WHAT WE SHOULD KNOW ABOUT TIRES:
HOW TIRES ARE TESTED AND RATED
This is the second four articles on automobile tires by JohnThomson.
Few consumer products on the market today offer the reliability and value of modern automobile tires. Their dependability is very high, especially considering their complexity and the demanding driving and road conditions to which they are subjected during their lifetime. But how the tire is used, that is, the driver’s personal driving style, the type of vehicle driven, where it is driven, and what kind of tire maintenance is performed routinely, can influence tire life more than the brand name and the price paid.
When they buy tires, many people rely on recommendations from service stations or tire dealers where they have been treated well in the past. Jaguar owners generally opt for high-performance tires, since they own high-performance automobiles. Purchasers can buy from a store that offers a variety of brands at discount prices, or make their decisions based on their loyalty to certain manufacturers whose tires have previously given good service.
Regardless of where they are bought—or how much is paid—all tires sold today are required to meet federal safety standards. The purpose of this article is to examine those safety standards and the manner in which the results of testing a tire against those standards appear as markings on the side of each and every tire.
The National Highway Traffic Safety Administration (under the direction of the Department of Transportation) oversees the Uniform Tire Quality Grade System, a labeling program that provides comparative manufacturer information. Tires are subjected to a series of government-mandated tests that measure performance in treadwear, traction, and temperature resistance. All testing is done by the manufacturer.
The Uniform Tire Quality Grading System provides buyers with tire information in three major categories:
Each tire manufacturer performs its own tests in these areas, following government prescribed test procedures. Each manufacturer then assigns grades to each particular tire, and those grades are set forth on the sidewall (the space on the side of the tire between the tread and the bead) of each tire that is manufactured. This disclosure is known as Uniform Tire Quality Grade Labeling.
Treadwear grades typically range from 60 to over 500, in twenty point increments, comparing treadwear to a reference standard of 100. For example, you might see TREADWEAR 300 in raised letters on a tire sidewall as its treadwear rating. The higher the grade, the better the treadwear.
Treadwear ratings are determined on a 400 mile government test course covering specified sections of public roads near San Angelo, Texas. A group of not more than four test vehicles travels the course in a convoy so that all tires experience the same conditions. Tread groove depths of the tires being tested are measured after each 800 miles. The same procedure is followed for a set of control or "course monitoring" tires. Upon completion of the 7,200 mile test, the rating results of both tests are compared, and the tires being tested are assigned a treadwear rating by the tire manufacturer.
It's important to remember that the actual life of any tire is determined by the road surface quality, driving habits, inflation, wheel alignment and the rotation it experiences. To receive a treadwear grade, a tire is tested under controlled conditions on a government prescribed test course which does not necessarily simulate the actual application for which a given tire is designed to perform. Note that because of the limitations of these test parameters, there is no reliable way to correlate treadwear grade points to actual miles of wear.
The best way to use treadwear ratings when buying tires is to compare one rating to another. For instance, a tire with a treadwear grade of 400 might be expected to last twice as long as a tire that has a grade of 200.
Traction grades indicate the measurement of a tire's ability to stop a car in straight-ahead motion on a wet test surface pavement. It does not measure straight-ahead acceleration. Traction grades range from "AA" to "C," with AA being the best attainable grade. For example, on a tire’s sidewall you might see TRACTION A in raised letters as its traction rating.
Traction ratings are established on government-maintained skid pads. Twenty measurements are taken with an industry standard "control" tire on a wet asphalt surface and averaged. The same number of measurements are then made on a wet concrete surface. Corresponding measurements are then made on the tires being tested. A tire that has Traction Grade A means that the tire performed well on both surfaces. A tire that has Traction Grade B means that the tire performed well on at least one of the surfaces. A tire that has Traction Grade C means that the tire performed poorly on one or both of the surfaces.
It's important to remember that traction rating tests are performed only for straight-ahead sliding on concrete and asphalt surfaces that have a specified degree of wetting that simulates most road surfaces in a rainstorm. The ratings that result from these tests do not apply to cornering traction or peak values of straight-ahead braking force like those experienced in non-skid braking tests.
Temperature grades range from "A" to "C," with A being the best grade. Temperature grades represent a properly maintained tire's ability to dissipate heat under controlled indoor test wheel conditions. For example, you might see TEMPERATURE A in raised letters on a tire’s sidewall as its temperature rating.
Temperature ratings are determined by running tires on an indoor roadwheel test under specified load conditions. Successive 30 minute runs are made in 5 mph increments starting at 75 mph and continuing until the tire fails. A tire is graded C if it meets the minimum Department of Transportation performance of 85 miles per hour for 30 minutes. Grades of B and A represent higher levels of performance, 99 mph and 114 mph, respectively, for 30 minutes.
Other Uniform Labeling Conventions
The Department of Transportation Certification "DOT" must appear on each tire sold in the United States. After the DOT initials will appear a coded serial number that sets forth the tire manufacturer, manufacturing plant, tire size, and date of manufacture. Tire manufacturers must maintain this information, and tire dealers are supposed to record this information when they sell you a tire. This serial number information was instrumental in pinpointing the particular plant and date of manufacture of the Firestone tires that were the subject of the recent massive tire recall.
Mud and snow labeling is noted if the tire is rated for safe performance in mud or snow. This is designated by raised letters that appear after the tire size, and may take the form of either "M/S" or "M+S" or "M&S."
Tire construction information such as tread ply and sidewall ply, including tire ply composition and materials, must also appear as a label. For example, a tire that has two plies of rayon and four plies of fiberglass in the tread and two plies of rayon in the sidewall would state: "Tread: 2 Plies Rayon + 4 Plies Fiberglass Sidewall: 2 Plies Rayon" as its tire construction.
Manufacturers also have a number of labeling conventions, including tire dimensions, load indexes, and speed ratings. We will look at these manufacturer labeling conventions in the next article.