1. Tire and Wheel Theory
Chapter 61

2. Objectives Describe how a tire is constructed
Understand the various size designations of tires
Tell the design differences between radial and bias tires
Be able to select the best replacement tire for a car

3. Introduction Service technician should be able to:
Discuss aspects of tire design
Help customers make safest choice
Tires and wheels important safety and service specialty area

4. Tire Construction Several layers of rubber, cords, two rings of wire
Casing (carcass) – internal tire structure
Ply – metal or fabric rubberized cord
Provide strength
Ends of plies wrap around steel bead
Bonded to side of tire
Beads – coils of wire at side edges
Chafing strips protect beads from rim
Belt – cord structure made of plies
Under the tread only
Tread – section of tire that rides on the road

5. Tire Cord and Tire Ply Design
Rubber must be reinforced with fabric, fiber, steel cords
Bias-ply tires have plies that cross at 35-45° angles
Ride softer, but wear faster
Radial-ply tires have casing plies that run across the tire from bead seat to bead seat
Longer tread life, better grip, improved fuel economy
Larger footprint gives better grip

6. Tire Tread Grooves in tread allow traction on wet surfaces
Allow tire to flex without squirming
Design is a compromise
Sipes – small grooves in tread like knife cuts
Clear water off the road
Ribs pump water through grooves to back of tire
Different tread patterns for different driving conditions
Asymmetrical patterns improve wet performance

7. Tire Tread
Tire tread depth gauge.

8. Tire Tread
Wear indicators are also called wear bars. When tread depth is down to the legal limit of 2/32", bald strips appear across the tread.

11. Tire Tread Material Rubber must be vulcanized (heated) to be stable
Chemicals added to natural rubber to improve performance

12. Tubeless Tires and Traction
Inner liner bonded to tire
Seals air into tire
Thicker than liner on tube-type tire
Tubeless tires safer than tube-type
Does not go flat immediately when punctured
Traction
How well tire grips the road
Affected by:
Road surface, contaminants
Tread material, inflation pressure, tread width, etc.

13. Tire Sidewall Markings
Tire size listed on sidewall
Profile is the tire’s height
Aspect ratio is height-to-width ratio
Load index – maximum load at designated speed rating
Related to strength of sidewall plies
Speed rating indicates better handling characteristics
New speed ratings developed for speeds over 168 mph

16. Some tires use letters at the end of the tire size (suffixes) to indicate special applications including the following. LT = light truck ML = mining and logging MH = mobile home ST = special trailer TR = truck

17. Service Description P205/75R x 15 92H 205 cross-sectional width in mm 75 aspect ratio R radial construction 15 rim diameter in inches 92 load index H speed rating (130 mph/210 km/h)

18. Load Rating
How much weight a tire can safely support at a specified air pressure
Amount of load determined by area of tire and air pressure in it
Vehicle’s gross weight rating (GVW or GVWR) includes weight of vehicle, passengers, luggage
Curb weight – weight of vehicle without people
DOT symbol indicates the tire meets DOT safety standards

19. Load Index

20. Ply Rating / load range

21. speed rating Letter Maximum Rated Speed L 120 km/h (75 mph) MQ
160 km/h (99 mph) R
170 km/h (106 mph) S
180 km/h (112 mph) T
190 km/h (118 mph) U
200 km/h (124 mph) H
210 km/h (130 mph) V
240 km/h (149 mph) W
270 km/h (168 mph) Y
300 km/h (185 mph) Z
Open-ended*
*The exact speed rating for a particular Z-rated tire is determined by the tire manufacturer and may vary according to size. For example, not all Brand X Z-rated tires are rated at 170 mph, even though one size may be capable of these speeds.
speed
rating

22. Tread Wear The tread wear grade is a comparison rating based on the wear rate of a standardized tire, tested under carefully controlled conditions, which is assigned a value of 100. A tire rated 200 should have a useful life twice as long as the standard tire's. A tire wear rating of 100 is equal to 10,000 miles

23. Traction Traction performance is rated by the letters AA, A, B, or C, with AA the highest. IMPORTANT NOTE: The traction rating is for wet braking distance only! It does not include cornering traction or dry braking performance.

24. All-Season Tires Radial tires have more traction on snow
Specially designed pockets and slots
Labeled with mud and snow designation

25. HIGH-FLOTATION TIRE SIZES These tires are usually larger than conventional tires and usually require a wider than normal wheel width. High-flotation tires have a size designation such as 33 x 12.50R x 15LT: 33 = approximate overall tire diameter in inches = approximate cross-sectional width in inches R = radial-type construction 15 = rim diameter in inches LT = light truck designation

26. Temperature Resistance Temperature resistance is rated by letters A, B, or C, with A the highest rating.

27. ALL-SEASON TIRE DESIGNATION Most all-season tires are rated and labeled as M & S, MS, or M + S, and therefore must adhere to general design features as specified by the Rubber Manufacturers Association (RMA). One design feature is that the tire has at least 25 percent void area. This means that the tread blocks have enough open space around them to allow the blocks to grab and clean themselves of snow and mud.

28. Snow Tires Snow tires have deeper tread grooves
Should be installed on all four wheels
Most manufacturers recommend against chains
Cable chains are not as effective as conventional

29. Low Pressure Monitoring
Run-flat tire has stiffer sidewall, tighter bead
Can partly support the car even with no air
Can be driven up to 70 miles without damage
All new vehicles since 2006 have low-pressure warning system
Low tire pressure monitored using direct or indirect method

30. Low Pressure Monitoring

31. DOT TIRE CODE All tires sold in the United States must be approved by the U.S. Federal Department of Transportation (DOT). NOTE: Most race tires are not DOT-approved and must never be used on public streets or highways. The DOT code includes letters and numbers such as MJP2CBDX264. The first two letters identify the manufacturer and location. For this example, the first two letters (MJ) mean that the tire was made by the Goodyear Tire and Rubber Company in Topeka, Kansas. The last three numbers are the build date code. The last of these three numbers is the year (1994), and the 26 means that it was built during the 26th week of Starting with tires manufactured after January 1, 2000, the tire build date includes four digits rather than three digits. The new code such as “3496” means the 34th week of 2006 (“3406”).

33. UTQG System

34. Tire Quality Grading and Tread Wear
UTQG system rates tread wear, traction, temperature dissipation ability
Tread wear varies with:
Wheel alignment
Road surface texture
Tire rotation maintenance
Vehicle speed and braking practices
Weight of vehicle
Size of tire
Tread wear ratings range from 100 to 500, in increments of A tire wear rating of 100 is equal to 10,000 miles

35. Traction Grade First letter in tire rating is traction grade
Indicates stopping ability on wet asphalt and concrete
Braking only in straight ahead direction
Second letter is temperature grade
Tire’s resistance to generating heat
Ability to dissipate heat at highway speeds
Compact spare tire is for temporary use only
Many limited to 31 mph for 31 miles

36. Changing Tire Size
If tire size changed, substitute tire with equal or greater load-carrying capacity
As diameter of tire increases, load capacity increases
If lower profile tire installed, wider tire and larger diameter wheel used
Overall diameter of replacement tire must be +2% to -3% of original tire
When tire diameter changed, speedometer must be recalibrated

37. Outside diameter is calculated by adding the wheel diameter to the cross-sectional height of the tire, multiplied by 2.

38. Wheels Wheels have two parts: center (flange) and rim
Drop center (rim well) facilitates removal and installation of tire
Safety beads keep the tire bead on bead seat
Hub-centric – center of wheel has machined counterbore that pilots on machined area of hub
Stud-centric – wheels locate on wheel studs

39. Rim components

40. Wheel Offset Offset is a very important variable in wheel design
Wheel Offset Offset is a very important variable in wheel design. If the center section (spider) is centered on the outer rim, the offset is zero. Wheel offset is often referred to as ET, which stands for Einpress Tieffe in German.

42. On four-lug axles and wheels, the measurement is simply taken from center to center on opposite studs or holes.

43. On five-lug axles and wheels, it is a little harder
On five-lug axles and wheels, it is a little harder. One method is to measure from the far edge of one bolt hole to the center of the hole two over from the first.

44. Custom Wheels Mag wheels use magnesium silicon alloy
Expensive, not corrosion resistant
Custom wheels for street use are single piece castings of light alloy aluminum
Weather resistant coating
Wheel offset – difference between rim centerline and mounting surface of the wheel
Negative offset increases track width of tires
Positive offset found on front-wheel-drive cars

45. Negative Offset The wheel has a negative offset if the center section is inboard (or “dished”) from the wheel centerline.

46. Back Spacing Back spacing, also called rear spacing or backside setting, is the distance between the back rim edge and the wheel center section mounting pad. Back spacing can be measured directly with a ruler.

47. Lug Studs and Lug Nuts Number of lug studs depends on vehicle load
Bolt pattern 6-5½ is six-bolt pattern spaced around 5½ inch circle
Lug nuts may be standard or metric
Lug nuts have serrated shank to remain tight during tightening
Lug nuts for cast wheels are long and thick
Must be used with a washer

48. If replacement wheels are used on any vehicle, check with the wheel manufacturer as to the proper type and style lug nut.

49. Tire Valve Stems Passenger car valve stems usually rubber
Designed for pressures less than 62 psi
Spring loaded valve core screwed into valve stem
Some have a gasket to prevent air loss past valve core
Short stem used when there is a hub cap
Long stem accommodates full wheel covers

52. Most vehicles have recommended tire inflation figures written in the owner's manual or on a placard or sticker on the door post or glove compartment.

54. Proper inflation