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

car

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

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

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

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

• Tire tread depth gauge.

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.

Tire Tread Material

• Rubber must be vulcanized (heated) to be stable

• Chemicals added to natural rubber to improve performance

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.

Tire Sidewall Markings

• Tire size listed on sidewall– Profile is the tire’s 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

Some tires use letters at the end of the tire size (suffixes) to indicate special applications including the following.

LT = light truckML = mining and logging

MH = mobile homeST = special trailer

TR = truck

Service Description

P205/75R x 15 92H205 cross-sectional width in mm

75 aspect ratioR radial construction

15 rim diameter in inches92 load index

H speed rating (130 mph/210 km/h)

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

• Vehicle’s 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

Load Index

Ply Rating / load range

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.

speed rating

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

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

HIGH-FLOTATION TIRE SIZES

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 inchesR = radial-type construction15 = rim diameter in inchesLT = light truck designation

Temperature Resistance

Temperature resistance is rated by letters A, B, or C, with A the highest rating.

ALL-SEASON TIRE DESIGNATION

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

mud.

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

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

• 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 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

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

multiplied by 2.

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

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 zero.

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

holes.

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

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 ruler.

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 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

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