on the right margin.
To see what others have said, click either the phrase “x comments” or the Rule Map–>Testing Requirements–> SAFETY/WET TRACTION
The “safety” rating that will be included on the proposed new consumer information tire label will be based on traction — specifically, a measure of the tire’s ability to stop on wet pavement.
NHTSA proposes to base a tire’s safety rating on the results of test procedures currently required under NHTSA’s Uniform Tire Quality Grading Standards (UTQGS) tire rating system. The agency intends to use these existing test procedures because the Energy Independent and Security Act of 2007 requires that the new tire efficiency consumer information rule be finalized by the end of 2009. However, NHTSA proposes that the UTQGS test results be normalized to a 0 to 100 point scale for display on the new label, to make the safety/wet traction rating comparable to the other two ratings.
In the UTQGS procedure, a tire is tested on both wet asphalt and wet concrete. The tire is attached to the axle of a trailer and towed behind a vehicle at 40 miles per hour. The trailer’s brakes are locked, causing the tire to skid. Sensors on the axle calculate the resulting braking forces, yielding a measurement called the sliding coefficient of friction. This measurement becomes the basis for the tire’s safety rating. See Rating Forumlas.
Of course, most modern vehicles would not allow such a skid to occur. New cars and trucks are usually equipped with Antilock Braking System (ABS), Electronic Stability Control (ESC), or both. Therefore, a tire’s sliding coefficient of friction may suggest that a tire is less safe — in terms of wet traction — than it may actually be. For this reason, NHTSA is considering basing a tire’s safety rating on its peak, as opposed to its sliding, coefficient of friction. This measurement, also calculated during the UTQGS test procedure, may more accurately represent a tire’s safety when used with a vehicle with ABS, ESC, or both.
A potential drawback of basing a tire’s safety rating on its peak coefficient of friction is that many older vehicles still in use are not equipped with ABS or ESC. NHTSA found that a non-ABS equipped vehicle required a significantly longer wet stopping distance than an ABS equipped vehicle using the same tire. Therefore, a tire safety rating based on peak coefficient of friction could be misleading to owners of older vehicles.
Recognizing the complexities of a single traction rating that will be applied to both older and newer vehicles, NHTSA is considering basing the safety rating on a composite of four measurements: peak coefficient of friction on wet asphalt, peak coefficient of friction on wet concrete, sliding coefficient of friction on wet asphalt and sliding coefficient of friction on wet concrete. These four friction values could be weighted equally, or differently. The agency is looking for comment on the idea of a composite rating, and on how the four elements ought be weighted.
Because asphalt and concrete surfaces can vary from day to day and place to place, NHTSA proposes to require tire manufactures to adjust their testing results by correlating to a “standard reference tire.” These adjustments would be accomplished using the formula described at Rating Formulas.