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Tires



A tire is a tubular corded carcass covered with rubber or synthetic rubber, which is mounted on a wheel and inflated to provide traction for moving a vehicle and for assisting the brakes in stopping it. Today`s tires, when properly inflated, will absorb bumps on a road's surface and give a safe, comfortable ride, while providing a reassuring grip on the road at all speeds.


TIRE TYPES AND MATERIALS



There are two basic types of tire: the tubeless tire for passenger cars and light-duty trucks; and those requiring inner tubes for medium and heavy-duty trucks. Government regulations require that every passenger tire be stamped either "tubeless" or "tube-type," but more than 90% of the passenger cars are tubeless. "Belted" would indicate that a tire has extra strength in its construction. In addition, if the tire were a radial, it would be designated as such - a necessary precaution because radials should not be mixed with other tire types except when the radials are used as the pair on the rear axle.


Bias Ply Tires



There are three general methods of arranging or laying down the tire plies. They may be laid down "on the bias," "on the bias and belted," or "radially."

The standard, and least expensive, tire is a bias ply. In this type of ply, the cord strips are arranged diagonally (i.e., at a bias) to the center line of the tread and alternate plies are reversed to cross at a 30 or 40 degree angle. The result is a uniformly firm body, which will wear satisfactorily at moderate speeds, with sidewalls that can stand curb bruises. In fast driving or hard turning, however, the tread elements squirm together and spring apart, producing heat that weakens the tires.


Bias Belted Tires



Bias Belted Tires are the same as Bias Ply Tires except that they have additional layers or cords - or "belts" - circling the tire beneath the tread. This adds some strength to the tires.


Radial Ply Tires



Radial Ply Tires (or just "Radials") have ply cords which run across the center line of the tread and around the tire. The two sets of belts are at right angles to each other. Most belts are made of steel wire ("Steel Belted Radial"), but others are made of ployester or other substances. Most tires today are radial tires.


Drag Racing Tires ("Slicks")



Drag Racing Tires (or more commonly called "Slicks") are a special type of tire used in drag racing where maximum acceleration in a straight line is essential. They are made of a soft compound of rubber, which affords better traction, but they tend to wear out very quickly (around 100 miles). The sidewalls of slicks are designed for straight-line performance, rather than cornering. In fact, as they are perfectly smooth with no tread pattern, they provide almost no traction in turns and thus are not street legal. Slicks are made of a soft compound, which are designed to give way to any stresses. The are also known as "Wrinklewalls", as they actually deform when you stomp on the accelerator. The power from the engine goes through the transmission and rear-end components, through the axles, and is applied to the rear tires. The slicks try to rotate, but are resisted by the friction of the ground. The tire ends up spinning faster at the center of the wheel than at the outer edge near the ground, which results in the tire wrinkling around a portion of the bottom of the tire. The physics behind the wrinkle is this: when the slick wrinkles, the tire creates a larger contact patch with the ground, or area where the surface of the tire and the ground are in contact. Looking at it from the ground's perspective, you would see the original contact patch, and then when power is applied, you hold onto what you had, and the tire forces more of itself down onto the ground on the front. In simple terms, the tire does flatten out, creating a larger contact patch, and thus better traction. What if that first application of power is too much for the ground's friction to hold onto, though? Well, that's simple too: the tires spin, and you lose.

The soft compound of the tire is composed of not just a special formula of rubber, but of some traction-aiding chemicals as well. To activate these chemicals, you have to heat the tires up. The usual method is a burnout. Heating the slicks releases some of the chemicals, and the tires become sticky to the touch. This sure doesn't make them last any longer, but consistent traction is a key to winning races. Not heating the tires is going to lead to traction deficiency, but over-heating the tires can do the same. If too many of the chemicals are released and get too hot, they can actually make the tire slippery.


UPGRADING YOUR TIRES



Standard tires, bias or radial belted, are compounded to provide the best overall combination of tread mileage, traction in both wet and dry conditions, durability in hot and cold driving, stability in turns, resistance to road hazards, etc. Specialty tires will trade one or more of these attributes for improvements in specific areas. Drag slicks with no tread pattern obviously would have no traction in wet conditions, and terrible handling in any kind of driving other than a straight line, but do provide optimum traction on a clean dry surface. DOT approved drag race tires provide some limited driving capability, (albeit very marginal) while maintaining good traction on clean roads. BF Goodrich has introduced a radial tire with improved straight line traction, and the normal good handling of the radial design. In general, long mileage tires are generally weak in straight line traction. If you plan to use conventional tires for drag racing, look for a relatively soft tread compound, a minimum of tread openings, a wide and flat tread, and a larger diameter. Increasing tire diameter by one inch will always increase traction more than increasing the tread width by one inch.


Increasing the Size of the Tires



It is generally accepted that larger diameter tires are preferred for greater performance. This is generally due to two reasons:

  1. They have a larger contact patch. For the same tire width, air pressure, tire construction, and material, a larger diameter tire will have more rubber on the road because the patch is longer from front to back.
  2. They apply the force more parallel to the ground. The force is applied at a tangent to the surface of the wheel. With a larger tire, more of the force will be available to move the car forward.

However, larger diameter tires do have some drawbacks as well. Some of these include:
  1. Larger diameter tires tend to be heavier. The tire is part of your rotated mass and part of the unsprung weight so heavier is bad.
  2. They have more inertia because the rotating mass is further from the center of the tire. Thus, more energy is wasted just to overcome the tires' increased inertia.
  3. They require more room in the wheel well which may mean having to "jack" the car up, which is bad for weight transfer.

  4. At some point, the disadvantages outweigh the advantages so choose the tire size that is appropriate for your vehicle and your wheels.

    As for width, increasing the tire width directly increases the contact patch which increases traction (good) and inertia (bad). Wider tires are best for maximum traction on hard launches (i.e. drag racing) but will result in a decrease in handling if mounted on the same wheel as the original tire.


    READING A TIRE



    The markings on a tire will give the weight (load), size designation, serial number, tire type, carcass cords, DOT, and profile size. Other markings on the tire will probably be the brand name and the name of the tire. Original muscle car tires were expressed as an alpha-numeric code. An example is as follows:

    Alpha Numeric: G78-14


    The "size designation" is a combination of one letter and four numbers and is a three-part code. The letter denotes the sidewall-to-sidewall width of the tire; the two numbers grouped with the letter refer to the height-to-width ratio; and the numbers following the dash indicate in inches the diameter of the wheel rim the tire will fit. Therefore, the above tire is "G" in width, and the sidewall height is 78% of the width. It is designed to be mounted on a 14" wheel.

    Modern tires are expressed in a metric designation such as the following example:

    Metric: P225/60R15 92H


    The first component is the type of tire. A "P" stands for Passenger tire. A "L/T" stands for light truck. This refers to the type of vehicle that this tire is suitable for. This only applies to tires in the U.S. European tires generally don't have this designation. The next three numbers refer to the width of the tire in millimeters. The two digit number after the slash refers to the sidewall height as a percentage of the tire width. The "R" designates that this is a Radial tire, which is important because radial tires should never be mixed with non-radial tires. The two digit number after the R denotes the wheel diameter in inches (yes, it is supposed to be metric but the standard measurement system still lives on). The final two or three digit number refers to the load rating of the tire. Multiply this by four to determine the maximum weight the four tires on the car can reasonably handle. Note that this includes the weight of the vehicle and all passengers and cargo. The load rating can be interpreted by using the following chart:


    Tire Load Rating
    Load Rating Maximum Load (Lbs.) Maximum Load (Kg)
    71 761 345
    72 783 355
    73 805 365
    74 827 375
    75 853 387
    76 882 400
    77 908 412
    78 937 425
    79 963 437
    80 992 450
    81 1019 462
    82 1047 475
    83 1074 487
    84 1102 500
    85 1135 515
    86 1168 530
    87 1201 545
    88 1235 560
    89 1279 580
    90 1323 600
    91 1356 615
    92 1389 630
    93 1433 650
    94 1477 670
    95 1521 690
    96 1565 710
    97 1609 730
    98 1653 750
    99 1709 775
    100 1764 800
    100 1764 800
    101 1819 825
    102 1874 850
    103 1929 875
    104 1984 900
    105 2039 925
    106 2094 950
    107 2149 975
    108 2205 1000
    109 2271 1030
    110 2337 1060


    The last letter refers to the speed rating of the tire. This is defined as the fastest speed that this tire is proven to endure reliably. Note that older tires sometimes had the speed code before the "R, as in 205/75SR15." This speed code can be interpreted by the following chart:

    Tire Speed Rating
    Speed Rating Maximum Speed (MPH) Maximum Speed (KPH)
    N 87 140
    P 93 150
    Q 99 160
    R 106 170
    S 112 180
    T 118 190
    U 124 200
    H 130 210
    V 149 240
    Z 149 + 240 +
    W 168 270
    Y 186 300


    In addition to the above information, all modern tires have the following information stamped on their sidewalls:

    Treadware: The treadwear grade lets you compare how long different tires would last if driven by the same driver under the same road conditions and if the tire is maintained properly. A tire rated 100 will last approximately twice as long as one rated 50. Most tires are rated between 150-300. Note that this is not the same as the mileage warranty on the tire. That is set by the manufacturer and is not regulated by the U.S. Federal government.

    Traction: The traction rating, scored A, B, or C, tells you how well the tires can stop your vehicle on wet roads. An "A" rated tire has the best traction.


    Temperature Resistance: This rating, also scored A, B, or C, measures how well the tire will resist overheating during sustained high speed use. In general, the lower the running temperature, the less likely the tire will fail. A tire graded "A" represents the best performance, and is better than a "B" tire.


    TIRE ROTATION



    Tires should be rotated approximately every 5,000 miles to ensure even wear. The following diagram shows the recommended tire rotation pattern, assuming that you are not rotating the spare tire (because it is a temporary spare or it is not on a matching rim). For a rear wheel drive car, move the rear tires to the front on the same side, and the front tires to the rear on the opposite side. For a front wheel drive car, move the front tires to the rear on the same side and the rears to the front on the opposite side. If you are able to rotate your spare tire with the others, insert the spare into the right rear position and take whatever tire that was supposed to go there as your spare. Also note that anytime you purchase a new tire, it should be as similar as possible to the other tires and should always be placed on the rear axle. Ideally, all four tires should be replaced at the same time to ensure that handling will not be negatively affected by differences in the tires.


    Tire Rotation Pattern



    TIRE VALVE



    The tire valve is really an air check that opens under air pressure and closes when pressure is removed. The inner valve or "valve core," acts as a check valve for the air. Positive sealing is provided by the "valve cap," which contains a soft rubber washer or gasket. It is this gasket, pressed against the end of the "valve stem," that seals the air in the tire. The careless practice of operating tires without the valve cap should not be followed, because, without the valve cap in place, there is usually a slow leak of air from the tire, causing the tire to run in an underinflated condition. If air should leak out around the base of the valve, it will be necessary to install a new tire valve assembly. This is easily accomplished with a special lever-type tool.


    TIRE CONVERSION CHART



    When converting changing wheels and tires, it is important to keep the overall tire diameter as close as possible to the original. As muscle cars originally came with skinny bias-ply tires, there is a large performance gain to be made by switching to wider, more aggressive radial tires. Use the following chart to determine the optimal tire size for your muscle car. Note that the wider tires may need wider rims than your stock rims.


    Tire Conversion Chart
    Overall Wheel Diameter 78 series 14" Bias Ply 70 series 14" Bias Ply 60 series 14" Bias Ply 78 series 15" Bias Ply 70 series 15" Bias Ply 60 series 15" Bias Ply 75 series 14" Radial 70 series 14" Radial 60 series 14" Radial 75 series 15" Radial 70 series 15" Radial 60 series 15" Radial 50 series 15" Radial
    21.8" - - - - - - - - 165/60 R14 - - - -
    22.3" - - - - - - - - 175/60 R14 - - - -
    22.7" - - - - - - - - 185/60 R14 - - - -
    23.0" - - - - - - - - - 135/75 R15 - - -
    23.1" - - - - - - - 165/70 R14 - - - - 205/50 R15
    23.2" - - - - - - - - 195/60 R14 - - - -
    23.6" - - - - - - - 175/70 R14 - 145/75 R15 - - -
    23.7" - - - - - - - - 205/60 R14 - - 185/60 R15 -
    23.9" - - - - - - - - - - - - 225/50 R15
    24.2" A78-14 - - - - - - 185/70 R14 215/60 R14 155/75 R15 - 195/60 R15 -
    24.6" - - - - - - - - 225/60 R14 - - - -
    24.7" B78-14 - - - - - - 195/70 R14 - 165/75 R15 - 205/60 R15 -
    24.8" - - E60-14 A78-15 - - - - - - - - -
    24.9" - - - - - - 185/75 R14 - - - - - -
    25.1" - - - - - - - - 235/60 R14 - - - -
    25.2" C78-14 - - - - - - - - - 185/70 R15 215/60 R15 -
    25.3" - - F60-14 - - - - 205/70 R14 - - - - -
    25.4" - - - B78-15 - - - - - - - - -
    25.5" D78-14 - - - - E60-15 195/75 R14 - - - - - -
    25.6" - - - - - - - - - - - 225/60 R15 -
    25.7" - - - - - - - - - - 195/70 R15 - -
    25.8" - E70-14 G60-14 C78-15 - - - - - - - - -
    25.9" - - - - - F60-15 - 215/70 R14 - 185/75 R15 - - -
    26.0" E78-14 - - - - - - - - - - - -
    26.1" - - - - - - 205/75 R14 - - - - 235/60 R15 -
    26.2" - F70-14 - D78-15 - - - - - - - - -
    26.3" - - - - - - - - - - 205/70 R15 - -
    26.4" - - H60-14 - E70-15 G60-15 - 225/70 R14 - - - - -
    26.5" F78-14 - - - - - - - - 195/75 R15 - - -
    26.6" - - - E78-15 - - - - - - - 245/60 R15 -
    26.7" - - - - - - 215/75 R14 - - - - - -
    26.8" - F70-14 - - - - - - - - - - -
    26.9" - - - - F70-15 - - - - - 215/70 R15 - -
    27.0" - - - - - - - 235/70 R14 - - - 255/60 R15 -
    27.1" G78-14 - - - - H60-15 - - - 205/75 R15 - - -
    27.2" - - - F78-15 - - - - - - - - -
    27.3" - - - - - - 225/75 R14 - - - - - -
    27.4" - - - - - J60-15 - - - - 225/70 R15 - -
    27.5" - H70-14 - - G70-15 - - - - - - - -
    27.7" - - - G78-15 - - - - - 215/75 R15 - - -
    27.8" H78-14 - - - - - - - - - - - -
    27.9" - - - - - L60-15 235/75 R14 - - - - - -
    28.0" - - - - - - - - - - 235/70 R15 275/60 R15 -
    28.1" - - - - H70-15 - - - - - - - -
    28.2" J78-14 - - - - - - - - - - - -
    28.3" - - - - - - - - - 225/75 R15 - - -
    28.4" - - - H78-15 J70-15 - - - - - - - -
    28.7" - - - J78-15 - - - - - - - - -
    28.9" - - - - L70-15 - - - - 235/75 R15 - - -
    29.1" - - - - - - - - - - 255/70 R15 - -
    29.3" - - - L78-15 - - - - - - - - -








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