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|Problems with Studded Tires|
Tire studs are small metal protrusions inserted into winter tires to improve tire-road friction in snow or ice conditions. In addition to this friction improvement studded tires also have an unwanted and detrimental side effect: increased pavement wear.
Studded tires have been around in one form or another since about 1890. Modern studded tires first took off in popularity in the 1950s in Scandinavia where they were used to increase traction on icy roads. Studded tires then took hold in the U.S. in the 1960s and enjoyed growing popularity into the 1970s. The 1970s saw the first significant research efforts aimed at assessing their safety and pavement wear effects. The late 1970s through the 1990s produced technical improvements in studded tires as well as widespread governmental restrictions on their use in an effort to limit pavement wear.
The typical studded tire (see Figure 1) is a winter tire with between 60 and 120 inserted small metal studs. These studs are usually made of tungsten carbide (an extremely hard metal), weigh 1.7 - 1.9 grams each and protrude about 1.2 - 1.5 mm from the tire surface. Studs are made by embedding a small metal "jacket" into the tire then inserting a tungsten carbide pin into the jacket (see Figure 2). This pin is tapered so that it will move back into the jacket when the dynamic forces acting on it reach a certain critical level. If this subtle stud movement is properly calibrated the result is a tire stud that maintains a near-constant protrusion length even as the rest of the tire wears down. This type of stud is called a controlled protrusion (CP) stud.
Although studded tires increase traction on icy surfaces, in the early 1970s various studies throughout the U.S. and internationally began to identify significant problems with studded tires:
Scheible, R. (October 2002). An Overview of Studded and Studless Tire Traction and Safety. WSDOT Research Report WA-RD 551.1. Washington State Transportation Center (TRAC), Washington State Department of Transportation. http://www.wsdot.wa.gov/traveler/wintertravel/Studded_Tire_Report_Final_Nov_2002.pdf
Figure 7: Two cores taken from the same pavement. The core on the left was taken from the wheelpath and subjected to heavy studded tire wear. The core on the right was not from the wheelpath and was thus not subjected to much studded tire wear. The difference in height between the two cores is due almost entirely to studded tire wear.
Studded tires cause significant pavement damage. When they strike the pavement surface, each small tungsten carbide pin causes a small amount of pavement material to dislodge from the overall pavement structure. This pavement damage is influenced by the following five factors:
Many studies have been done to quantify studded tire pavement wear and they have given widely varying results. Based on data summarized by Angerinos et al. (1999), a rough rule-of-thumb would be that studded tire wear causes about 0.1 inches of wheelpath wear per million studded tire passes. This may not seem significant, however many major urban U.S. highways experience a directional traffic volume in excess of 100,000 vehicles per day, of which anywhere from about zero to 60 percent of the automobiles within this traffic are equipped with studded tires in the U.S. In Finland, studded tire use is estimated at above 90 percent for automobiles.
Studded tire pavement damage has resulted in the following:
In sum, studded tires do increase tire-pavement friction on icy roads but they also may reduce tire-pavement friction on non-icy roads and they definitely do increase pavement wear. A typical passenger car produces negligible pavement damage (typically < 0.0001 ESALs per car) and can thus be ignored in pavement structural design. However, the same typical passenger car equipped with studded tires becomes a significant pavement damage concern (in the form of studded tire ruts) and must be accounted for through use restrictions, maintenance costs and mix design.