Tire Basics

It's partway through the race. The leader is slowly losing his advantage, as the other cars gain, when from out of nowhere, the 36th-place car-some six laps down-runs down that dominant car and passes him on the outside like he was standing still. How? Tires. That backmarker has just come out of the pits with fresh rubber. The leader's tires are 60 laps old. New rubber can make a dramatic difference in a vehicle's performance, as evidenced on the race track, where the new tires can turn a zero into a hero. The same holds true on the street, where the right tires can bring an older car back to life, tighten up a sloppy riding and handling SUV, quell apprehension on a wet, curvy backroad, and silently perform years of faithful service. The wrong tires, on the other hand, can make you think the shocks and bushings have failed, the alignment is out of spec, the steering is demon-possessed, a band of screaming bobcats have taken up residence in the wheelwells, and the water on the road is a foot deep. There's much more to tires than the meets the eye. Designing circles
Tires are the single most important component on a vehicle, one that seemingly requires an advanced engineering degree to fully understand. And yet, despite advanced computer simulations, designing and developing a tire remains as much art as science and involves as much compromise as an international peace accord. With a tire, improvement in one performance characteristic almost always comes at the expense of another. For example: Reducing rolling resistance (for better fuel mileage) or increasing tread life usually decreases grip, wet or dry. The opposite is true too: A race car wearing race tires can corner at 1.5g without aero aids, but gas mileage is of almost no concern and the rubber is worn out within 150 miles. Sometimes, the interactions don't work the way tire designers' computers predict. This is due partially to the tire literally being cooked (or "cured") in the manufacturing process: Polymers, sulphur, carbon black, oils, waxes, resin, and structural components evolve and develop new properties. Some of this evolution continues over the tire's life, especially if the tire is overheated. With the latest tire production methods, which lay individual cables of material rather than sheets of fabric or steel, there are more individual components in a tire than in a car engine. An engine can be torn down to measure and test every component for design and material compliance. Once reassembled, it's as good as new. Disassemble a tire, and even experts have a devil of a time accurately measuring the basic components, much less figuring out which compounds evolved into what. (Imagine cutting apart a cake in an effort to determine whether free-range or coop-raised eggs were used.) "Born on" dating
Among the factors that influence tire performance is how recently the tire was crafted. Just like Budweiser beer, tires bear a "born on" date. Imprinted on one or both sidewalls is an alphanumeric code of a dozen or so letters and numbers that begins with "DOT." Tires produced starting in 2000 have a four-digit birthdate code at the end of the DOT number. The first two numbers indicate the week, and the last two the year. So, for example, a DOT number ending in 0205 means the tire was produced the second week of January 2005. This date is important because rubber deteriorates with age. A tire that's been in service for five or six years should be replaced regardless of tread depth. Cut a year or two off that for vehicles that are not garaged during the day or are run in areas of high ground-level ozone. Is "plus sizing" a plus?
Not long ago, 17-inch-diameter wheels were limited to upper-end sports cars, while 20-inchers were strictly for lunatic-fringe tuners. Now, many minivans come standard on 17s, and 20-inch "dubs" are original equipment on pickups. If you're buying a new car, should you go for the optional 19s? If your considering an "upgrade" to your current vehicle, should you go to 20s, 24s or 26s? Here's the short answer: If show is more important than go and the roads in your region are so smooth you think a pothole is something that causes pasta sauce to leak onto the stove, then, sure, go for the bigger size. Just be sure to keep the OE wheels for when you want to sell the car because those certain-to-no-longer-be-trendy versions will be lucky to bring 50 cents a pound at the aluminum recycler. The long answer is that there's no performance advantage or even steering-feel improvement to be gained by increasing wheel diameter beyond about 17 or 18 inches for most cars. When asked what diameter Formula One wheels would go to if the current 13-inch mandate were removed, the head of Michelin's F1 program predicted 17 to 19 inches, adding "certainly not 20 or beyond." If you were to select larger-diameter aftermarket wheels that weigh more than stock or have a different offset than the stock units, or if the new tires are of a significantly different diameter or lower load rating, the vehicle's handling, ride, emergency capability, and safety could be severely impacted. Auto manufacturers spend millions of dollars on their vehicles' suspension engineering and chassis tuning, including factory tire fitment. Any change you make after you buy a vehicle means a trade-off must be made. The more dramatic the change, the more severe the compromise. The best general guidance regarding plus-sizing is to look to the automaker's own offerings. If a car is available with 18s, then rest assured, the suspension and braking is engineered to work effectively with this heavier setup.   |   Michelin, the company that invented the radial tire, has rolled out a radical new concept called the Tweel. As this prototype demonstrates, the traditional rubber tire and solid wheel duo is replaced by a tread bonded to a strong loop of composite material. The vehicle's weight hangs from a series of polymer plastic spokes that connect this loop to the hub. The flexible Tweel can bear the load of a pneumatic tire four times its size, it can cushion bumps five times better than a pneumatic tire, and the contact area can be nearly doubled for improved handling. Noise, mass, and rolling resistance have to be improved before this promising new technology is ready for high-speed, high-load passenger-car applications. How long can they go?
The greatest factor in tire longevity is the roads on which you drive. Tire companies have mapped out average tread life by county and discovered that tread life is inversely proportional to local elevation. It's great in the flatlands of west Tennessee and eastern Colorado, but poor in the Smokies and Rockies. It's also better for those who do most of their driving on roads with long straights and gentle curves. And it's worse the farther south you go, due to the heat. The next factor is how you drive. If the plush toys and ball caps on your rear package shelf remain motionless under all driving conditions, you might get the claimed 70,000 miles from your tires. If you know on which interstate transition ramps it's possible to more than double the recommended speed, and you measure 60-foot times leaving stoplights, the same tires might not last 30,000 miles. Next is the type of tires you have. Ultra-high-performance tires rarely last more than 25,000 miles, even if the ball caps never budge. Ultimately, maintenance is the key to getting the most mileage and performance from your tires. Each month make sure pressures are at least what the vehicle manufacturer recommends, rotate positions every 3,000 miles, diligently check for signs of uneven wear, and correct any vehicle problems that can cause poor wear. As we've illustrated, tires are much more sophisticated than they appear. These black doughnuts are the only parts of the car that make contact with the ground, and they're relied upon to keep the expensive automobile and invaluable occupants safe. Whether you're buying a new vehicle or replacing old tires, it pays to have a solid understanding of this essential automotive component. Reading sidewalls
A tire's sidewall is packed with information. Each manufacturer's Web site has a detailed key translating the figures, but this quick primer will help you decipher the information.   |   112 0504 Ic Tire Side Z Tire manufacturers have a size "window" in which to fit a nominal dimension. While all of Maker A's 245/40R17s will be identical, they may be ever-so-slightly wider, narrower, shorter, or taller than Maker B's 245/40R17s. A tire focused on performance will likely be in the wide and tall corner of the window, while one that prioritizes low cost will likely be in the narrow and short corner. Since the size windows overlap, some makers use one size mold to produce a pair of dimensions. "M+S" means the tire meets the standards for an all-season tire. This does not imply the tire performance rivals a dedicated snow tire, just that it's better in snow than one without the M+S designation. Widely misunderstood, "Max Load/Max Pressure" is not the pressure at which the tire will burst. Instead, increasing pressure beyond this point results in no additional load-carrying capacity beyond that stated. Uniform Tire Quality Grade is shown as numbers or letters after the words "treadwear," "traction," and "temperature," this is the tire manufacturer's self-assessment of how this tire compares to its other offerings. The treadwear number, determined by field testing on a government-specified course, is a percentage of a control tire graded at 100 points. Thus, a tire marked "200" means it lasts twice as long one graded "100." The traction rating comes from a straight-line wet (more like damp) grip test (not really stopping distance, as its performed on a traction trailer). "AA" is best. And temperature indicates the tire's resistance to heat generated by running at high speed. "A" is best. So many high-quality tires get "AA" and "A" in the last two categories, that they are of relatively little use, except to convince you not to buy cheap tires. The right tire for you
Selecting the right tire is a daunting task, surpassing even that of a novice snow skier from the flatlands choosing downhill skis: In the ski store, he finds row after row of seemingly identical skis, labeled with, to him, meaningless features and benefits. At least with skis, however, you can pick your favorite color. To choose the right tire for your needs, start with understanding what performance category you have now and then decide where you want to end up. Passenger: Round and black. They hold air, last a long time, provide a comfortable ride, and don't cost too much. A step up from this is sometimes called "traditional luxury." Touring: While comfort and long-life are top priorities in this category, precise steering feel and a more controlled ride are important as well. Grand touring: Sometimes called "performance luxury," these tires offer sportier steering feel, tauter ride, and enhanced grip. High-performance: Grip, both wet and dry, and crisp steering feel are at least as important as tread life and comfort. Ultra-high-performance: Grip is king, and precise steering feel is queen. Tread life and ride comfort take a back seat. Some separate out the stickiest street tires into a "max performance" category. If you like the way your car currently feels, stay with the same tire category or, ideally, the same tire. If you'd like to enhance performance, consider stepping up a category or two. Automakers typically fit their vehicles with specially formulated tires designed to work with the handling, noise, fuel-economy, and bottom-line pricing goals set forth by their engineering teams. If your priorities differ, such as favoring more performance over saving a few bucks, then aftermarket tires can make a big difference in your driving experience.