Tire and Wheel Theory Chapter 61

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

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

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

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

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

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

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 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

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

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

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

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

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

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

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

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

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

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

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