Tire and Wheel Theory

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Tire and Wheel Theory

• Chapter 61

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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
  car

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Introduction

• Service technician should be able to
• Discuss aspects of tire design
• Help customers make safest choice
• Tires and wheels important safety and service
  specialty area

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Tire Construction

• Several layers of rubber, cords, two rings of
  wire
• Casing (carcass) internal tire structure
• Ply metal or fabric rubberized cord
• Provide strength
• Ends of plies wrap around steel bead
• Bonded to side of tire
• Beads coils of wire at side edges
• Chafing strips protect beads from rim
• Belt cord structure made of plies
• Under the tread only
• Tread section of tire that rides on the road

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Tire Cord and Tire Ply Design

• Rubber must be reinforced with fabric, fiber,
  steel cords
• Bias-ply tires have plies that cross at 35-45
  angles
• Ride softer, but wear faster
• Radial-ply tires have casing plies that run
  across the tire from bead seat to bead seat
• Longer tread life, better grip, improved fuel
  economy
• Larger footprint gives better grip

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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

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Tire Tread

• Tire tread depth gauge.

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Tire Tread

• Wear indicators are also called wear bars. When
  tread depth is down to the legal limit of 2/32",
  bald strips appear across the tread.

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Tire Tread Material

• Rubber must be vulcanized (heated) to be stable
• Chemicals added to natural rubber to improve
  performance

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Tubeless Tires and Traction

• Tubeless tires
• Inner liner bonded to tire
• Seals air into tire
• Thicker than liner on tube-type tire
• Tubeless tires safer than tube-type
• Does not go flat immediately when punctured
• Traction
• How well tire grips the road
• Affected by
• Road surface, contaminants
• Tread material, inflation pressure, tread width,
  etc.

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Tire Sidewall Markings

• Tire size listed on sidewall
• Profile is the tires height
• Aspect ratio is height-to-width ratio
• Load index maximum load at designated speed
  rating
• Related to strength of sidewall plies
• Speed rating indicates better handling
  characteristics
• New speed ratings developed for speeds over 168
  mph

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Some tires use letters at the end of the tire
size (suffixes) to indicate special applications
including the following.LT light truckML
mining and loggingMH mobile homeST special
trailerTR truck
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Service DescriptionP205/75R x 15 92H205
cross-sectional width in mm75 aspect ratioR
radial construction15 rim diameter in inches92
load indexH speed rating (130 mph/210 km/h)
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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 in it
• Vehicles gross weight rating (GVW or GVWR)
  includes weight of vehicle, passengers, luggage
• Curb weight weight of vehicle without people
• DOT symbol indicates the tire meets DOT safety
  standards

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Load Index
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Ply Rating / load range
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Letter Maximum Rated Speed
L 120 km/h (75 mph)
M 130 km/h (81 mph)
N 140 km/h (87 mph)
P 150 km/h (93 mph)
Q 160 km/h (99 mph)
R 170 km/h (106 mph)
S 180 km/h (112 mph)
T 190 km/h (118 mph)
U 200 km/h (124 mph)
H 210 km/h (130 mph)
V 240 km/h (149 mph)
W 270 km/h (168 mph)
Y 300 km/h (185 mph)
Z Open-ended
The exact speed rating for a particular Z-rated tire is determined by the tire manufacturer and may vary according to size. For example, not all Brand X Z-rated tires are rated at 170 mph, even though one size may be capable of these speeds. The exact speed rating for a particular Z-rated tire is determined by the tire manufacturer and may vary according to size. For example, not all Brand X Z-rated tires are rated at 170 mph, even though one size may be capable of these speeds.
speed rating
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Tread WearThe tread wear grade is a comparison
rating based on the wear rate of a standardized
tire, tested under carefully controlled
conditions, which is assigned a value of 100. A
tire rated 200 should have a useful life twice as
long as the standard tire's. A tire wear
rating of 100 is equal to 10,000 miles
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TractionTraction performance is rated by the
letters AA, A, B, or C, with AA the
highest.IMPORTANT NOTE The traction rating is
for wet braking distance only! It does not
include cornering traction or dry braking
performance.
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All-Season Tires

• Radial tires have more traction on snow
• Specially designed pockets and slots
• Labeled with mud and snow designation

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HIGH-FLOTATION TIRE SIZESThese tires are
usually larger than conventional tires and
usually require a wider than normal wheel width.
High-flotation tires have a size designation such
as 33 x 12.50R x 15LT33 approximate overall
tire diameter in inches12.50 approximate
cross-sectional width in inchesR radial-type
construction15 rim diameter in inchesLT
light truck designation
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Temperature ResistanceTemperature resistance is
rated by letters A, B, or C, with A the highest
rating.
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ALL-SEASON TIRE DESIGNATIONMost all-season
tires are rated and labeled as M S, MS, or M
S, and therefore must adhere to general design
features as specified by the Rubber Manufacturers
Association (RMA).One design feature is that the
tire has at least 25 percent void area. This
means that the tread blocks have enough open
space around them to allow the blocks to grab and
clean themselves of snow and mud.
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Snow Tires

• 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

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Low Pressure Monitoring

• Run-flat tire has stiffer sidewall, tighter bead
• Can partly support the car even with no air
• Can be driven up to 70 miles without damage
• All new vehicles since 2006 have low-pressure
  warning system
• Low tire pressure monitored using direct or
  indirect method

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Low Pressure Monitoring
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DOT TIRE CODEAll tires sold in the United
States must be approved by the U.S. Federal
Department of Transportation (DOT).NOTE Most
race tires are not DOT-approved and must never be
used on public streets or highways.The DOT code
includes letters and numbers such as
MJP2CBDX264. The first two letters identify the
manufacturer and location. For this example, the
first two letters (MJ) mean that the tire was
made by the Goodyear Tire and Rubber Company in
Topeka, Kansas. The last three numbers are the
build date code. The last of these three numbers
is the year (1994), and the 26 means that it was
built during the 26th week of 1994. Starting
with tires manufactured after January 1, 2000,
the tire build date includes four digits rather
than three digits. The new code such as 3496
means the 34th week of 2006 (3406).
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UTQG System
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Tire Quality Grading and Tread Wear

• UTQG system rates tread wear, traction,
  temperature dissipation ability
• Tread wear varies with
• Wheel alignment
• Road surface texture
• Tire rotation maintenance
• Vehicle speed and braking practices
• Weight of vehicle
• Size of tire
• Tread wear ratings range from 100 to 500, in
  increments of 20 A tire wear rating of 100 is
  equal to 10,000 miles

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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
• Tires resistance to generating heat
• Ability to dissipate heat at highway speeds
• Compact spare tire is for temporary use only
• Many limited to 31 mph for 31 miles

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Changing Tire Size

• If tire size changed, substitute tire with equal
  or greater load-carrying capacity
• As diameter of tire increases, load capacity
  increases
• If lower profile tire installed, wider tire and
  larger diameter wheel used
• Overall diameter of replacement tire must be 2
  to -3 of original tire
• When tire diameter changed, speedometer must be
  recalibrated

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Outside diameter is calculated by adding the
wheel diameter to the cross-sectional height of
the tire, multiplied by 2.
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Wheels

• Wheels have two parts center (flange) and rim
• Drop center (rim well) facilitates removal and
  installation of tire
• Safety beads keep the tire bead on bead seat
• Hub-centric center of wheel has machined
  counterbore that pilots on machined area of hub
• Stud-centric wheels locate on wheel studs

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Rim components
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Wheel OffsetOffset is a very important variable
in wheel design. If the center section (spider)
is centered on the outer rim, the offset is zero.
Wheel offset is often referred to as ET, which
stands for Einpress Tieffe in German.
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On four-lug axles and wheels, the measurement
is simply taken from center to center on opposite
studs or holes.
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On five-lug axles and wheels, it is a little
harder. One method is to measure from the far
edge of one bolt hole to the center of the hole
two over from the first.
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Custom Wheels

• Mag wheels use magnesium silicon alloy
• Expensive, not corrosion resistant
• Custom wheels for street use are single piece
  castings of light alloy aluminum
• Weather resistant coating
• 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

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Negative OffsetThe wheel has a negative offset
if the center section is inboard (or dished)
from the wheel centerline.
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Back SpacingBack spacing, also called rear
spacing or backside setting, is the distance
between the back rim edge and the wheel center
section mounting pad. Back spacing can be
measured directly with a ruler.
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Lug Studs and Lug Nuts

• Number of lug studs depends on vehicle load
• Bolt pattern 6-5½ is six-bolt pattern spaced
  around 5½ inch circle
• Lug nuts may be standard or metric
• Lug nuts have serrated shank to remain tight
  during tightening
• Lug nuts for cast wheels are long and thick
• Must be used with a washer

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If replacement wheels are used on any vehicle,
check with the wheel manufacturer as to the
proper type and style lug nut.
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Tire Valve Stems

• Passenger car valve stems usually rubber
• Designed for pressures less than 62 psi
• Spring loaded valve core screwed into valve stem
• Some have a gasket to prevent air loss past valve
  core
• Short stem used when there is a hub cap
• Long stem accommodates full wheel covers

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Most vehicles have recommended tire inflation
figures written in the owner's manual or on a
placard or sticker on the door post or glove
compartment.
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Proper inflation