire performed in a certain manner.

Original developments

The fast recorded inventor of the pneumatic tire was Robert William Thompson. The patent specification, No. 10,990, is dated June 10, 1846 and has a description as follows: "The nature of my said invention consists in the application of elastic bearings round the tires of wheels of carriages, for the purpose of lessening the power required to draw the carriages, rendering their motion easier and diminishing the noise they make in motion (ref. 1)."

The re-invention of the pneumatic tire is attributed to the 10-year-old son of John Boyd Dunlop who pleaded with his father for some device to make his solid tired tricycle run more smoothly and faster. Again, as in the first invention, rolling resistance was a concern, and some test was used to demonstrate the efficacy of the invention. In the first invention, the force required to draw a carriage was measured. In the second, a rolling test was employed, first rolling the solid tire, then rolling the pneumatic tire and noting how far they rolled before they toppled over (refs. 1 and 2).

So, in the descriptions both of the invention of the tire and the re-invention of the tire, testing is the proof of the design. And so it continued for years as improvement after improvement to the fledgling product was made. At first a couple of simple tests were all that was needed to demonstrate the advantages of any new idea for improvement. This is so for all new products; when the product is not very sophisticated, small samples and simple tests are enough to demonstrate improvements. As a product matures in its life, the major gains have been made and so each new idea becomes an increment rather than a leap in the properties or features. This in turn means that more samples need to be tested to higher accuracy in order to be certain that the new idea is indeed an improvement. When there are uncontrolled variables in the testing, the required testing is compounded. This became clear as tire manufacturers needed fleets of vehicles to prove the durability and wear resistance of their new tires.

Vehicle fleet testing

Testing became a large business within the tire business. Test tracks were built and facilities constructed to handle all the testing work. Large fleets of vehicles were needed, as well as drivers, mechanics, tire changers, inspectors, track maintenance people and clerks to handle the data that were generated. Again many improvements were made to improve the efficiency of the testing departments, reduce the variability of the test results, control the test track surfaces and control any possible variation in the vehicles and driving. New methods of collecting data and measuring tires were developed. All of this was taking more and more manpower, time and, of course, money. Customers were pressed into service also; with the lure of tires for their fleets, taxi companies, hauling companies and shuttle services were convinced to use test tires and keep records of tire usage. These methods continue to be used to learn more about tires in actual use, and generate data for tire manufacturers, as well as fleet owners. To validate this data, most manufacturers survey the fleets and capture additional data, as well as subjective user opinion on tire performance. For the off-road and mining segment of tires, this is still the most cost effective means for obtaining data, as machines to test the operation of these size tires is extremely limited.

Machine testing

The use of roadwheels as testing machines was very cost efficient because a tire could run 24 hours a day, seven days a week without stopping for fuel or changing of drivers. In addition, many variables could be controlled in a test chamber that were impossible to control on the road, including the wearing of suspension and track surface changes, not to mention the fact that temperature and speed could be held constant (refs. 4-6).

In order to protect the public, the government mandated a number of tests that had to be performed on tires. The Code of Federal Regulations 49, paragraphs 109 and 119 lists a variety of tests that must be performed in order to assure that tires produced would perform safely on the highways. All of which meant that more tires needed to be tested in a variety of ways, from simple tests of strength and endurance to the Uniform Tire Quality Grade Labeling tests that need to be performed, and results molded on the sidewalls of tires to inform the public how a particular tire should perform in comparison to other tires in the market.

Again, to satisfy these additional requirements, more test wheels were needed and so the testing departments of the tire manufacturers responded by adding more test wheel positions, staff, buildings and, of course, budget requirements. These were easily justified by the requirements of the government so fires could continue to be produced and sold.

The very good fallout of this is that better data, that are more closel controlled, produced higher accuracy results. This helped in the battle of finding the ever smaller differences in the test tires produced. By now the tire is a reasonably mature product, and for noticeable improvements to be found, several increments of improvement must be added together or very accurate testing must be carried out.

The roadwheels are also being adapted for special testing such as accelerated indoor wear testing and special rolling resistance measurements (refs. 7 and 8). Other wheels were enclosed in special semi-anechoic chambers to allow for precise measurements of the noise generated by the tire and tread pattern. For a list of test procedures, see table 1.

Modeling efforts

For years, many people worked on trying to analyze this complex composite called a tire. A number of mathematical models were proposed to help determine how different parts of the tire worked and what was needed in the design. These relations still exist as guides for the design of several parts of the tire, notably bead sizes and belt strengths and angles. The problem with these methods was that they were not able to handle the complete tire as a system. Each part of the tire interacts with all the other parts to make a balanced system to handle the forces and displacements that are demanded by the conditions within which the tire must operate.

As the power of the computer was developing, tools such as finite element analysis were becoming feasible to use in the analysis of a tire. As these math models of tires were developed, of course the engineers turned to the testing department to help prove their models. So hand in hand the engineers in the research and development departments worked with the test engineers to develop tests that would give the necessary material parameters and test results to confirm the finite element models.

The tire models were now sophisticated enough to be able to predict tire performance characteristics such as footprint pressure, footprint shape, rolling resistance losses, temperature and deformations (ref. 9) (see figures 1-4). In fact, some tire companies have developed their tire models to the level that they can predict performance characteristics of a tire without building it (ref. 10). These models are then coupled with vehicle handling computer models to predict how a vehicle that has not been built will perform with a tire that has not been built (figure 5).

[FIGURES 1-5 OMITTED]

Extensions of these efforts have led to systems that are able to predict how a tire will wear on a particular test course on a vehicle that has only been built in the computer.

Future

With the continued rapid development of computer power and speed, it is easy to predict that tire modeling will become more sophisticated, accurate, user friendly and widespread in its use to make continued incremental improvements in the performance of passenger and truck tires. With some further research in material models and finite element models, it can be seen that the use of this modeling can be extended to giant tires and aircraft rims. The improvements in machine testing such as the wear test machines, high speed force and moment machines, and specific test machines that can simulate acceleration and cornering, the ability of a tire manufacturer to understand and design a special tire to a particular environment is almost here.

The coupling of computer control and data acquisition to special test machines will soon give information about things like rolling resistance over speed ranges (ref. 8). With the advances and developments of materials used in tires, a tire that will last the life of the automobile is almost here. The real future of the pneumatic tire really depends on the customer. Technology is either here or on the doorstep that will provide nearly everything that the customer desires. Testing will always be here ready to help in the development or prove the developments.

Conclusions

The pneumatic tire was initiated from a customer request. Continued development of tires is a result of more customer requests. Technology advances in materials, computers, modeling and testing have provided the means for continued advances in the performance properties of tires. These include specialized test track surfaces and test methods, special data acquisition methods, new developments in specialized equipment, and advanced analysis and modeling software. In the future, it appears that these advances will continue, and will continue to provide tires with enhanced safety, performance and life.

(This article is based on a paper presented at the May 2000 Akron Rubber Group meeting.)

Table 1 - listing of test procedures

High speed               Endurance              Noise ride handling

SAE high speed           Indoor pulley wheel    Noise subjective
ECE speed rating           step load            Noise road drum
Indoor pulley            Indoor wheel low PSI   Ride
  wheel speed            Bead chafe             Handling
                         Indoor wheel           J turn
                           durability           Wet handling
                         Indoor wheel           Dry handling
Measurements               ozone/cold box       Groove wander
                         Accelerated tire       Force and moment
Plunger energy           Endurance/gravel       Impact cleat test
Burst                    Tire endurance
Bead seat pressure       Field service          Sidewall
Bead fit                 Oscillating slip
Static dimensions          durability           Curb scuff
Footprint pressure                              Curb impact
  distribution/motion    Traction
Thermography                                    Treadwear
Leak down                Wet traction
Immersion tank test      Dry traction           Indoor wheel irregular
Holography               UTQG traction          Consumer treadwear
Wear                     Hydroplane             UTQG treadwear
                         Snow traction          Diagonal fleet
Belt                     Snow handling          Cut and chip
                         Ice traction
Bead roll-off            Ice stopping
Figure 8 belt breakage   Ice cornering

Rolling resistance

67" wheel
Twin roll
48" wheel

References

(1.) The History of the Pneumatic Tire, Eric Tompkins, Eastland Press, 1981.

(2.) Woehrle, William, "A history of the passenger car tire," Automotive Engineering, Sept. 1995, pp. 71-75.

(3.) Buchholz, Kami, "Tier 1 suppliers speak on testing and validation," Automotive Engineering, vol. 105, July 1997, pp. 31-34.

(4.) Ortoli, David, "How to automate tire testing laboratories," Elastomerics, Feb. 1988, pp. 14-18.

(5.) Yost, Kevin, "Tire design and development," Automotive Engineering, June 1992, pp. 21-25.

(6.) Kline, S.M., "Model reference adaptave control of a tire test machine," Tire Science and Technology, vol. 20, no. 3, 1992, pp. 181-194.

(7.) Stalnaker, D., et al, "Indoor simulation of tire wear: Some case studies," Tire Science and Technology, vol. 24, 1996, pp. 94-118.

(8.) Draft of SAE J2452 Stepwise Coast Down Methodology for Measuring Rolling Resistance.

(9.) Fujikawa, T., Uchiyama, Y. and Yamaguchi, I., "Tire model to predict treadwear," Tire Science and Technology, vol. 27, no. 2, 1999, pp 106-125.

(10.) Williams, A. Roger, Tire technology: Future developments," Materials World, vol. 3, no. 12, Dec. 1995, pp. 588-591.

Jon Gerhardt, University of Akron