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 vehicle

3. Introduction Service technicians often advise customers
Aspects of tire design
Help make the safest choice
Tires and wheels
Important safety and service specialty area

4. Tire Construction Layers of rubber, cords, and two rings of wire
Casing (i.e., carcass): internal tire structure
Ply: metal or fabric rubberized cord
Ends of plies wrap around steel bead
Bonded to side of tire
Beads: coils of wire at side edges
Belt: cord structure made of plies
Under the tread only
Tread: section of tire that rides on the road

5. Tubeless Tires Inner liner bonded to tire Safer than tube-type
Seals air into tire
Thicker than liner on tube-type tire
Safer than tube-type
Does not go flat immediately when punctured

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

9. Tire Tread Material Tread material calls for compromise
Hard materials: might wear longer but not provide sufficient traction
Soft materials: must provide sufficient wear
Hysteresis describes rubber’s energy absorption characteristics
High hysteresis: quiet running, and better wet and dry grip
Low hysteresis: good lateral stability, low rolling resistance, and minimized tread wear

10. Tire Cord and Tire Ply Design
Rubber must be reinforced with fabric, fiber, or steel cords
Tire ply design
Bias-ply tires have plies
Cross at degree angles
Ride softer, but wear faster
Radial-ply tires have casing plies
Run across the tire from bead seat to bead seat
Longer life, better grip and fuel economy

11. Tire Sidewall Markings
Tire size: listed on sidewall
Profile: tire’s height
Aspect ratio: height-to-width ratio
Cars may be equipped with a spare tire
Temporary and many have limit speeds
Speed rating indicates better handling characteristics
Load index: maximum load at speed rating
New speed ratings have been developed for speeds over 168 mph

13. 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
Gross weight rating (GVW or GVWR) includes weight of vehicle, passengers, and luggage
Curb weight: weight of vehicle without passengers or luggage
DOT symbol signifies the tire meets DOT safety standards

14. All-Season Tires Specially designed pockets and slots
Labeled with mud and snow designation

15. Snow Tires and Tire Pressure Monitoring System
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
Run-flat tire have stiffer sidewall and tighter bead
Can partly support the car even with no air
Low-pressure warning system
Installed on all cars since 2006
Direct or indirect TPMS

17. Uniform Tire Quality Grading
UTQG system rates tread wear, traction, and temperature dissipation ability
Tread wear varies
Wheel alignment
Road surface texture
Tire rotation maintenance
Vehicle speed and braking practices
Weight of vehicle
Size of tire
Ratings range: 100 to 500 in increments of 20

18. 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

19. Changing Tire Size
Tire size is changed: substitute tire with equal or greater load-carrying capacity
Diameter of tire increases: load capacity increases
Lower profile tire installed: wider tire and larger diameter wheel is used
Overall diameter of replacement tire: must be positive two to three percent of original tire
Tire diameter changed: speedometer must be recalibrated

21. Wheels Wheels have two parts Drop center (i.e., rim well) Safety beads
Center (i.e., flange) and rim
Drop center (i.e., rim well)
Facilitates removal and installation of tire
Safety beads
Keep the tire bead on bead seat
Methods to center wheels on hub
Hub-centric: center of wheel has machined counterbore that pilots on machined area of hub
Stud-centric: wheels locate on wheel studs

22. Custom Wheels Several types Wheel offset
Aluminum wheels: cast, forged, or rolled
Race cars: use alloy wheels
Custom wheels for street use: single piece castings of light alloy aluminum
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

23. Lug Studs and Lug Nuts Lug studs Lug nuts
Number of lug studs depends on vehicle load
Different lug bolt patterns are used
Lug nuts
May be inch-standard or metric
Have serrated shank to remain tight during tightening
Lug nuts for cast wheels are long and thick
Must be used with a washer

24. Tire Valve Stems Passenger car valve stems are usually rubber
Designed for pressures less than 62 psi
Valve stems have a screw-on dust cap
Some have a gasket that prevents air loss past the valve core