Quote:
From the
Tire Rack Tech FAQ
Quote:
Traction Grades
UTQG Traction Grades are based on the tire's straight line wet coefficient of traction as the tire skids across the specified test surfaces.
Traction
Grades Asphalt g force Concrete g force
AA Above 0.54 0.41
A Above 0.47 0.35
B Above 0.38 0.26
C Less Than 0.38 0.26
So, to answer your question, I'd expect the AA rated All-Season to have better slow speed Wet stopping power.
The quotation is accurate, but the description of NHTSA Traction is incomplete. Because it is incomplete, the premises do not support the conclusion.
The NHTSA procedure to measure Traction (capitalized because it is a defined term) measures the coefficient of friction of a tire mounted on a wheel that has been mechanically locked and then the tire, with the wheel locked, has been dragged -- with the same little contact patch in contact with the pavement -- across the tarmac a distance of 29 to 59 feet (0.5 to 1.0 seconds after locking the wheel while the trailer on which the tire is mounted is being dragged at 40 mph) before the measurement of the coefficient of friction commences.
Most modern cars are equipped with antilock braking systems and would not allow the NHTSA test procedure to be replicated at the wheel in real world conditions.
The major difference between all season tires and "summer" tires is that all season tires retain water on their tread -- they are made with a tread compound that has an affinity to moisture -- which aids traction in light snow, while the tread compound of "summer" tires retains rubber's natural aversion to water and sheds moisture quickly. Braking on wet pavement is better the drier the interface between tread and pavement at the contact patch is, and summer tires are better at keeping the interface dry, "wiping" the pavement, if you will, like windshield wipers. All season tires, retaining a film of moisture (as they are designed to do), retain a lubricating layer of water between tire and pavement.
By dragging a rotationally stable tiny contact patch across the pavement, the NHTSA test procedure for Traction obviates the all season tire's disadvantage
vs. a "summer" tire, because in those 29 to 59 feet the film on the tread gets scrubbed off and, through frictional heating, evaporated.
For that reason, it is invalid to compare a Traction A or AA all season tire against a Traction A "summer" tire, at least when the fitment will be on a vehicle equipped with ABS.