Carcass
If you've ever wondered what the term "radial" means, your answer can be found in the cutaway to the right. Note the four different visible layers of inter-woven material under the tire's rubber. These are the tire's plies, and serve to limit its flex underload. A belt runs the circumference of the tire, just underneath the tread, limiting radial runout (the deviation of a tire's shape from a perfect circle) and "centrifugal growth" brought on by high speed. The ply just above the inner-most layer of rubber at an angle of 90 degrees from the direction of travel, is called the "radial" ply, and serves to limit lateral runout (confusing, we know) that happens as the tire's sidewall is put underload from the weight of the car. Before radial design became the industry standard, tires were commonly constructed of two "bias" plies, diagonally-opposed at 60 degrees from the direction of travel, that limited each type of runout, but at a cost; when stressed, the plies would rub across each other, creating friction that increased rolling resistance, and robbed power. Despite decreased stiffness compared to a bias tire, the flexible sidewalls of a radial tire provide more inertia that serve to decrease rolling resistance. Crisscrossing plies are added to the circumference of this ADVAN Sport to further increase rigidity.
Tire plies were first constructed from steel, but have long-since been replaced with lightweight Aramid fibers like Twaron and Kevlar (although belts still remain of steel), which are also very expensive materials; to cut costs, many cheap-o tires are manufactured with these materials used sparingly.
Sidewall
A tire's sidewall is never considered as much as its contact patch and compound, but nonetheless plays a crucial role in providing traction, as well as performing the other duties of a tire: maintaining shape under duress of load and speed, remaining durable, decreasing rolling resistance and providing traction in wet conditions. Without reinforcements, tire sidewalls would bulge and flex under load, meaning a worse handling characteristic. Also, increased sidewall flex means increased rolling resistance and more energy that has to be taken away from acceleration to turn the wheels (think of pedaling a bike with a flat tire as opposed to one that's fully inflated). Looking at our cutaway, we see the ADVAN Sport features a radial that wraps around the bead and extends almost to the surface of the tire for an extra-strong sidewall-expect the cheap-o tires call it quits on the radial right after, the bead.
Decoding A Tire
If you're like most, the strange markings found on the sides of automobile tires are a bit of a mystery. You probably know where the tire's size is and maybe even where to find its maximum pressure, but can you tell your tire's speed and load limits? What about it's Uniform Tire Quality Grading System (UTQG) rating or the date of its manufacture? A simple glance at the sidewall of all tires sold in the United States can tell you all this and more. Watch as we dissect this Toyo Proxes R1R, to find out how.
The tire's size is easy to find and typically follows a pattern of width/sidewall/wheel diameter. Here we see the tire's width is 265mm, its sidewall measures 35 percent of that (called the aspect ratio) and it is designed for an 18-inch wheel. The "ZR" designation means the tire is a UHP radial, and the "93W" stamping is a rating of the load and speed capacity that the appropriate charts will show means our tire is good for up to 1,433 pounds and 168 mph.
Above the tire's safety warning are its UTQG ratings. Tread-wear is assessed by comparing the tire's expected tread life to that of a government "control tire" with a rating of 100. Our tire's tread-wear rating of 140 tells its tread should last about 40 percent longer than that of a tire with a rating of 100, under ideal driving conditions. Traction on wet pavement is assessed and rated lowest to highest by the designations: AA, A, B, or C. In our case, the traction AA rating tells us what we knew when we bought the tire-will fare better on a racetrack than a wet road. Finally, the tire's resistance to high temperature, (brought on by sustained aggressive driving), is expressed through a rating of A, B or C; this time highest to lowest. The "A" temperature designation on our tire tells us the R1R can withstand a lot of abuse before failing.
Here, we can see three things: The arrows and "Right" designation tell us Toyo's Proxes R1R is a directional tire, meaning it needs to be mounted on each wheel in a way specific to the direction of travel. Underneath that, the DOT designation is the beginning of the DOT code all tires sold in the US must carry. The CX marking identifies where the tire was manufactured. In this case, our R1R was made in the company's Sendai, Japan plant. The WD letters that follow are an internal code for the tire's size, and EFL is a designation for the mold batch the tire was a part of. The remaining four numbers tell us this tire was produced in the 31st week of 2007.