After drawing up a list of products to be tested, CR employees, acting anonymously, go about purchasing tires and wheels from retailers. (The publication, not unlike its readers, likes to buy wherever it obtains the best price.)
Retail merchants aren't told the tires will be used for testing purposes and special care is taken to purchase stock from existing inventories. This is done to assure that the tires to be tested are representative of those available to consumers. Enough wheels also are acquired to eliminate the need for repeated mounting and demounting of test tires.
Before the testing phase gets under way, Mr. Petersen and Ms. Stockburger consult with tire manufacturers one last time to assure nothing has or will change in regard to each product's continued availability.
Fortunately, for CR's purposes, tire lines seem to have a life cycle of about four years-long enough to permit the publishing of meaningful test results, Ms. Stockburger said.
Once that is accomplished, a suitable test vehicle is brought into the picture. ``We look for a vehicle that best represents the segment we've chosen to focus on,'' she said. ``For this year's tests, we're using a Ford Focus SVT''-a 2003 model.
Tire testing usually runs from the first of the year through most of June.
Before testing gets into full swing, CR's staffers drive on the tires for 200 miles in order to break them in. The evaluation process then begins in earnest, commencing with non-destructive type tests, such as traction testing on snow and ice and hydroplaning resistance carried out on the water-covered straightaway. Once these have been completed, more destructive testing is carried out. These include such tests as dry braking and cornering.
A rating scale of 1 to 5 is used (with five being excellent). After testing is completed and the scores in various performance areas are tabulated, the results are forwarded to CR's Yonkers headquarters where statisticians rank the tires. In doing so, they also determine whether the scoring differences between individual tires are statistically significant.
The findings then are turned over to the magazine's editors who prepare them for publication. Once the resulting article is ready for print, Mr. Petersen and Ms. Stockburger check it over one last time for technical accuracy.
To rank No. 1 in CR's listings, tires don't have to be the absolute best performer in every test-just be among the top scorers in most categories, the pair said. They cautioned that CR's rankings should be compared only to those of other tires in the group tested.
Thus, readers shouldn't attempt to compare the rankings of similar type tires from year to year or the magazine's findings regarding one classification of tires with those pertaining to another.
A ``good'' rating for snow traction in all-season tires might equal a ``poor'' rating in the case of winter tires, Ms. Stockburger said. Performance differences among the various vehicles also make it impossible to draw valid product comparisons from year to year and test to test, Mr. Petersen added.
Not all CR's tire testing is done for the purpose of ranking competitive products. One such exception was a test comparing the snow traction of all-wheel-drive vehicles equipped with all-season tires to that of front-wheel-drive cars alternately shod with true winter tires and all-seasons.
In that comparison, the greatest performance differences between the two types of vehicles occurred during acceleration rather than when stopping on loose snow. Not surprisingly, the all-wheel-drive cars with all-season tires performed best-accelerating from 5 to 20 mph in slightly more than 40 feet, compared with 60 feet in the case of the front-wheel-drive cars shod with winter tires, and more than 90 feet for the same vehicles equipped with all-seasons.
The magazine's resulting recommendation to consumers: Consider all-wheel-drive if you live in a snowy area. But a front-wheel-drive vehicle equipped with four true winter tires will get most motorists through the winter and provide the greatest ``bang for the buck.''
Another study done by CR's East Haddam test crew explored the adverse effects of treadwear on tire traction performance. To conduct that test, the two researchers shaved about half the original tread from new test tires and then compared their performance with that of identical tires having full treads.
The results of the shaving produced significant decreases in snow traction, ice braking with ABS, wet braking and hydroplaning resistance. Based on that data, CR advised consumers not to wait until their tires wear down to 2/32nds if they should experience a falloff in any of those performance areas.
Another significant test conducted two years ago by Mr. Petersen and Ms. Stockburger furnished ammunition for Consumers Union advocacy of so-called ``direct'' vs. ``indirect'' tire pressure monitoring systems in new cars. The monitors originally were mandated by the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act starting with 2004 model-year vehicles.
Critics, including Consumers Union and most tire makers, contend that indirect monitoring systems, which depend on the vehicle's ABS to detect tire underinflation, won't warn drivers if all four tires lose pressure at the same rate.
In exploring the extent of this problem, the pair used a total of 109 tires-three identical tires for each of the 26 all-season models represented-and mounted them on brand new rims.
After water-testing all the assemblies to eliminate the possibility of bead or other type of leakage, they set the final inflation pressure at 30 psi and placed the tires in a temperature-controlled room where they would be stored for one year.
During the first six months, air pressure in the tires was checked monthly with a precision gauge. The tires then were left unattended for the remaining six months. At the one-year point, the two researchers checked them for pressure loss.
The Michelin X-1 lost the least amount of pressure-just under 4 psi. However, other tires did not do as well. One, the Kelly Explorer, lost more than 13 psi over the one-year period. What's more, CR's testers found great consistency in air loss among the three tires representing each model. Almost all of them lost pressure at approximately the same rate.
The bottom line, in Ms. Stockburger's words, was that indirect air pressure monitors, as favored by the auto makers and initially approved by the National Highway Traffic Safety Administration (NHTSA) ``are not going to pick up on the fact that all four Kelly Explorers, after one year, are now 40-percent underinflated.''
As a result, Consumers Union wrote NHTSA stating it was disappointed with the decision to allow the use of indirect monitors and expressing the hope that the agency would reconsider requiring the use of direct monitors at some future time. Ironically, that time may come sooner than anyone anticipated.
A U.S. Appeals Court in New York Aug. 7 overturned NHTSA's tire pressure monitoring requirement and remanded it back to the agency for further rulemaking, declaring it ``contrary to law and arbitrary and capricious.'' NHTSA reportedly has the option to appeal the decision to the full Second Circuit Court.
The court order resulted from a lawsuit brought in 2002 by three public advocacy groups, Washington-based Center for Auto Safety and Public Citizen as well as the New York Public Interest Group. They argued against NHTSA's decision to allow car makers to opt for an indirect monitoring system, contending such systems are accurate only part of the time.