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The Modern Tire
Compiled by Ali A. Karim (Jeddah, KSA)
History and Evolution
A modern tire contains up to 50 basic raw materials, from carbon black to zinc oxide, 15 synthetic rubber combinations, man-made fibers, and high-strength alloy steels. The material must be strong enough to support cornering, acceleration, and braking loads, yet supple enough to absorb bumps. The manufacturing process that converts all these raw materials into a modern tire is just one step short of pure magic. It wasn’t always that way; Fred Flintstone’s "tire" was nothing but a chunk off the old stone wall.

The first real breakthrough was the invention of the pneumatic tire, which dates to Robert William Thomson’s English patent of 1845. That patent’s description of the function of a pneumatic tire still applies. It reads in part: "the application of elastic bearings around the tires of the wheels of carriages…a hollow belt composed of some air or water tight material…inflating it with air whereby the wheels will, in every part of their revolution, present a cushion of air to the ground."

The first pneumatic tires were built for bicycles. Things got better, of course, and in 1896, a Leon Bollee automobile was offered with Michelin pneumatic tires as original equipment (OE) - the first vehicle so fitted.

Early in tire development and through the ‘30s, the cords used in tire construction were generally cotton-based. In 1937, Michelin introduced a revolutionary truck tire called the Metallic, whose four plies of steel cord replaced as many as 10 plies of cotton. This truck tire was the beginning of steel-belt construction and changed the industry.

In the ‘50s, Michelin introduced the first passenger car radial, the Michelin X. Many refused to ride in a car equipped with the X tire because it looked under-inflated, but the little Michelin with the bulge in the sidewall turned out to be the forerunner of a superior breed of tire. The radial revolution had begun in the earnest. Passenger car tires are almost universally radial construction now, and the range of tires available is staggering.

Technical Aspects
Technology has now progressed to the point where tire and car engineers collaborate to tune tire and car to work in unison. This makes it imperative for the car owner to understand that changing to different tires is not as simple as it appears. An improper choice can negatively affect vehicle performance and, worst case, even safety.

Ultra-high-performance tires have made enormous strides in traction (grip) and stability, but like most other things, tire design is the art of compromise. Traction on dry pavement requires the maximum amount of rubber in contact with the pavement. A thin tread reduces heat buildup at high speeds, which allows the tires to live at sustained high speeds. On a wet surface, however, a tire without tread has all the stability of the stock market. Street tires have tread patterns, sacrificing ultimate traction for wet stability. Such compromises exist in all areas of tire design. Lower aspect ratios (ratio of the height of the tire’s cross section, to its width) increase response, steering feel, and stability, but degrade ride comfort. Softer, ‘stickier’ rubber compounds can increase traction dramatically, but wear much faster. Stiffer sidewalls increase steering precision and feel, but transmit excessive road shock to the vehicle. Radial construction allows the contact patch to maintain a better grip on the road surface, compared to less compliant, belted tires, and so on. Every parameter settled on by the tire engineer requires at least two compromises in other areas.

Tires designed for low-traction (wet) conditions have diametrically opposite requirements. They need deep tread grooves to channel the water away from the contact patch, high-void ratio (ratio of tread to grooves) to cut through the water and down to the pavement, and high aspect ratios to increase ground clearance. The long, thin contact patch also reduces the tendency for the tire to aquaplane (a dangerous phenomenon where the tire water-skis, losing all traction). In addition to the High Performance and Luxury tires, new tread designs and rubber compounding techniques have helped in the development of All Season tires which are good on wet as well as on dry roads, and even manage acceptable performance on the snow. These new generation all-season tires have had a lot of energy expended on making sure they’re not noisy. Tires make noise in two ways: a low frequency noise from the tire striking the road and a high frequency sound from the air being squeezed out of the tread spaces as the tire compresses, rolling over the surface.
As we have seen, tires are more than just round and black; they act as a spring in both the vertical and horizontal planes, with a significant effect on vehicle dynamics. Rest assured that whatever direction tire technology follows, the industry will have a substantial effect on the way our vehicles interact with the world, and as tire and chassis engineers get better at tuning the tire / car systems to work together, some dramatic improvements will surface and today’s ultra-high-performance tires will pale in comparison to those of the future.

Selection
At tire replacement time, an owner faces a bewildering array of choices, and getting good product information isn’t always easy. The simplest solution is to stay with the same brand and size as supplied by the manufacturer. These tires have good all-round capabilities, but aren’t outstanding at one task. If an owner is willing to give a little on some tire parameters, he can gain significantly on others to better tailor the tire to his exact needs. Those willing to explore the cornucopia of tires available can increase cornering power, directional stability, braking potential, wet traction, and durability, while altering a car’s appearance dramatically. But don’t rush out with wallet in hand just yet – there are some definite pitfalls. The engineers who designed your car took many factors into consideration when specifying original-equipment (OE) tires, from both size and capability standpoints. Changing tire / wheel size has a ripple effect that can hurt many areas of performance, and in some instances actually may be dangerous. It is recommended that any tire / wheel upgrades be done with the help of a competent shop that thoroughly understands all the ramifications of the changes and guarantees its work. The upgrade does not have to be for handling or styling reasons – many people upgrade tires to increase load-carrying capacity; obtain better traction in mud, snow, or ice; or just for safety reasons.

Tire Code Nomenclature

Today’s tires have typical designations on their side-wall which can be understood as follows:

Section Width / Aspect Ratio, Rim Diameter, Load Index, Speed Symbol

for example: 205 / 60 R13 85 H

205 : Section Width (measured in millimeters)

60 : Aspect Ratio (ratio of the height of the tire’s cross section, to its width)

R13 : Rim Diameter (measured in inches)

85 : Load Index (Service Description, refer to Load Index table)

H : Speed Symbol (Service Description, refer to Speed Symbol table)

Load Index Table:

Load Index

Load (kg)

Load (lbs)

Load Index

Load (kg)

Load (lbs)

Load Index

Load (kg)

Load (lbs)

75

387

853

89

580

1279

103

875

1929

76

400

882

90

600

1323

104

900

1934

77

412

908

91

615

1356

105

925

2039

78

425

937

92

630

1389

106

950

2094

79

437

963

93

650

1433

107

975

2149

80

450

992

94

670

1477

108

1000

2205

81

462

1019

95

690

1521

109

1030

2271

82

475

1047

96

710

1565

110

1060

2337

83

487

1074

97

730

1609

111

1090

2403

84

500

1102

98

750

1653

112

1120

2469

85

515

1135

99

775

1709

113

1150

2535

86

530

1168

100

800

1764

114

1180

2601

87

545

1201

101

825

1819

115

1215

2679

88

560

1235

102

850

1874

Speed Symbol Table

Speed Symbol

Speed (kph)

Speed (MPH)

Speed Symbol

Speed (kph)

Speed (MPH)

P

150

94

U

200

124

Q

160

100

H

210

130

R

170

106

V*

Over 210

Over 130

S

180

112

V

240

149

T

190

118

Z

Over 240

Over 149

Note: Consult tire manufacturer for maximum speed.

Final Word
The remaining question of brand (and model) selection is possibly the most difficult of all. Wading through the tire manufacturers’ claims is a formidable task, made more difficult by the glowing terminology that makes even the most miserable rim-protector sound like an Indy 500 pole-sitter. The bottom line is to treat tire buying as you would a new-car or other major purchase. Do research before starting. Know what brands and sizes are in your price range. Get out there, examine the tires, check their availability, and compare prices. Ask questions and try to get the manufacturers’ brochures. The more informed you are, the more satisfied you are going to be with your purchase. The right set of tires can make a tremendous amount of difference in your car.

Source : Motor Trend - Performance Car Buyer’s Guide

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