tire and wheel theory

© 2012 Delmar, Cengage Learning

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

– 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• 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


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

– 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

– 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

– 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

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