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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.

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Tire size and rating systems

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  1. Tire size and rating systems

  2. 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.

  3. 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.

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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.

  10. 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.

  11. 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.

  12. 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

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

  14. 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.

  15. 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

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. 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

  21. 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

  22. 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]

  23. 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

  24. 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

  25. 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

  26. 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

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

  28. 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

  29. 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.

  30. 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

  31. 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

  32. 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]

  33. 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.

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

  35. 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.

  36. 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.

  37. 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

  38. 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.

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

  40. 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.

  41. 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.

  42. 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

  43. 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.

  44. 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

  45. 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

  46. 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]

  47. 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.

  48. 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.

  49. 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.

  50. 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.

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