Tire and Wheel Theory

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					Tire and Wheel Theory

      Chapter 61
• 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
• 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
                    Tire Construction
• Several layers of rubber, cords, two
  rings of wire
    – Casing (carcass) – internal tire
    – Ply – metal or fabric rubberized
        • Provide strength
• Ends of plies wrap around steel
    – 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
                  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°
   – 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
• Larger footprint gives better grip
                           Tire Tread
• Grooves in tread allow traction on
  wet surfaces
   – Allow tire to flex without
   – 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
Tire Tread
   • Tire tread depth
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
            Tire Tread Material
• Rubber must be
  (heated) to be
• Chemicals
  added to natural
  rubber to
              Tubeless Tires and
• 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.
              Tire Sidewall Markings
• Tire size listed on sidewall
   – Profile is the tire’s height
   – Aspect ratio is height-to-width
• Load index – maximum load at
  designated speed rating
   – Related to strength of sidewall
• Speed rating indicates better
  handling characteristics
• New speed ratings developed for
  speeds over 168 mph
Some tires use letters at the end of the
 tire size (suffixes) to indicate special
  applications including the following.
             LT = light truck
        ML = mining and logging
           MH = mobile home
           ST = special trailer
                TR = truck
       Service Description

        P205/75R x 15 92H
 205 cross-sectional width in mm
          75 aspect ratio
       R radial construction
    15 rim diameter in inches
           92 load index
H speed rating (130 mph/210 km/h)
                          Load Rating
• How much weight a tire can
  safely support at a specified air
• Amount of load determined by
  area of tire and air pressure in it
• Vehicle’s gross weight rating
  (GVW or GVWR) includes
  weight of vehicle, passengers,
   – Curb weight – weight of
     vehicle without people
• DOT symbol indicates the tire
  meets DOT safety standards
Load Index
Ply Rating / load range
                         Letter                          Maximum Rated Speed
speed                      L                      120 km/h (75 mph)

rating                     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
                     Tread Wear
 The 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

Traction 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.
              All-Season Tires
• Radial tires have more
  traction on snow
• Specially designed
  pockets and slots
• Labeled with mud and
  snow designation
 These 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 15LT:
          33 = approximate overall tire diameter in inches
        12.50 = approximate cross-sectional width in inches
                    R = radial-type construction
                     15 = rim diameter in inches
                     LT = light truck designation
            Temperature Resistance
Temperature resistance is rated by letters A, B, or C, with A the
                       highest rating.

Most 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
                  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
          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
Low Pressure Monitoring
                      DOT TIRE CODE
 All 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”).
UTQG System
           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
                  Traction Grade
• First letter in tire rating is traction grade
   – Indicates stopping ability on wet asphalt and
   – Braking only in straight ahead direction
• Second letter is temperature grade
   – Tire’s 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
              Changing Tire Size
• If tire size changed, substitute tire with equal or
  greater load-carrying capacity
• As diameter of tire increases, load capacity
• 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
Outside diameter is calculated by adding the wheel
 diameter to the cross-sectional height of the tire,
                  multiplied by 2.
• 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
Rim components
Wheel Offset

Offset is a very important
variable in wheel design.

If the center section
(spider) is centered on
the outer rim, the offset is

 Wheel offset is often
referred to as ET, which
stands for Einpress
Tieffe in German.
  On four-lug axles and wheels, the measurement is
simply taken from center to center on opposite studs or
 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.
               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
                   Negative Offset

The wheel has a negative offset if the center section is
   inboard (or “dished”) from the wheel centerline.
Back 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
               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
If replacement wheels are used on any vehicle, check
with the wheel manufacturer as to the proper type and
                    style lug nut.
              Tire Valve Stems
• Passenger car valve stems usually rubber
  – Designed for pressures less than 62 psi
• Spring loaded valve core screwed into valve
• 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
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.
Proper inflation