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
Tread
Sidewall & carcass
Liner
Bead gum strip
Cable liner
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.

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 Polyester Found on 98% of OEM and replacement tires
Nylon
Rayon
Steel bead cables
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

8. Cord construction
Bias
Tires are generally divided into 3 categories by the type of cord construction used
Bias ply
Bias belted
Radial ply
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 /R S
Speed Rating
Load Rating
Rim Width
Construction
Aspect Ratio
Section Width
Application

24. P-metric system - Application code
P /R S
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

25. P-metric system - Section width
P /R S
225 mm
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.

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

28. P-metric system - Aspect ratio
P /R 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 /R 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 /R 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 /R 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 /R 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 /R 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.) 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

35. P-metric system - load rating
P /R 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 /R 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 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
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.
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.

50. Tire size – floatation method
32 x 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.

51. Tire size – floatation method
32 x R 15 LT
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.

52. Tire pressure
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.

53. Tire 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.

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

55. Temporary 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

56. Excessive tire pressure
Causes the tread to crown.
This places more weight in the center of the tread.
Can cause tire failure
Causes premature tread wear

57. Insufficient tire pressure may cause
Cause the tire to bow – placing most of the weight on the edges of the tread
Excessive heat in the tread results in tire failure
Poor fuel economy
Premature tire wear

58. Wheel construction Automotive wheels are generally made of:
Stamped & welded steel - inexpensive
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.

59. Aluminum 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.

60. Magnesium 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

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

62. One 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

63. Wheel 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

64. Lug 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

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]

66. Lug 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

67. Wheel dimensions – wheel offset
Wheel center
Hub flange
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.
Offset

68. Wheel 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]
15 inch

69. Wheel dimensions - rim width
8 inch
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

70. Wheel 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.
4 ½ inch

71. Wheel dimensions - backspacing
Hub flange
Rim edge
Distance from the hub flange mounting surface to the outside edge of the rim
Necessary to determine fender clearance
Backspace

72. Wheel dimensions - dish
Inside rim edge
Hub flange
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
Dish

73. Tire height
Tire height is the distance from the pavement to the top of the tire.
Typically about 26 inches for a passenger car.
26 inch
P /R S

74. Plus 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.
26 inch
22.6 inch
P /R S

75. Plus 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.
26 inch
22.6 inch
P /R S

76. Plus 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.