Tire and wheel lesson

Chapter 36

Wheels and Tires Theory

NATEF Tasks

• There are no NATEF tasks in this chapter.

Knowledge Objectives

(1 of 2)

• K36001 Explain the principles of tire distortion and center of

gravity.

• K36002 Describe the functions and features of wheels and

tires.

• K36003 Describe purpose, function, and types of a wheel

and wheel lug nuts/studs.

• K36004 Describe the components of a vehicle’s tire.
• K36005 Decipher the tire markings on the sidewall of each

tire.

Knowledge Objectives

(2 of 2)

• K36006 Describe the common safety features found on

today’s tires.

• K36007 Describe the purpose, function, and types of tire

pressure monitoring systems.

• K36008 Describe the purpose and function of run-flat tires.
• K36009 Describe the purpose and function of self-sealing

tires.

• K36010 Describe the purpose and function of space saver

spare tires.

Skills Objectives

• There are no Skills Objectives in this chapter.

Introduction

• The contact between the tire

and the road surface plays a
large role in determining how
a vehicle handles and rides

• Today’s newer, lightweight

vehicles are more sensitive
than ever to minor tire or
wheel issues

Wheels and tires provide the connection between
the vehicle and road surface.
DaveAlan/E+/Getty

Principles

(1 of 2)

• K36001 Explain the principles of tire distortion and

center of gravity.

• Tires by themselves have very little rigidity
• Wheels are very rigid and have no way of absorbing the

unevenness of the road surface

• Tires on most passenger vehicles are called pneumatic tires

as they are filled with pressurized air

• Recent vehicles use tubeless tires
• The air pressure gives the tire its shape when the weight of

the vehicle is sitting on it

Principles

(2 of 2)

The strands in the tire are stretched tight by the air pressure inside the tire allowing it to support
the weight of the vehicle. At the same time, the strands keep the tire from ballooning.

Tire Distortion

(1 of 4)

• During cornering, centrifugal force acts on a vehicle to

produce a side force

• The side force is the pressure on the wheel that pushes it

toward the outside or inside of the rim as the vehicle makes
a turn

• When the roads are slippery, or the vehicle is traveling too

fast, the tire is unable to grip the road surface adequately to
generate enough friction to overcome the side force

• The tire provides this opposing force by being able to distort

while still gripping the road

Tire Distortion

(2 of 4)

Tires distort during cornering, providing an opposing
force that allows a vehicle to make a turn.

Slip angle is the difference in angle
between the direction the tire is pointed
and the direction the tire is going.

Tire Distortion

(3 of 4)

• When a vehicle is driven into a turn with a decreasing

radius, both slip angle and cornering force increase until the
tire slides

• Slip angles depend on the weight distribution within the

vehicle, the wheelbase, the tire track, and the overall length
of the vehicle

• During cornering, centrifugal force puts more weight on the

outside wheels, acceleration puts more weight on the rear
wheels, and deceleration or braking puts more weight on
the front wheels

Tire Distortion

(4 of 4)

Understeer occurs when the front wheel slip
angle is larger than the rear slip angle.

Oversteer occurs when the rear wheel slip
angle is larger than the front slip angle.

Center of Gravity

• The balance point of the

entire vehicle

• Its actual position depends

on the location of the
engine and transmission

• Static weight distribution is

changed laterally by
centrifugal force when the
vehicle is turning and in a
fore-and-aft direction
during acceleration or
braking

The vehicle’s center of gravity.

Wheels and Tires

(1 of 3)

• K36002 Describe the

functions and features of
wheels and tires.

Wheels
• K36003 Describe purpose,

function, and types of a wheel
and wheel lug nuts/studs.

• The rim of a wheel is the outer

circular lip of the metal on which
the inside edge of the tire is
mounted

Rim of the wheel.

Wheels and Tires

(2 of 3)

• Rim width is the distance

across the inside of the rim
flanges at the bead seat

• The bead seat is the edge of

the rim that creates a seal
between the tire bead and the
wheel

• Rim flange is the exterior lip

that holds the tire in place

Drop-center rim.

Wheels and Tires

(3 of 3)

• Rim diameter is the distance

across the center of the rim
from bead seat to bead seat

• In most stock wheels, the

drop center is closer to the
front side of the wheel

• In deep dish wheels, the drop

center is closer to the rear of
the wheel
Deep dish wheel.

Wheel Offset

• The offset of a wheel

is the distance from its
hub mounting surface
to the centerline of the
wheel

• Typically used to bring

the tire centerline into
close alignment with
the larger inner wheel
bearing

Zero, positive, and negative offset.

Wheel Studs and Lug Nuts

(1 of 3)

• Wheels are fastened to the rims by wheel studs and lug

nuts

• Highly stressed by loads
• Made from heat-treated, high-grade alloy steel
• Types of wheel retaining stud or nut include tapered seat,

flat seat with washer, flat seat without washer, or ball seat

Wheel Studs and Lug Nuts

(2 of 3)

A. Tapered seat lug nut and wheel. B. Flat seat with integrated washer and wheel.
C. Flat seat with separate washer and wheel. D. Ball seat.

Wheel Studs and Lug Nuts

(3 of 3)

• Most lug nuts and studs

are right-hand threaded

• The wheel center is the

part of the wheel
containing the holes for
the lug studs

• The wheel center also

provides the required
offset from the centerline
of the wheel to the face
of the mounting flange

Left-hand lug nut and stud identified by the “L” on the
stud.

Bolt Pattern

• Refers to the number and

spacing of the lug nuts or
wheel studs on the wheel
hub on the wheel rim

• The number of studs

determines the pattern

• The exact number and

pattern of studs vary
depending on the vehicle
type and manufacturer
design

The bolt pattern refers to the number and spacing of lug
nuts or wheel studs on the wheel hub on the wheel rim.

Tires

• K36004 Describe the

components of a vehicle’s
tire.

• The air in the tire supports

the vehicle’s mass, and the
tire tread provides frictional
contact with the road surface

• Cords of synthetic strands or

fabric have high tensile
strength

• Fewer plies makes the tire

more flexible

The tire itself is generally composed of the
tread, sidewalls, inner liner, and bead.

Tire Valve Stems, Cores, and Caps

• Valve stem allows a tire to

be inflated and then
automatically close to
prevent air from escaping

• A valve core is threaded into

the valve stem and is a
Schrader valve

• A valve stem cap is used to

keep debris out of the valve
stem

Components of a tire valve.

Types of Tire Construction

(1 of 2)

• Two types of tire construction: bias-ply and radial ply
• Radial tires are by far the most common
• Bias-ply tires:

– Body ply cords extended diagonally from bead to bead at 30 to 40

degree angles, with successive plies laid at opposing angles

– Design provides a strong, stable casing, but with relatively stiff

sidewalls

– Stiff sidewalls can distort the tread and partially lift it off the road

surface while cornering

– Two or more layers of casing plies

Types of Tire Construction

(2 of 2)

• Radial tires:

– Body ply cords are laid across

the tread extending from bead to
bead and the cords end up
parallel to each other at
approximately 90 degrees to the
centerline of the tread

– Extremely durable and maintain

good traction

– Flexible sidewalls because of

their construction

– Run cooler than a comparable

bias-ply tire, when properly
inflated

Radial tires.

Tread Designs

(1 of 2)

• Differing tread patterns give manufacturers the ability to

design tires for special applications
– Directional tread patterns: designed to provide a range of attributes

during particular driving conditions

– Nondirectional tread patterns: designed in such a way that the tire

can be mounted on the wheel for any direction of rotation

– Symmetric tread patterns: have the same tread pattern on both

sides of the tire

– Asymmetric tread patterns: have a tread pattern that is different

from one side of the tire to the other

– Directional and asymmetric tread patterns: are both directional and

asymmetric

Tread Designs

(2 of 2)

Tread designs: A. Directional tread pattern. B. Nondirectional tread pattern. C. Symmetric
tread pattern. D. Asymmetric tread pattern. E. Directional and asymmetric tread pattern.

Tire Markings

(1 of 2)

• K36005 Decipher the tire

markings on the sidewall
of each tire.

• All tires meeting legislative

codes must have the
following information clearly
marked on the sidewall:
– Manufacturer or brand name
– International Organization for

Standardization (ISO) tire class

– Section width
– Aspect ratio

Typical sidewall markings.

Tire Markings

(2 of 2)

– Type of tire construction
– Wheel diameter
– Speed rating designation
– Maximum air pressure
– Load index
– Uniform Tire Quality

Grading (UTQG) system

– Department of

Transportation compliance
symbols and serial
numbers, including date of
manufacture code

Maximum pressure that the tire is designed for.
Always make sure this pressure is higher than the
recommended pressure on the tire placard.

Tire Sizes and Designations

(1 of 2)

• The vehicle manufacturer designates the recommended

size and load rating of the tires to be used on each vehicle

• The bead diameter must match the rim diameter, the section

width must be suitable for use on the rim, and the overall
tire size must allow sufficient clearance

• The aspect ratio of a tire is the ratio of its height to its width
• The lower a tire’s aspect ratio, the wider the tire is in relation

to its height

• Low-profile tires have very short sidewalls and can be

difficult to remove and install

• They have a higher speed rating

Tire Sizes and Designations

(2 of 2)

Tire profile. B. Low-profile tire.

Tire profile. A. Standard-profile tire.

Tire Ratings for Tread Wear, Traction, and

Temperature

• UTQG rating provides information on

tread wear, traction, and
temperature
– Tread wear grade comes from testing

the tire in controlled conditions

– Traction grade is a letter-based indicator

system based on the tire’s ability to stop
a vehicle on wet concrete and asphalt in
a straight-line situation

– The temperature grade of a tire is a

letter based on a test performed by the
tire manufacturer and overseen by the
US government

Typical UTQG ratings on a tire sidewall.

Tire Date of Manufacture Coding

• As part of DOT regulations,

a tire manufacture date
code must be stamped on
the sidewall of every tire

• A three or four digit code

denotes when the tire was
manufactured

• Never use tires more than

six years old

The DOT tire date manufacturing code is a four-digit code.

Tire Safety Features

• K36006 Describe the common safety features found on

today’s tires.

• A tire pressure monitoring system (TPMS) is used to

monitor the air pressure in each tire and warn the driver if
one or more tires is low

• Run-flat tires are designed to be driven for a reasonable

amount of time when the tire pressure is low or empty

• Self-sealing tires resist leaks and help maintain air pressure
• A spare tire is designed to be used for reduced speeds and

distances so that a driver can get the vehicle to a service
facility

Tire Pressure Monitoring Systems

(1 of 4)

• K36007 Describe the purpose, function, and types of

tire pressure monitoring systems.

• In addition to increased fuel consumption and tire wear, long

periods of driving with low tire pressures can cause
additional stress on the tire sidewalls

• Under-inflation can lead to a tire blowout or tread separation
• The automated tire pressure monitoring system (TPMS)

provides a means of reliable and continuous monitoring of
the vehicle tire pressure

Tire Pressure Monitoring Systems

(2 of 4)

The tire pressure monitoring system (TPMS).

Typical pressure sensors used in TPMS.

Tire Pressure Monitoring Systems

(3 of 4)

• Monitors the tires for low air pressure and alerts the driver

when one or more tires are lower than (or in some cases,
higher than) the designated thresholds

• Designed to ignore normal pressure variations caused by

changes in ambient temperature

• Two basic configurations to monitor the vehicle’s tire

pressures: direct and indirect

• The use of a centrifugal switch in the sensor allows the

sensor to go to sleep when the vehicle stops

Tire Pressure Monitoring Systems

(4 of 4)

• A direct TPMS can be of two

types:
– In one-way communication, the

TPMS sensor can only transmit to
the receiver

– In a two-way communication TPMS,

the sensor can receive as well as
transmit signals

• Indirect tire pressure monitoring

systems use anti-lock braking
systems to measure the
difference in the rotational speed
of the four wheels

Band-type TPMS sensor.

Run-Flat Tires

(1 of 2)

• K36008 Describe the

purpose and function of
run-flat tires.

• Enable a driver to maintain

vehicle control if a tire suffers
a rapid pressure loss when in
motion

• Enable the driver to continue

the journey within specified
speed and distance limits

Run-flat tire construction.

Run-Flat Tires

(2 of 2)

• TPMS is normally mandatory for all run-flat technology

applications to monitor the drivability of the vehicle’s run-flat
tires

• Generally two to three times heavier than their conventional

counterparts

• The design features of run-flat technology generally focus

on two aspects of operational use: rigidity and heat
resistance

• EH2 rims have a wider, or more extended, safety hump than

a standard safety rim

Self-Sealing Tires

(1 of 2)

• K36009 Describe the purpose and function of

self-sealing tires.

• Designed to repair most small tread-area punctures

instantly and permanently

• Feature standard tire construction with the addition of a

flexible and malleable lining inside the tire in the tread area

• Provide a seal around the object when the tire is punctured

and then fill in the hole in the tread if the object is removed

• Still require a TPMS to detect any low-pressure conditions

on 2008 and newer vehicles

Self-Sealing Tires

(2 of 2)

Self-sealing tire protects against small punctures.

Space-Saver Spare Tires

• K36010 Describe the

purpose and function of
space saver spare tires.

• Designed for emergency use

only
– Designed to get the driver to a

service center to have the
regular tire fixed or purchase a
new one

• Normally require a specially

charged canister for inflation
when being installed

Typical space-saver tire.

Summary

(1 of 6)

• Poorly maintained wheels and tires decrease effective

handling and may lead to steering and suspension
problems.

• Tire distortion refers to the tire’s cornering force countering

the side force that occurs when a vehicle corners, to create
a slip angle.

• Center of gravity refers to the balance point of the vehicle,

which is determined by location of the engine and
transmission.

• Wheel offset refers to the distance from the hub mounting

surface to the centerline of the wheel.

Summary

(2 of 6)

• Wheel offset can be zero, positive, or negative.
• Tire and wheel assemblies must be balanced to prevent

both static and dynamic imbalance.

• Tire and wheel components include the wheel or rim, wheel

studs and nuts, wheel center, and tires.

• A Schrader valve is a one-way valve used in a valve stem.
• Wheels usually have a deep well—a widened area on one

side of the wheel.

• Types of rims include steel, one-piece alloy, two-piece alloy,

multi-piece alloy, custom, spinning, split, semi-drop well,
drop well, and safety.

Summary

(3 of 6)

• Wheel studs and lug nuts fasten the wheels to the rims.
• Wheel retaining studs or nuts can be tapered seat, flat seat

with washer, flat seat without a washer, or ball seat types.

• Tires provide the wheel with coverage and protection and

absorb shock from road surfaces.

• Tires are composed of treads, sidewalls, inner liners, and

beads.

• Synthetic fabric cords are used to create plies, giving the

tire strength and flexibility.

• Tires are most commonly either cross-ply or radial (used by

most passenger vehicles).

Summary

(4 of 6)

• Tire pressure monitoring systems (TPMS) come in two

types: direct and indirect.

• Direct TMPS systems use pressure sensors in each wheel

and transmit the pressure readings wirelessly.

• Indirect TPMS systems use wheel speed sensors to indicate

wheel speed, which increases as tires lose pressure.

• TPMS sensors can be part of the tire’s valve stem or

mounted with a band around the inside of the wheel.

• TPMS sensors may have a centrifugal switch that turns

them on only when the vehicle is moving.

Summary

(5 of 6)

• Most TPMS systems need to be manually calibrated to

identify which pressure sensor is mounted inside each
wheel.

• Run-flat tires allow a vehicle to be driven for up to 50 miles if

it experiences a sudden loss of pressure.

• A TPMS is normally mandatory for all run-flat technology

applications.

• Self-sealing tires have a lining that serves as a puncture

sealant that can permanently seal most punctures from
nails, bolts, or screws up to 3/16" (or 5 mm) in diameter.

Summary

(6 of 6)

• Space-saver spare tires are designed to get the driver to a

service center to have the regular tire fixed or purchase a
new one.

• Some vehicles have miniature or collapsible space-saver

spare tires as spares. These normally require a specially
charged canister for inflation when being installed.

• Other vehicles have small, temporary spare tires that have

been inflated to a much higher pressure than normal road
tires because of their smaller size.