1

Tire size and rating systemsTire size and

rating systems

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Forces acting on tiresForces 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.

• 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.

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Contact patchContact 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.

• 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.

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Tire Terminology:Tire Terminology:

• 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

• 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

• 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

• 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

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Components of tire rubber 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.

• 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

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Cord materialCord 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

• 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

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Cord materialCord 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

• 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

• Polyester – Found on 98% of

OEM and replacement tires

• Nylon• Rayon

Steel bead cables

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Cord constructionCord construction

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

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

Bias

Bias Belted

Radial

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Bias ply tires 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.

• 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.

• Because of the rigidity of the sidewall the tread must flex more as the tire rolls.

• Tread flexing increases rolling resistance and tire wear.

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Radial tiresRadial 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.

• 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.

• 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.

• 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.

Belt

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Bias-belted tiresBias-belted tires

• Bias belted tires have cord plies running at an angle to the centerline.

• They also have a reinforcing belt.

• 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.

• The bias belted tire is considered an improvement over the regular bias ply tire but is generally inferior to a radial ply tire.

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Belt materialBelt 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

• 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

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Belt materialBelt material

• Steel - found on 98% of OEM and replacement tires

• Fiberglass• Aramid fiber

• Steel - found on 98% of OEM and replacement tires

• Fiberglass• Aramid fiber

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Tire treadTire 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.

• 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 compound must be hard enough to reduce rolling resistance and have a long tread life and yet soft enough to provide good traction.

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Tread rubber compoundsTread 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

• 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

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Tread blocksTread 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.

• 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.

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Tread designTread 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

• 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

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Unidirectional tread designUnidirectional 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.

• 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.

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Tread DesignTread 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

• 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

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Tread wear indicatorTread 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

• 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

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Tire sidewallTire 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

• 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

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P-metric tire size/rating systemP-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]

• 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]

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P 225 70/R 15 87 S

P-metric tire size/rating systemP-metric tire size/rating system

ApplicationSection Width

Aspect Ratio

Construction

Load Rating

Rim Width

Speed Rating

The P-metric tire rating is broken down into 7 sections

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P-metric system - Application codeP-metric system - Application code

• The first digit is reserved for the intended use of the tire C - indicating commercial or truck tireT - indicating temporary or spare tireLT - indicates light truckP - indicating passenger car

• The first digit is reserved for the intended use of the tire C - indicating commercial or truck tireT - indicating temporary or spare tireLT - indicates light truckP - indicating passenger car

P 225 70/R 15 87 S

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P 225 70/R 15 87 SP-metric system - Section widthP-metric system - Section width

• 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.

• 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

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Section heightSection 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.

• 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

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Aspect ratioAspect ratio

• The ratio of section height divided by section width is called Aspect Ratio.

• The ratio of section height divided by section width is called Aspect Ratio.

157

mm

225 mm

225 mm157 mm

= .70

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P-metric system - Aspect ratioP-metric system - Aspect ratio

• 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

• 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 225 70/R 15 87 S

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P 225 70/R 15 87 SP-metric system - Aspect ratioP-metric system - Aspect ratio

• 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.

• 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.

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P-metric system - cord construction P-metric system - cord construction

• A letter follows the aspect ratio that represents tire constructionR - indicates radial constructionD - indicates bias ply constructionB - indicates bias belted construction

• A letter follows the aspect ratio that represents tire constructionR - indicates radial constructionD - indicates bias ply constructionB - indicates bias belted construction

P 225 70/R 15 87 S

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P-metric system – rim diameterP-metric system – rim diameter

• 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

• 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 in

ch

P 225 70/R 15 87 S

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P 225 70/R 15 87 SP-metric system – rim diameterP-metric system – rim diameter

• 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]

• 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]

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P 225 70/R 15 87 SP-metric system - load ratingP-metric system - load rating

• 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.

• 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.

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P-metric system - load ratingP-metric system - load rating

74 827

75 853

76 882

77 908

78 937

79 963

80 992

81 1019

82 1047

83 1074

84 1102

85 1135

86 1168

87 1201

88 1235

89 1279

90 1323

91 1356

Load Index

Max Load (lbs.)

Load Index

Max Load (lbs.)

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P 225 70/R 15 87 SP-metric system - load ratingP-metric system - load rating

• 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.

• 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.

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P-metric system - speed ratings P-metric system - speed ratings

• 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.

• 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 225 70/R 15 87 S

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P-metric system - speed ratingsP-metric system - speed ratings

S = 112 mphT = 118 mphU = 124 mphH = 130 mph V = 149 mph W = 168 mphY = 186 mph

S = 112 mphT = 118 mphU = 124 mphH = 130 mph V = 149 mph W = 168 mphY = 186 mph

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Replacement tiresReplacement 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.

• 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.

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UTQG grading systemUTQG grading system

• Uniform Tire Quality Grade • A three digit code that gives the

consumer information on – Treadwear– Traction– Temperature Tread wear

• Uniform Tire Quality Grade • A three digit code that gives the

consumer information on – Treadwear– Traction– Temperature Tread wear

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UTQG - treadwearUTQG - 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.

• 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.

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UTQG - tractionUTQG - 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.

• 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.

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UTQG - temperature resistanceUTQG - 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

• 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

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DOT Safety Standard codesDOT 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.

• 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.

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DOT Safety Standard codesDOT Safety Standard codes

UJNX = (optional) Brand and significant characteristics of the tire

5008 = Date of manufactureB9 = Manufacturers plant code

YR = Manufacturers tire size code

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DOT Safety Standard codesDOT Safety Standard codes

• The last section of the DOT code is the date of manufacture.

• 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

• 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

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Asymmetrical tire orientationAsymmetrical 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]

• 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]

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Unidirectional tire orientationUnidirectional 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.

• 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.

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M + S designation 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.

• 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.

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Snow tiresSnow 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.

• 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.

• 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.

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Tire size – floatation methodTire size – floatation method

• 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.

• 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.

32 x 10.5 R 15 LT

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Tire size – floatation methodTire size – floatation method

• The first part of the tire code is the tire height in inches.

• The second part is the section width in inches.

• The third part is the construction – the same codes are used as in the P-metric system.

• The fourth part is the rim diameter in inches• The last part is the application – normally LT

for light truck.

• The first part of the tire code is the tire height in inches.

• The second part is the section width in inches.

• The third part is the construction – the same codes are used as in the P-metric system.

• The fourth part is the rim diameter in inches• The last part is the application – normally LT

for light truck.

32 x 10.5 R 15 LT

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Tire pressureTire pressure

• The maximum pressure on the tire sidewall is not the pressure that the tires should be inflated to.

• The maximum pressure on the tire sidewall is not the pressure that the tires should be inflated to.

• The correct operating tire pressure is based on the tire load rating and the weight applied to the tire.

• Most manufactures install tires with about 50% more load capacity than is normally needed.

• A vehicle that will normally have 900 lbs of weight at each wheel will have tires with a load rating of about 1350 lbs.

• The correct operating tire pressure is based on the tire load rating and the weight applied to the tire.

• Most manufactures install tires with about 50% more load capacity than is normally needed.

• A vehicle that will normally have 900 lbs of weight at each wheel will have tires with a load rating of about 1350 lbs.

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Tire pressureTire pressure

• The proper tire pressure will be found on a placard on the driver’s door or door jamb or in the owner’s manual.

• Occasionally on a placard is located on the glove compartment lid.

• It may also be found in the vehicle shop manual or AllData.

• The proper tire pressure will be found on a placard on the driver’s door or door jamb or in the owner’s manual.

• Occasionally on a placard is located on the glove compartment lid.

• It may also be found in the vehicle shop manual or AllData.

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Checking tire pressureChecking tire pressure

• The gauge should retain the pressure reading after you remove it from the tire stem but can be easily disturbed by any rapid motion of you hand.

• The gauge should retain the pressure reading after you remove it from the tire stem but can be easily disturbed by any rapid motion of you hand.

• The pressure should be read as you hold the end of the pencil gauge firmly against the tire valve.

• The pressure should be read as you hold the end of the pencil gauge firmly against the tire valve.

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Temporary loadsTemporary loads

• When carrying additional loads in a vehicle the tire pressure is generally increased

• The pressure should never exceed the maximum pressure rating of the tire shown on the sidewall

• When carrying additional loads in a vehicle the tire pressure is generally increased

• The pressure should never exceed the maximum pressure rating of the tire shown on the sidewall

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Excessive tire pressure Excessive tire pressure

• Causes the tread to crown.

• Causes the tread to crown.

• This places more weight in the center of the tread.

• Can cause tire failure• Causes premature tread wear

• This places more weight in the center of the tread.

• Can cause tire failure• Causes premature tread wear

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Insufficient tire pressure may causeInsufficient tire pressure may cause

• Excessive heat in the tread results in tire failure• Poor fuel economy• Premature tire wear

• Excessive heat in the tread results in tire failure• Poor fuel economy• Premature tire wear

• Cause the tire to bow – placing most of the weight on the edges of the tread

• Cause the tire to bow – placing most of the weight on the edges of the tread

58

Wheel constructionWheel construction

• Automotive wheels are generally made of:1. Stamped & welded steel - inexpensive2. Cast aluminum alloy - expensive

• As the aluminum rim has to be much thicker than the equivalent wheel made of steel there is little if any weight difference between steel and aluminum.

• The principle reason for installing aluminum wheels is for appearance.

• Automotive wheels are generally made of:1. Stamped & welded steel - inexpensive2. Cast aluminum alloy - expensive

• As the aluminum rim has to be much thicker than the equivalent wheel made of steel there is little if any weight difference between steel and aluminum.

• The principle reason for installing aluminum wheels is for appearance.

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Aluminum wheelsAluminum wheels

• Aluminum rims will require much greater care in servicing due to the softness of the metal

• They corrode very rapidly and cannot withstand the impact loads of potholes and curbs as well as steel wheels

• Although aluminum alloy wheels are slightly lighter than steel wheels there offer relatively little handling performance improvements over steel rims.

• The only reason for using aluminum is appearance.

• Aluminum rims will require much greater care in servicing due to the softness of the metal

• They corrode very rapidly and cannot withstand the impact loads of potholes and curbs as well as steel wheels

• Although aluminum alloy wheels are slightly lighter than steel wheels there offer relatively little handling performance improvements over steel rims.

• The only reason for using aluminum is appearance.

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Magnesium wheelsMagnesium wheels

• Older high performance and racing cars often used magnesium alloy wheels that are were lighter than aluminum - this reduced unsprung weight , improving handling

• Advances in aluminum alloys and casting technology have made it possible to produce wheels that are nearly as light as magnesium at far lower cost and in the process have made magnesium wheels obsolete.

• There term MAG wheels is a holdover from the 1960s

• Older high performance and racing cars often used magnesium alloy wheels that are were lighter than aluminum - this reduced unsprung weight , improving handling

• Advances in aluminum alloys and casting technology have made it possible to produce wheels that are nearly as light as magnesium at far lower cost and in the process have made magnesium wheels obsolete.

• There term MAG wheels is a holdover from the 1960s

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Plastic wheelsPlastic wheels

• Some manufactures are experimenting with plastic polymer composite wheels - we will probably see this type of wheel in the near future.

• Some manufactures are experimenting with plastic polymer composite wheels - we will probably see this type of wheel in the near future.

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One piece / two piece wheelsOne piece / two piece wheels

• Cast aluminum wheels can be made in one piece but are often constructed out of a center disc section, riveted or bolted to the rim section

• This allows the manufacture to produce modular wheels

• Modular wheels allow for different rim offset without redesigning the entire wheel - only the center section needs to be replaced

• Steel rims have a center disc welded to the rim - this makes them ‘one piece’ wheels

• Cast aluminum wheels can be made in one piece but are often constructed out of a center disc section, riveted or bolted to the rim section

• This allows the manufacture to produce modular wheels

• Modular wheels allow for different rim offset without redesigning the entire wheel - only the center section needs to be replaced

• Steel rims have a center disc welded to the rim - this makes them ‘one piece’ wheels

63

Wheel center holeWheel center hole

• A center hole is needed to allow the wheel to be clamped onto an older tire mounting machine.

• The center hole may also be used to align the center of the wheel with the hub axis.

• When the wheel is center by the center hole a machined ridge on the hub flange fits tightly into the center hole

• This is the most common way of centering the wheel on the axle hub

• The wheel must have the proper center hole diameter if it is to fit onto the hub

• A center hole is needed to allow the wheel to be clamped onto an older tire mounting machine.

• The center hole may also be used to align the center of the wheel with the hub axis.

• When the wheel is center by the center hole a machined ridge on the hub flange fits tightly into the center hole

• This is the most common way of centering the wheel on the axle hub

• The wheel must have the proper center hole diameter if it is to fit onto the hub

64

Lug stud centered wheelsLug stud centered wheels

• After market wheels and truck wheels are often centered by the lug nut studs.

• This allows the wheel to fit on more than just one hub.

• Centering of the wheel on the lug holes is not as precise as centering on the hub as the lug holes are generally stamped or cast with the rim and not necessarily machined to align perfectly with the wheel center axis.

• Passenger cars will normally use 4 or 5 lug nuts - light trucks may use as many as eight

• Since different manufactures use a variety of bolt circle diameters - rims cannot be easily interchanged

• After market wheels and truck wheels are often centered by the lug nut studs.

• This allows the wheel to fit on more than just one hub.

• Centering of the wheel on the lug holes is not as precise as centering on the hub as the lug holes are generally stamped or cast with the rim and not necessarily machined to align perfectly with the wheel center axis.

• Passenger cars will normally use 4 or 5 lug nuts - light trucks may use as many as eight

• Since different manufactures use a variety of bolt circle diameters - rims cannot be easily interchanged

65

Drop center Drop center

• The center of the rim must have a slight depression to allow the tire bead to be fit over the edges of the rim flange during tire mounting

• Without a drop center tire mounting would be impossible

• If a steel band is bolted over the drop center to support the tire beads the vehicle can be driven a short distance with a flat tire , at slow speeds [“run flat” rim]

• The center of the rim must have a slight depression to allow the tire bead to be fit over the edges of the rim flange during tire mounting

• Without a drop center tire mounting would be impossible

• If a steel band is bolted over the drop center to support the tire beads the vehicle can be driven a short distance with a flat tire , at slow speeds [“run flat” rim]

Drop center

66

Lug stud holesLug stud holes

• The lug nut holes in steel rims are flared outward

• When the lug nuts are properly torqued the steel around the nuts is compressed inward - somewhat like a Belleville spring - this holds a constant tension on the lug nut preventing them from loosening

• Aluminum wheels often use a hard steel insert embedded in the wheel lug nut bore -

• This prevents the lug nuts from gouging the wheel and helps to prevent the nuts from loosening

• Alternatively - some aluminum rims use special lug nuts with flat washers to prevent loosening

• The lug nut holes in steel rims are flared outward

• When the lug nuts are properly torqued the steel around the nuts is compressed inward - somewhat like a Belleville spring - this holds a constant tension on the lug nut preventing them from loosening

• Aluminum wheels often use a hard steel insert embedded in the wheel lug nut bore -

• This prevents the lug nuts from gouging the wheel and helps to prevent the nuts from loosening

• Alternatively - some aluminum rims use special lug nuts with flat washers to prevent loosening

67

Wheel dimensions – wheel offsetWheel dimensions – wheel offset

• The center section of the wheel is flat allowing the rim to come in even contact with the axle flange.

• This flange contact area is normally offset outboard a small amount from the wheel centerline.

• This helps position the weight of the vehicle directly over the wheel bearing.

• When the flange is closer to the outside of the rim it is called positive offset.

• This is common on front wheel drive cars as additional room is required in the wheel well area for the constant velocity joints and brake calipers.

• The center section of the wheel is flat allowing the rim to come in even contact with the axle flange.

• This flange contact area is normally offset outboard a small amount from the wheel centerline.

• This helps position the weight of the vehicle directly over the wheel bearing.

• When the flange is closer to the outside of the rim it is called positive offset.

• This is common on front wheel drive cars as additional room is required in the wheel well area for the constant velocity joints and brake calipers.

Wheel center

Hub flange

Offset

68

Wheel dimensions - Rim diameterWheel dimensions - Rim diameter

• Measured from the bottom of the bead flange

• Normally 13, 14 or 15 inches for passenger cars

• Light trucks may be larger

• Some European cars use metric rim diameter dimensions

• Size may be stamped on rim [as in 14jj]

• Measured from the bottom of the bead flange

• Normally 13, 14 or 15 inches for passenger cars

• Light trucks may be larger

• Some European cars use metric rim diameter dimensions

• Size may be stamped on rim [as in 14jj]

15 in

ch

69

Wheel dimensions - rim width Wheel dimensions - rim width

• May range from 4 inches up to 10 inches for passenger cars

• Normally measured in 1/2 inch increments

• Measured from bead flange to bead flange

• May range from 4 inches up to 10 inches for passenger cars

• Normally measured in 1/2 inch increments

• Measured from bead flange to bead flange

8 inch

70

Wheel dimensions - bolt circle diameterWheel dimensions - bolt circle diameter

• Measured in inches• Measured from

center of the wheel to center of lug nut hole

• Direct measurement of a 5 bolt circle diameter requires calculations using trigonometry.

• Measured in inches• Measured from

center of the wheel to center of lug nut hole

• Direct measurement of a 5 bolt circle diameter requires calculations using trigonometry.

4 ½

inch

71

Wheel dimensions - backspacingWheel dimensions - backspacing

• Distance from the hub flange mounting surface to the outside edge of the rim

• Necessary to determine fender clearance

• Distance from the hub flange mounting surface to the outside edge of the rim

• Necessary to determine fender clearance

Rim edge

Hub flange

Backspace

72

Wheel dimensions - dishWheel dimensions - dish

• Distance from the hub mounting surface to the inside edge of the rim

• Necessary to determine suspension and inner fender clearance

• Center hole diameter• Required to insure

proper fit and centering on axle hub

• Distance from the hub mounting surface to the inside edge of the rim

• Necessary to determine suspension and inner fender clearance

• Center hole diameter• Required to insure

proper fit and centering on axle hub

Inside rim edge

Hub flange

Dish

73

Tire heightTire height

• Tire height is the distance from the pavement to the top of the tire.

• Typically about 26 inches for a passenger car.

• Tire height is the distance from the pavement to the top of the tire.

• Typically about 26 inches for a passenger car.

26 i

nch

P 195 75/R 15 87 S

74

Plus sizingPlus sizing

• Installing new 50 series tires on the original 15 inch rim will result in a tire that is almost 4 inches lower.

• There is now a speedometer error of 9 mph.

• Since the footprint is shorter the tire has less traction.

• Installing new 50 series tires on the original 15 inch rim will result in a tire that is almost 4 inches lower.

• There is now a speedometer error of 9 mph.

• Since the footprint is shorter the tire has less traction.

26 i

nch

P 195 50/R 15 87 S

22.6

inc

h

75

Plus sizingPlus sizing

• Installing new 50 series tires on a new 17.5 inch rim restores the tire back to it’s original height.

• Normally when custom tires and rims are installed the rim width will be increased as well.

• Installing new 50 series tires on a new 17.5 inch rim restores the tire back to it’s original height.

• Normally when custom tires and rims are installed the rim width will be increased as well.

26 i

nch

P 195 50/R 17.5 87 S

22.6

inc

h

76

Plus sizingPlus sizing

• Increasing rim width will also increase overall tire height.

• Rim width is limited by fender and suspension clearance.

• Vehicles with wishbone suspension systems are limited in tire and rim selection by the proximity of the upper ball joint to the tire.

• Increasing rim width will also increase overall tire height.

• Rim width is limited by fender and suspension clearance.

• Vehicles with wishbone suspension systems are limited in tire and rim selection by the proximity of the upper ball joint to the tire.