Tire size and rating systems

Tire size and rating systems

Forces acting on tires • The tires must support the weight of the vehicle. • The forces of braking, cornering and acceleration must also be transmitted from the vehicle to the road through the tires. • The tires also act as springs to cushion the vehicle from road shock.

Contact patch • The contact area of the tire is only a few square inches. • All vehicle control - acceleration, braking and cornering must be accomplished through this contact area.

Tire Terminology: • Tread - the area in contact with the road • Sidewall- the area between bead and tread • Bead - area where tire is in contact with the rim • Bead cable - steel cables embedded into the sidewall area - for reinforcement • Cord [carcass] - woven fabric that is wrapped around the tire - imbedded in the rubber -from bead cable to bead cable • Liner - an air tight rubber coating on the inside surface of the tire • Belt - found on radial & bias belted tires - required to reinforce the cord material on radials and bias belted tires

Components of tire rubber • Sources of rubber • Synthetic rubber compounds made from petroleum. • Natural rubber [Latex] from rubber plants. • Soybean oil can be used as a replacement for petroleum based synthetic rubber. Tread Sidewall & carcass Liner Bead gum strip Cable liner

Cord material • Must be strong and flexible • Arranged in layers or plies • At one time the number of plies denoted the strength of the tire - modern 2 ply tires however are just as strong as 4 ply tires - reducing the number of plys reduces friction between layers making the tire last longer

Cord material • The cord material is woven into a fabric that is wrapped around the steel bead cables. • Multiple layers of fabric called plies are used to support and reinforce the tire. • Passenger car tires will typically have only two plies. • Truck tires will have 4 or more plies • Polyester • Found on 98% of OEM and replacement tires • Nylon • Rayon Steel bead cables

Cord construction • Tires are generally divided into 3 categories by the type of cord construction used • Bias ply • Bias belted • Radial ply Bias Bias Belted Radial

Bias ply tires • Bias ply tires have the cord material running at an angle relative to the tire centerline. • Alternate layers are at opposite angles. • By alternating the angles of the cord layers the sidewall and tread are very strong and rigid. • Because of the rigidity of the sidewall the tread must flex more as the tire rolls. • Tread flexing increases rolling resistance and tire wear.

Radial tires • Radial tires have all the cord material running at right angles relative to the tire centerline • The cords are parallel to each other. • This makes the sidewalls much more flexible - there is no diagonal bracing as in a bias ply tire that would cause the sidewalls to be ridged. Belt • A belt of high strength synthetic fiber or steel mesh must be embedded below the tread to prevent the tire cords from separating. • The belt adds rigidity to the tread - reducing rolling friction.

Bias-belted tires • Bias belted tires have cord plies running at an angle to the centerline. • They also have a reinforcing belt. • The bias belted tire is considered an improvement over the regular bias ply tire but is generally inferior to a radial ply tire.

Belt material • Must be stiffer and stronger than the cord material • Prevents the tire tread from squirming as it rolls through the contact area • Also prevent the formation of standing waves in the tread area as the tire rolls

Belt material • Steel - found on 98% of OEM and replacement tires • Fiberglass • Aramid fiber

Tire tread • Made of a mixture of natural [latex] and synthetic rubber [butyl rubber] • Deep grooves are molded into the tread to allow water to escape tread area - prevents hydroplaning. • Tread compound must be hard enough to reduce rolling resistance and have a long tread life and yet soft enough to provide good traction.

Tread rubber compounds • Rubber compound is a compromise between soft and hard rubber formulations • Soft rubber will have excellent traction - but tire mileage will be poor • Hard rubber will have poor traction but will have very good tire mileage

Tread blocks • For maximum traction in mud and snow the tread is broken up into a series of blocks that provide a ‘bite’ when driving on loose surfaces. • This type of tread pattern has a lot of rolling resistance so fuel economy is reduced. • Blocks are arraigned in varying in sizes and shapes to help reduce road noise.

Tread design • Smooth tread with small grooves gives the tread maximum dry traction. • Lateral grooves called ‘sipes’ help pump water away from the contact patch when the car is driven on rain covered roads. • Aggressive tread patterns tend to make a lot of tire noise Summer tire

Unidirectional tread design • For increased resistance to aquaplaning many tire manufactures have developed a tread designs where the water is channeled diagonally to the side and rear of the tire. • This type of tire can rotate in only one direction. • A rotation arrow is usually embedded in the sidewall.

Tread Design • Most tires sold in the northeast are a combination of snow and conventional patterns - called all season tires • Random size tread block patterns in modern all season tires have reduced tire noise [drone] to an acceptable level

Tread wear indicator • Embedded in the grooves are slightly raised sections that will show up as bars when the tread is worn down to the legal limit - called tread wear indicators

Tire sidewall • Made from mixture of natural and butyl rubber • Softer and more flexible than tread rubber • Whitewalls and raised white letters have rubber compound containing titanium dioxide embedded in the sidewall area

P-metric tire size/rating system • Modern tires use an international tire rating system to identify the tire size and load capacity. • Standards for the P-metric system are set by the ISO [International Standards Organization]

P-metric tire size/rating system • The P-metric tire rating is broken down into 7 sections P 225 70/R 15 87 S Speed Rating Load Rating Rim Width Construction Aspect Ratio Section Width Application

P-metric system - Application code • The first digit is reserved for the intended use of the tire C - indicating commercial or truck tire T - indicating temporary or spare tire LT - indicates light truck P - indicating passenger car P 225 70/R 15 87 S

P-metric system - Section width P 225 70/R 15 87 S • The second group has 3 digits and indicates the section width in millimeters. • Section width is the distance between sidewalls measured midway between bead and tread - with the tire at normal inflation pressure. • Normally expressed in millimeters • Normally slightly larger than the width of the rim. 225 mm

Section height • The distance from the tread to the bead - with the tire at normal inflation pressure • This dimension is not printed on the sidewall or part of the P-metric code but it can be calculated multiplying the section width times the aspect ratio. 157 mm

Aspect ratio 225 mm • The ratio of section height divided by section width is called Aspect Ratio. 157 mm = .70 225 mm 157 mm

P-metric system - Aspect ratio P 225 70/R 15 87 S • Aspect ratio is sometimes called “Series” • A tire with a 70 aspect ratio has a sidewall that is 70% of the section width. • The decimal point is normally left off. • Usually rounded to the nearest five [%] • Most standard equipment tires are 70 series • Tires with 60% aspect ratios or less are often called “low profile” tires

P-metric system - Aspect ratio P 225 70/R 15 87 S • Decreasing the aspect ratio generally increases lateral traction. • Lowering the aspect ratio diminishes ride quality as the tire can no longer cushion the movement of the suspension. • The is no noticeable improvement in handling with aspect ratios lower than 50. • Aspect ratios of 70 and 80 offer the best ride quality.

P-metric system - cord construction P 225 70/R 15 87 S • A letter follows the aspect ratio that represents tire construction R - indicates radial construction D - indicates bias ply construction B - indicates bias belted construction

P-metric system – rim diameter P 225 70/R 15 87 S • Expressed as the diameter at the point where the rim contacts the tire bead • Normally expressed in even inches • Some high performance rims use 1/2 inch increments • Some European cars are using metric sized rims 15 inch

P-metric system – rim diameter P 225 70/R 15 87 S • Smallest rim currently used on a few sub compact vehicles is 12 inches in diameter • 13, 14, and 15 inch rims make up about 98% of the passenger tire rims on the market • Rim diameter is commonly stamped on steel rims [example 14jj would indicate a 14 inch rim]

P-metric system - load rating P 225 70/R 15 87 S • The load rating index is the a number designating the maximum load [weight] the tire can carry safely at it’s rated speed. • In the P-metric tire size system the load rating is a number between 75 and 115 just before the last digit [letter] in the tire rating.

P-metric system - load rating Load Index Max Load (lbs.) Load Index Max Load (lbs.)

P-metric system - load rating P 225 70/R 15 87 S • Modern tires also have the maximum load in pounds and kilograms at the tire maximum recommended inflation pressure molded into the sidewall - near the bead.

P-metric system - speed ratings P 225 70/R 15 87 S • Normally expressed as a letter representing the maximum sustained speed that the tire can driven at without developing excessive heat. • Speed rating is needed for driving in Europe where speed limit on some superhighways is based on the tires installed on the car. • An ‘S’ rated tire can be driven at sustained speeds of up to 112 mph without overheating is the minimum standard for European passenger cars.

P-metric system - speed ratings S = 112 mph T = 118 mph U = 124 mph H = 130 mph V = 149 mph W = 168 mph Y = 186 mph

Replacement tires • Replacement tires must have the same or higher load index and speed rating as the OEM tire. • Optional tire sizes are often listed on the tire placard.

UTQG grading system • Uniform Tire Quality Grade • A three digit code that gives the consumer information on • Treadwear • Traction • Temperature Tread wear

UTQG - treadwear • The treadwear grade is a comparative rating based on the wear rate of the tire as compared to a reference tire that has a number of 100. A tire graded 150 would wear one and one-half times as well in the government test procedure as a the reference tire graded at 100. • The reference tire expected to last about 30000 miles - if properly driven and maintained.

UTQG - traction • Traction is graded from highest to lowest as ‘AA’, ’A’, ‘B’, and ‘C’. • The letter indicates a tire’s ability to stop on wet pavement.

UTQG - temperature resistance • The temperature grade is a measure of how resistant to heat buildup the tire is under high speed driving and driving with heavy loads. • A letter from ‘A’ to ‘C’ • ‘A’= excellent ……… ‘C’ = mediocre

DOT Safety Standard codes • At the base of the sidewall there is a DOT [Department of Transportation] code. • There are 4 sections to the code after the letters DOT. • In the first section the first two letters indicate the manufacturer and plant code.

DOT Safety Standard codes UJNX = (optional) Brand and significant characteristics of the tire B9 = Manufacturers plant code 5008 = Date of manufacture YR = Manufacturers tire size code

DOT Safety Standard codes • The last section of the DOT code is the date of manufacture. • The first two digits are the week of the year. • The second two digits are the last two digits of the year • 2112 would indicate that the tire was made in week 21 of 2012. • If the last section has only 3 digits the tire was made prior to 2000 • 436 would indicate that the tire was made in the 43rd week of 1996

Asymmetrical tire orientation • Asymmetrical tires have non-symmetrical tread pattern where the pattern on the inside edge is different than on the outside. • These tires have mounting label embedded in the sidewall indicating which way the sidewall faces [inside or outside]

Unidirectional tire orientation • Tires designed for driving in heavy rain often have an angular tread design the pumps water through the sipes and groves to the rear of the tire contact patch. • Do to this design the tire can rotate in only one direction. • And arrow is embedded in the sidewall to indicated the proper installation on the vehicle.

M + S designation • All season tires have a M+S designation following the P-metric designation. • Tires that do not have the M+S designation are considered summer or touring tires. • In some areas of the country you will not be allowed to drive on some roads during winter unless you have tires with the M+S designation.

Snow tires • Snow tires are inferior to conventional tires when driven on dry or rain soaked roads in warm weather conditions. • Most states require that snow tires can only be used during the winter months and must be replaced with summer or M+S tires for the rest of the year. • Studs can be added to most snow tires to improve performance on ice. • The mountain and snowflake symbol indicate that the tire is designed as a true snow tire. • Snow tires are specifically designed with aggressive tread patterns for maximum traction in snow.

Tire size – floatation method 32 x 10.5 R 15 LT • Truck tires normally use the ‘Floatation’ method of tire sizing instead of the P-metric system. • In the flotation system the unit of measure is inches.

Tire size and rating systems