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Tires are the most neglected and underrated performance aspect. The best way to emphasize their contribution to overall performance is to state their obvious role: tires are the absolute only thing touching the ground. Engine power, braking power, steering response, all these performance aspects are channeled through the tires: when any of these aspects exceed the tires performance, trouble follows in the form of a skid (power skid, brake skid or cornering skid). Regardless of the quality or condition of tires, proper inflation and wheel alignment must be maintained. Not all tires are created equal, the following technical aspects are to be considered when choosing: construction and size.

Construction

Tires are designed for a purpose, and careful observation can guide the informed consumer through all the marketing hype that surrounds tires. First and foremost, the intended use of tires will be reflected by the type of rubber used and the sculpting of the rubber.

Rubber

Rubber tires tend to get softer as they get hotter, while they tend to get harder as they get colder. For proper traction, tires need to be hard enough to keep their shape but soft enough to grip the ground. The type of rubber used can be evaluated by touching tires. Tires intended for use in hot weather (above 10 Celcius) will have a harder rubber compound (feels almost like wood to the touch at room temperature), whereas tires intended for use in cold weather (below 10 Celcius) will have a much softer rubber compound (feels almost spongy to the touch at room temperature). While being driven in hot weather, tires will warm-up and soften: the harder tires will soften enough to provide traction, whereas softer tires are already too soft at room temperature and will soften even more while being driven in hot weather, they will wear-out faster and lack lateral stability. While being driven in cold weather, tires cool down because of the cold pavement and become harder that at room temperature: the softer tires will harden enough to provide traction (even on ice), whereas harder tires are already too hard at room temperature and will harden more while being driven in cold weather and will provide little traction. No known rubber suitable for tires remains hard enough to be driven above 20 Celcius, yet soft enough to be driven below 0 Celcius. Therefore, while driving in a location where temperatures rise above 20 Celcius and drop below 0 Celcius, two sets of tires will be needed, one for each weather condition. To help consumer choose between compounds for tires, tires are rated following tests under standard conditions and these ratings are written on the side of tires directly. Dry-pavement traction and temperature-resistance are rated using a lettered-scale (with AA and A being the best ratings), while the treadwear (what distance can the tires be driven before they wear-out) is rated using a number scale (larger numbers are better). These three ratings together are called the UTQG (uniform tire quality grading). A good tire would have A or AA for traction, A for temperature-resistance, and at least 400 for treadwear.

Sculpting

On dry pavement, "slick" tires (no sculpting) provide the best traction, as they have more rubber in contact with the pavement than scuplted tires of the same size. Sculting of the tread is important in non-ideal driving conditions (wet, muddy, snowy or slushy pavement). The role of sculpting is to force water, mud and snow away from the rubber so the tires can touch the pavement and provide traction. Water is best squirted through a multitude of narrow grooves in the sculpting, whereas mud and snow are better squirted through larger and deeper grooves in the sculpting. Therefore, unless driving in a location where mud is a common occurence on pavement, hot weather tires should only have thin grooves. Sculpting on cold weather tires comes in two flavors, as there are two kinds of cold weather: snow and ice that will never melt, and snow and ice that will melt eventually. For snow and ice that will never melt, there is no chance of driving on wet pavement, therefore tires with not only larger and deeper grooves, but also metal studs imbedded in the tread will provide the best traction. For snow and ice that will eventually melt, there will eventually be wet pavement to drive on before the weather gets hot enough for hot weather tires. Therefore, tires designed for this type of cold weather will have a mix of narrow grooves and wide and deep grooves, but no metal studs. Metal studs in this type of cold weather will raise tires off the pavement and eliminate traction. There are no tires suitable for all types of weather: the apelations "all-season" and "all-weather" are simply clever marketing.

Size

Proper sizing of tires is important. There is a sticker (usually near the driver door) stating the recommended inflation and size of tires, but the recommended size of tires can be modified. Indeed, there are a few factors to consider in choosing sizes for tires, and these are described by a series of letters and numbers on the side of tires. There are two groupings of letters and numbers. The first grouping contains three numbers (one of 3 digits and two of 2 digits) and at least one letter (for example, 175/70/R13), whereas the second grouping contains a number (2 or 3 digits) and a letter (for example, 82H).

First Grouping

The first grouping (a 3 digit number, two 2 digit numbers and a letter) states the size of the tires directly. The first number (3 digits) is the tread width in milimeters (this is the part of tires touching the ground). The second number (2 digits) is the sidewall height as a percentage of the tread width (this is the part of tires where the UTQG rating and sizing is written, as well as the brand name of the tires). The third number (2 digits) is the size of metal wheel (in inches) that the tires fit on. The letter indicates the type of tires. For example, 175/70/R13 refers to radial tires that have a tread 175 milimeters wide, a sidewall 70% of that and fit on 13 inch wheels.

Second Grouping

The second grouping contains a number (2 or 3 digits) that refers to the maximum weight tires can handle, and a letter refering to how fast tires can be driven. These are not measurements like the first grouping and refer instead to categories in a table.

Resizing of Tires

Resizing of tires (also known as plus-sizing) is replacing tires of the recommended size by other tires of a different size. What is important to understand, is that speedometers and odometers need tires of a specific circumference. Therefore, any tires that have the same circumference as the specified size will function, yet provide different handling characteristics. Unfortunately, the circumference of tires is not specified on tires and must be calculated using the formula (2*tw*sw/100)+(ws*25.4), where tw is the tread width (first number), sw is the side wall height (second number) and ws is the wheel size (third number). Any alternate size of tires will need a circumference within 1-2% of the specified size of tires. The handling characteristics of tires depend on their tread width, sidewall height and size of metal wheel. Traction in hot weather is improved by having more rubber touching the ground, while lateral stability is improved with shorter sidewalls. Therefore a bigger tread width is prefered, while the smaller sidewalls will need a bigger wheel to maintain the same circumference. Following our example, our 175/70/R13 tires have a circumference of 575.2 milimeters. Increasing tread width by 10 milimeters to 185 milimeters and wheel size by 1 inch to 14 inches, plugging these numbers into the formula and working backwards yields a sidewall height of 59.4. Therefore, tires of size 185/60/R14 would have almost the same circumference (577.6 milimeters), and since these tires fit on a metal wheel 1 inch bigger, this is considered the +1 size (the tires will be wider with skinnier sidewalls). The +2 size tires would fit on 15 inch metal wheels, and increasing tread width another 10 milimeters to 195 milimeters would yield a sidewall height of 49.8. Therefore, 195/50/R15 tires would have a similar circumference (576.0 milimeters) and be +2 size (the tires will be even wider, with even skinnier sidewalls). In cold weather, the amount of rubber touching the ground is less important than the pressure exerted by the tires. Therefore, cold weather tires are best with a narrower tread width, carrying the same weight on less rubber (therefore more weight per square inch of rubber). It is common to keep the recommended size of tires in cold weather and use +1 or +2 size tires in hot weather.