In the 1980s, trends in truck-tire configurations, tire types, and tire pressures began to concern U.S. pavement engineers because of the potential for pavement damage. As a result, a 1992 study sought to "(a) determine the extent and pattern of use of single and high-pressure tires in Oregon and (b) determine the pavement impacts of the use of single and high-pressure tires." C. A. Bell, S. U. Randhawa, and Z. K. Xu discussed the results of the study in "Impact of High-Pressure Tires and Single-Tired Axles in Oregon" (Transportation Research Record 1540). They concluded that no significant changes in tire pressures have occurred since 1986, nor is there any apparent need to control tire pressures. They further concluded that while a significant change in the use of single tires has not occurred, the practice of "partial singling out needs to be controlled and perhaps eliminated completely." "Singling out" is "the practice of using only one tire on axles having hubs for two tires."

METHODS

The study involved an Oregon-based literature review of single-tired axles and tire pressures, a data collection plan that included new collection sites and a larger sample size, a data analysis, and a results comparison with other studies to estimate trends. The data collection sites were five ports of entry (POE) identified by the Oregon State Highway Division (OSHD) as representative of the trucks using different highways in the state. Data were collected at all five sites at different times of the year "to determine seasonal trends in truck dynamics." A total of 634, 564, and 507 trucks were surveyed in March, June, and September of 1992, respectively.

RESULTS

The majority of trucks surveyed (72 percent) were the 3-S2 type. This is the truck configuration commonly called an "18-wheeler"--a "tractor" with a semi-trailer. Single units (smaller, one-piece vehicles) and trucks with trailers (a single-unit truck combined with one or more trailers) made up the remaining surveyed vehicles.

Most tires on surveyed trucks were radials, primarily 11-inch-wide tires and 22.5-inch or 24.5-inch wheels. Trucks with single tires on steering axles were not counted as singled-out trucks. At the five survey sites, 8.4 percent of the trucks had at least one axle using single tires, and most of these were singled out. This corroborated findings from studies conducted in 1986 and 1989. Results from the current survey suggested that singling out correlated with weight; most of the trucks using singling out were carrying lightweight products such as paper or food, or traveling empty. Of note was the "significant degree of singling out of tridem axles (about 40 percent in March and June surveys and almost 90 percent in the September survey), with a tendency for singling out to occur on the lead axle."

The tire pressure portion of the study measured actual and recommended tire pressures for both steering and nonsteering axles. Results revealed that "actual pressures were higher than the recommended pressures and have a much larger spread on both the steering and nonsteering axles." Actual pressures were skewed to the right, while recommended pressures tended to skew to the left; the difference reflected the significant number of tires carrying pressures higher than recommended. Results showed the mean tire pressure to be 109 pounds per square inch (psi) for steering axles, 102 psi for nonsteering axles, 105 psi for singled-out tires, and approximately 120 psi for wide-base tires. These results were similar to results from earlier studies. The current study also verified the declining trend in the use of bias tires--from 12.9 percent in 1986 to 9.9 percent in 1989 to 1.2 percent in 1992.

Pavement Impacts

Traffic loads and environmental conditions cause pavement damage. The damage caused by traffic is influenced by the total contact area between the tire and the pavement--more contact between the tire and pavement results in less damage to the pavement.

In analyzing trucks with five and six axles, results showed that "singling out of tandem axles does not appear to be particularly detrimental" to pavement, and "singling out of tridems is detrimental only when compared with a similarly loaded dual-tired tridem." As tire size increases, the damage potential from tridem axles decreases. For example, "tridem axles with wide-base tires can carry 42,000 lb and have a lower damage potential than a tandem axle loaded to 34,000 lb."

The study also compared trucks with seven and eight axles using wide-base single tires to those using regular dual tires on single axles. Results indicated that "in terms of the load carried, these trucks tend to be less damaging than those with fewer axles."

The partially singled-out tandem axle was found to be "particularly damaging." While this finding was significant, it "is not surprising, since a full 34,000-lb load can be carried legally with such an arrangement, resulting in two very concentrated wheel loads of 5,666 lb in proximity." In addition, the suspension system of a partially singled-out tandem axle truck "may not be capable of distributing the load evenly to all the tires in the group. Thus, higher loads could result on the singled-out tires in such situations." Analysis of partially singled-out tridem axles suggested that the potential pavement damage is not as severe as with the partially singled-out tandem axle.

CONCLUSIONS / RECOMMENDATIONS

Tire pressures and the use of single tires have not changed appreciably in Oregon since 1986. However, "the proportion of vehicles using single tires (7 percent) merits detailed evaluation, particularly those using partial singling out." The extent of the problem of partial singling out is such that the practice "needs to be controlled and perhaps eliminated completely." Eliminating this practice for trucks with tandem axles presents the simplest and least intrusive option, since the practice is rarely used.

Regardless of axle type, "the use of single tires results in a greater damage potential, relative to a similarly loaded dual-tired axle. Singling out of 11-in. dual tires is much more damaging than the use of wide-base tires."

Results also indicated that "tandem axles with single tires are less damaging than comparably loaded single axles with dual tires. Similarly, tridem axles with single tires are less damaging than similarly loaded tandem axles with dual tires. Overall, "singling out of all of the dual tires in a tandem or tridem axle is a less serious problem" and "should not be eliminated because in several instances such axles are less damaging than other alternatives." This practice may warrant some control, but the trend toward the use of larger axle groups should not be discouraged.

In addition, analysis showed that "wide-base single tires are potentially more damaging than comparably loaded dual tires"; however, "the use of wide-base tires is without doubt preferable to the use of singled-out conventional tires and incentives could be introduced to encourage their use on tridem axles." For both singled-out and wide-base tires, "Oregon may wish to consider a modification to the mileage tax rates and axle-weight mile schedules. Simple adjustments could be developed on the basis of the number of tires used in an axle group and the tire width."