Testing Radial Medium Truck Tires
On Nov. 1, 2000, the U.S. Tire Recall Enhancement, Accountability, and Documentation (TREAD) Act was signed into law, calling on the secretary of transportation to conduct rulemaking to revise and update the tire standards published in Title 49 Code of Federal Regulations (CFR) Part 571. This update was to involve Part 571.119, “New pneumatic tires for vehicles other than passenger cars,” also known as Federal Motor Vehicle Safety Standard 119 (FMVSS 119). The focus of the FMVSS 119 update was directed at truck tires used on vehicles having a gross vehicle weight rating greater than 4,536 kg (10,000 lbs).
As a result, the National Highway Traffic Safety Administration of the Department of Transportation began its truck tire research in the spring of 2003 on the current FMVSS 119 test standards. Current levels of performance and a study of increased levels of test stringency were included to provide a basis for a notice for proposed rulemaking for the regulatory testing of truck tires.
In response, a truck/bus tire test development task group was formed in 2004 within Subcommittee F09. 30 on Laboratory (Non-Vehicular) Testing, part of Committee F09 on Tires, to identify, propose and develop new and appropriate endurance and high speed roadwheel tests for load range F and above radial medium truck, or RMT, tires. In addition to ASTM laboratory roadwheel testing standards, the results of this work would be provided to NHTSA to support their research on RMT tire testing.
The work of the F09.30 task group has been a collaborative effort by cross-industry stakeholders. The voluntary contributions of personnel and resources from its members have enabled the task group to advance through its objectives and continue the work that is required. Over 14 tire industry-related organizations have been actively involved in the generation and analyses of the data and results obtained to-date.
The objective of the design of experiment, or DOE, conducted in Phase I was to benchmark tire operating temperatures on flat and curved surfaces for Class 8 long haul truck tires. These are the tires that are used on trucks typically referred to as tractor/semi-trailers or the “18-wheelers” that typically make runs of 800 km (500 miles) or more, operate nearly continuously for up to 11 hours, travel on interstates/turnpikes, and have a 36,000 kgs (80,000 lbs) gross vehicle weight rating.
The 295/75R22.5 and 275/ 80R22. 5 load range G sizes in three brands were selected for testing, and steer, drive and trailer designs were included. The standard tire test machine for the tire industry is a 1.7-m (67-inch) diameter roadwheel, and is used extensively for RMT testing versus flat surface road testing.
Several adverse effects of this size roadwheel are characteristic of the testing of truck tires because of the relative diameters involved (see Figure 1):
• Severe reverse curvature of the tire at the contact patch,
• Distortion of the tire contact patch shape and its pressure distribution,
• Over-deflection of the tire sidewalls.
For a laboratory roadwheel test to realistically evaluate tires for specific road operating conditions, adjustments must be made to mitigate the roadwheel diameter effects, i.e., to avoid atypical test removal conditions such as tread chunking due to rubber reversion. The reversion described here is the decomposition of the rubber caused by unrealistic, extremely high temperatures resulting from the roadwheel curvature.
In order to limit the tire temperatures during the roadwheel tests, modifications to various test parameters such as load, inflation, speed, or ambient temperature are possible, with decreased speed being among the most effective.
The approach was to develop a highway condition equivalency standard for laboratory testing based on modifications to load, inflation, and/or speed. The standard would be based on maintaining equivalent tire crown area temperatures (at the centerline, shoulder, and belt edge locations) between flat and curved test surfaces, and would be the basis for an accelerated and increased level of test stringency.
The work for Phase I was divided into three segments:
• Laboratory tire temperature trials conducted on a flat belt test machine,
• Road tire temperature trials conducted on a 14-km (9-mile), neutral-steer, oval test track
• Laboratory tire temperature trials conducted on a 1.7-m (67-inch) diameter roadwheel.
The DOE space (see Figure 2) was based on typical long haul truck operating conditions that were identified through a survey of the task group’s collective truck tire experience and data, and published data of the Technology and Maintenance Council of the American Trucking Association.
Three parameters were identified as input variables to the DOE, namely speed, load, and inflation pressure. The ambient temperature was to be held at a constant 38 °C (100 °F). However, the ambient temperature at the track could not be held constant, and, subsequently, ambient temperature was added as an input variable in the temperature modeling phase.
The DOE utilized two tire sizes, three tire brands and five tread designs. The right front steer, right rear inner dual drive, and right rear inner dual trailer positions were chosen as test positions with the drive anticipated to be the hottest tire temperature location.
Measured Tire Temperatures
Over 150 DOE runs were made during Phase I, which consisted of the flat belt, the track, and the roadwheel test surfaces. Approximately 1,200 temperature measurements were recorded.
Figure 3 illustrates the locations of the eight thermocouples for the temperature measurements. Four thermocouples were embedded on the DOT serial side of each tire and an additional set 180 degrees away were embedded in the non-DOT side of each tire. Figure 4 shows the tire thermocouples, and other instrumentation.
For the standard steady state temperature point, a time duration of 60 minutes was identified based on industry standards and experience for tire warm-up requirements, e.g., Society of Automotive Engineers standard SAE J1269, Rolling Resistance Measurement Procedure For Passenger Car, Light Truck, and Highway Truck and Bus Tires.
In some instances, steady state temperature projections were required due to:
• Data acquisition being stopped short of 60 minutes,
• Thermocouple failure before steady state was reached,
• Thermocouple data was too variable and/or demonstrated an atypical “shift” in the temperature curve.
Acceptable steady state temperature projections were generated through the use of a curve fitting routine that produced R2 values of 0.95 to 0.99.
The maximum steady state temperature for each pair of thermocouples at each tire location was calculated for each test run, i.e., the maximum of the two belt edge thermocouples, the maximum of the two centerline thermocouples, etc. Multiple linear regression was used to fit a model to each maximum temperature by utilizing published tire dimensions, test surfaces (flat and curved), and ambient/track temperature as factors, in addition to the DOE factors of load, speed, and inflation pressure, to account for the variety of steer, drive, and trailer tires and test conditions.
The models are, as with all models, limited by the data upon which they were built. Much of the unexplained model variation is due to tire construction and material differences between brands. The models do meet the statistical criteria for stability, power, normality and colinearity.
Considering these model limitations, their sources of error, and the predictive error, the models provide considerable insight into the roadwheel test condition effect on tire temperatures. They offer a tool to explore strategies and options in developing new laboratory endurance and high speed tests.
The regression summary in Table 1 shows the predicted average increase in tire temperature due to the increased severity of testing on the 1.7-m (67-inch) diameter roadwheel versus testing on the flat surface. It is in close agreement with the flat-to-curved measured temperature increases observed.
Inflation pressure changes have significant effects on the temperatures in the two critical areas of interest, the tread centerline and the belt edge. Higher inflation causes higher tread centerline temperature and lower belt edge temperature while lower inflation causes lower centerline temperature and higher belt edge temperature. This is true for both flat and curved surface testing.
Application of the Models for Equivalent Tire Temperatures
Toward the task group’s goal of developing a highway condition equivalency standard for use in laboratory endurance and high speed test development, the temperature prediction models have been used to compare tire temperatures for various operating conditions. These predictions included operating conditions outside of the original DOE space, e.g., FMVSS 119 speed and load of 56 km/h (35 mph) and 101 percent, respectively. The linearity of the models lends confidence to such predictions.
Predicted temperatures for the current FMVSS 119 laboratory endurance test were compared to predicted “maximum” and “severe” service road condition temperatures.
Temperatures for an inflation condition of 80 percent of maximum inflation were predicted because this is a minimum level of inflation identified by the Rubber Manufacturers Association and Occupational Safety and Health Administration as critical to the integrity of radial truck tires.
This inflation, combined with 100 percent of the sidewall stamped load, 120 km/h (75 mph) speed, and 38 °C (100 °F) ambient temperature, has been labeled as “severe service” and produced higher predicted belt edge temperatures than at “maximum service,” which is defined at the conditions above, but at 100 percent of the maximum inflation. This finding will be of special interest for the endurance test development work in Phase II.
Strain Energy Density Analysis
In addition to the temperature modeling for Phase I, preliminary finite element analysis was started for strain energy density characteristics at the belt edge, the tread/belt centerline, and the ply ending locations. The SED work will continue in the Phase II work.
From the Phase I work, the tire tread temperatures are critical factors in relevant roadwheel testing. In the course of the Phase II objectives for new, standardized laboratory endurance and high speed tests, it will be important to keep in mind that unrealistically high tread temperatures on a roadwheel will lead to tread chunking from rubber reversion.
Two important points to note are:
• Any potential roadwheel test conditions designed to increase belt edge temperature/severity will have the unintended consequence of increasing tread centerline temperatures.
• A roadwheel test inflation of 80 percent of the pressure at maximum tire load will serve to both increase belt edge temperatures over current FMVSS 119 levels and to help minimize tread centerline temperatures.
A direct translation of operating conditions from the flat surface to the curved surface based upon tread centerline temperature yields an average roadwheel highway equivalent speed of 68 km/h (42 mph) for highway severe service conditions of 120 km/h (75 mph) at 100 percent maximum load, 80 percent maximum inflation, and 38 °C (100 °F) ambient temperature.
The highway speed equivalency equation developed from the work over the DOE space studied is:
KPH Curved = -50.48 + KPH Flat -0.210 * NS mm
(NS mm = tread design non-skid depth at the centerline)
Roadwheel belt edge temperatures at the highway equivalent speed based on the tread centerline temperature match between the flat and curved surfaces are predicted to be significantly higher than current FMVSS 119 belt edge temperatures, i.e., +15 percent average.
To incorporate an acceleration factor into potential, new laboratory endurance tests, roadwheel speed increases over the “highway equivalent speed” will be required.
The extent to which roadwheel test speeds can be increased without causing tread rubber reversion will be determined by trials in Phase II.
In summary, due to the characteristic effects on the structure of long haul truck tires during testing on the 1.7-m (67-inch) diameter roadwheel:
• Simultaneous equivalence of laboratory tire temperatures at the centerline and at the belt edge to those from “severe service” road conditions was not possible within the designed Phase I conditions.
• Since tread centerline temperature is generally the highest tire temperature both on the highway and on the roadwheel, it has become the basis for a highway speed equivalency standard.
• An acceleration factor for laboratory belt edge durability evaluations via increased roadwheel speed may be possible but runs the risk of tread rubber reversion due to high tread centerline temperatures. //