Tire and Wheel Theory

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 • 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 • Tire cord • Rubber must be reinforced with fabric, fiber, or steel cords • Tire ply design • Bias-ply tires have plies • Cross at 35-45 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 • 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 • 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 • 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