System: Tires and Wheels

System: Tires and Wheels

• Compact spares
• Tire construction and design
• Tire matching rules
• Tire ratings and designations
• Tires - pressures, flats, and wear
• Wheel balancing and alignment
• Wheels and bearings

Compact Spares

Most car manufacturers now supply a compact spare tire. This tire is smaller in diameter and width than the four regular tires on the car. The tire has the advantage of taking up less space in the luggage area. It is designed only for temporary use. The compact spare is safe only for low speeds and short distances. Many are limited to 50 kilometers per hour for a distance of 50 kilometers.

CAUTION: Compact spare tires must never be used as a substitute for normal tires. Always observe the speed and distance warnings printed on the compact spare tire.

Tire Construction

We usually think of a rubber tire, but a tire is really constructed of a number of parts made from different materials. A cross section of a tire is shown below.

The main structural element of a tire is the two wire beads. Each bead is formed by wrapping together several steel wires to make a continuous hoop.

The tire's basic strength is determined by its internal structure, called a carcass or casing. The carcass is formed with layers of rubberized fabric called plies and belts. Each ply is a layer of rubber with metal or fabric cords embedded in its body. The cords may be made from a variety of materialsÑsuch as rayon, nylon or polyester, fiberglass, steel, aramid, or kevlar. The material used determines the tire's stability and resistance to bruises, fatigue, and heat. The ends of the plies are wound around the beads and bonded into the sides of the tire.

Belts are also cords made from rubber reinforced with metal or fabric. The belts do not, however, go all the way down to the bead. The belts fit under the tread area of the tire.

An inner liner is added to seal in the air, and is bonded into the carcass assembly. When the carcass is filled with the correct air pressure, a force is exerted evenly against all interior surfaces. This provides the tension in the carcass that resists and supports the weight of the vehicle. The tread section of the tire provides the traction on the road. The tread is designed to allow the air flow to cool the tire and to channel water during wet weather. The raised section of the tread is called ribs and the lowered section is called grooves. Treads are made of a rubber compound that is highly resistant to abrasion wear. The thick layer of tread rubber is especially compounded to withstand road wear. The spaces between the treads permit distortion of the tire on the road without causing scrubbing (back and forth movement) that would accelerate wear.

The sidewalls are made of a different grade of rubber designed to help absorb irregularities in the road and to protect the cord plies from damage. The sidewall rubber, which does not touch the road, is not as thick as the tread and is made of an abrasion-resistant rubber to provide maximum protection for the structural plies. Inflation pressure and the stiffness of the cords account for the rigidity of sidewalls; the sidewall rubber has almost nothing to do with it. In general, the more plies a tire has, the stiffer the sidewalls, and, therefore, the less cushioning the tire gives to the ride.

Air is sealed inside the tire and wheel assembly by the contact of the bead area against the wheel rim. A valve assembly fits through the wheel as shown below. The valve allows the tire to be filled with air. The body of the valve is made from rubber. A groove seals the body to the wheel rim. Inside the body is a valve core. The one-way valve core allows air to be forced into the tire but prevents it from going out. A pin at the top of the valve may be depressed against a return spring to let air out of the tire. A valve cap seals the valve and prevents damage to the pin.

Tire Designs

There are three basic tire constructions based on the arrangement of the body plies: bias, belted radial, and belted bias. For many years, the bias ply, or conventional tire, was standard on cars. The layers, or plies, were laid down on a bias crossing each other. Two- or four-body plies criss-cross at an angle.

The bias ply tire provided a smooth, quiet ride. However, it had one basic fault. Because both the sidewall and tread have the same basic reinforcement, there is excessive tread flex or squirm in corners that can reduce the tire's area of contact, or footprint. An unstable footprint results in less tread life and poor performance during cornering.

One solution to the unstable footprint problem was the introduction of the bias belted tire. The belted tire has a body construction similar to the bias design. In addition, it uses two or more reinforcing belts positioned under the tread. These reinforce the tread and help prevent excessive squirm and flex.

On radial tires the carcass plies are laid down at right angles to the circumference of the tire. These tires are called belted radial tires. The cords run from one side of the tire directly across to the other side, looping under the tread and returning to the center on the other side. Heavy belts are added under the tread, as in the bias belted design. Radials have a rigid tread and flexible sidewalls, resulting in a smooth ride with less rolling resistance and a stable footprint. With less rolling resistance, the tire also helps increase the car's fuel mileage.

Tire Designations

The tire's size and the amount of load a tire can support are indicated in a combination of numbers and letters imprinted on the tire sidewall. Tires are designated by their profile or aspect ratio, size, and load rating.

The profile of a tire is based on the width and height of a cross section of the tire as shown in the following illustration. The profile is determined by an aspect ratio. This is equal to the section height divided by the section width. The ratio is given as a percentage. The lower the aspect ratio number - for example, 60 or 50 - the wider the tire. Using the above example, a tire with a section height of 7 inches and a section width of 10 inches has an aspect ratio of 70% (7/10 = 0.70 or 70%). This means the section height is 70% of the section width. Tires with the same aspect ratio are often described as being in the same series. Tires with a 70 aspect ratio are called '70 series' tires.

The load rating on a tire describes the load in the pounds the tire can support at a specific tire inflation pressure. Each tire must be able to support a minimum of one quarter of the total car weight. If the car weighs 4,000 pounds, each tire must be capable of supporting 1,000 pounds.

The load that a tire will safely carry depends on its size, its load range, and its inflation pressure. Regardless of brand, all tires of the same size and load range are rated to carry the same load at any given inflation pressure.

Tire size is one factor that determines the amount of load it can carry. The size and load rating of the tire are often given together on the sidewall. The system used to describe tire size and load ratings has changed many times over the years. Currently, there are two common systems in use: all U.S. measurements and a combination of U.S. and metric measurements.

The U.S. measurement system uses the width/load letter scale shown below. The scale starts at V through Z. Then it begins at A and goes through N. The V-designated tire is only wide enough to support 650 pounds. The N-rated tire is wide enough to support 1,880 pounds. The letter is followed by a number, which shows the tire's aspect or profile ratio. This is followed by a number that shows what rim diameter the tire fits on. A typical size designation might be F78-15. The F is the width/load designation, the 78 is the aspect ratio, and the 15 means it fits on a 15-inch rim. Radial ply tires use an R in the designation, such as FR78-15.

The width/load rating replaces the ply rating system formerly used to denote tire load capacity. Although the old ply rating system has been phased out, both designations may be used on tire sidewalls. For example, load range B tires may be marked 4-ply rating/2-ply, or 4-ply; load range C tires, 6-ply rating/4-ply, or 6-ply; and load range D tires, 8-ply rating/4-ply, 8-ply rating/6-ply, or 8-ply.

The combination U.S. and metric system tire designation specifies the tire size and load rating in metric units and the rim size in inches. An example of a combination tire size designation is P 185/80 R 13. The letter P indicates the tire use appropriate for passenger car applications, 185 is the section width in millimeters, 80 is the aspect ratio, R indicates radial ply construction, and 13 is the wheel rim diameter stated in inches.

Uniform Tire Quality Grading and Speed Ratings

Tire manufacturers have adopted a system of tire grading called the Uniform Tire Quality Grading (UTQG). This system grades three tire qualities: tread wear, traction, and resistance to high temperature. The UTQG grades are shown by letters and numbers on the sidewall as shown below.

Relative tread wear is indicated by a number. The control number is 100, indicating approximately 30,000 miles under normal test driving conditions. Thus, a tire rated 080 can be expected to deliver about 20% less mileage than a tire rated 100. A tire rated 160 should wear 60% longer than one rated 100. Numbers range from 050, poor wear, to 270, exceptionally long wear. The traction or resistance to skidding and resistance to high temperature is rated with an A, B, or C. The A rating is best and C is the worst.

Tires designed for high performance cars have used a speed rating system. This system is used in the tire size designation. There are three common ratings: SR-maximum speed 112 mph, HR-up to 130 mph, and VR-up to 165 mph. The rating is specified in the tire size. For example, the tire may be marked 175SR 13. This means the tire is 175-millimeters wide, has a speed rating of 112 mph, and fits on a 13-inch rim.

Required Sidewall Information

Because tires are used in interstate transportation, the Department of Transportation (DOT) specifies that certain information must be imprinted on each tire. The information required by law is:

1. The size of the tire (F78-14).
2. The maximum permissible inflation pressure (32 psi).
3. The maximum load rating (1,500 pounds).
4. The generic name of each cord material in the sidewalls and tread areas.
5. The actual number of plies in the sidewall (2 plies) and, if different, in the tread area (4 plies).
6. The words tubeless or tube type, as applicable.
7. The word radial, as applicable.
8. The DOT manufacturing code is used to determine who made the tire, where it was made, and when.

Tire Mixing Rules

Tire construction, size, rating, and inflation have a great effect on the handling of the car. As a general rule, tires should be replaced with tires recommended by the car or tire manufacturer. To prevent handling problems that could result in loss of control of the car, observe the following rules:

1. Never mix size or construction types on the same axle.
2. Tires on the same axle should have about the same tread depth.
3. New tires should be installed in pairs on the same axle.
4. Follow manufacturer recommendations for the use of radial tires.
5. Follow manufacturer recommendations for use of tire types for trailer towing.
6. Follow manufacturer recommendations for use of types and sizes of snow tires.

CAUTION: Always follow tire or car manufacturer recommendations for types of tires to use and mixing of tire types. Using the wrong tires could cause the car to handle improperly and may result in an accident.

Rotate Tires

Moving the tires on a car to different positions periodically is called rotating the tires. Some car manufacturers recommend that tires be rotated at specific intervals. Others recommend that tires be rotated only if they are wearing unevenly. Tires wear unequally even under perfect mechanical and operating conditions. Tires powered by the drive train typically wear more than tires not providing driving traction.

CAUTION: If the vehicle has both radial and bias or bias belted tires, the radial tires should stay on the rear of the car. Mixing tire designs on the same axle can cause poor handling and dangerous braking.

The first step in tire rotation is to raise the car on a hoist or jack and jack safety stands. Follow the safety procedures for lifting presented previously. Next, inspect the tire markings to determine if the tires are bias ply or radial ply design.

Look up the tire rotation procedure and the wheel lug nut torque specification in the shop service or owner's manual. The vehicle manufacturer provides a tire rotation diagram to follow when rotating tires. Many cars have a compact-size, low-mileage spare that is only used in emergencies. In this case, follow a four-wheel rotation. Bias belted or bias ply tires can be criss-crossed from one side of the car to the other. Some tire and car manufacturers recommend against using this procedure for radial tires because moving tires from one side to another changes their direction of rotation and can lead to internal tire problems. A radial tire rotation moves the tires back and forth on the same side of the car. Prior to removing the wheels, it is a good idea to use chalk to mark each wheel to keep track of its position. Use the initials LF (left front), RF (right front), LR (left rear), and RR (right rear). Remove the wheel covers from each of the wheels. Use an air impact wrench to remove the lug nuts from each wheel as described previously. Tighten each wheel lug nut in the correct order and to the correct torque with a torque wrench as described previously.

WARNING: Always tighten the lug nuts to the correct torque specifications with a torque wrench. Using a non-torque-controlled air or electric impact wrench on lug nuts can cause wheel rim and brake rotor distortion.

Remove and Remount a Tire on a Rim

A worn or damaged tire must be replaced. Tires are removed and remounted on a rim with equipment called a tire changer. Tire changers all have the same basic parts and operate in the same basic way. The controls, however, are different for different models. Always follow the instructions provided with the tire changer you are using.

WARNING: Always follow the car or tire manufacturer's recommendations regarding replacement tires. The replacement tire must be the correct size, load rating, and type of construction. Never mix radial and bias type construction on the same axle.

The typical tire changer has a mount to hold the rim securely in position. It has a set of air-operated arms, which are used to unseat the tire bead from the rim. This unseating takes a great deal of force. The air-operated arms can provide this necessary force.

CAUTION: Always wear eye protection when deflating, dismounting, mounting, or inflating a tire. A sudden air blast could injure your eyes.

Before you dismount the tire, decide if the same tire will be installed on the rim. If you are removing the tire for a repair operation and will be remounting it, you should remount it in the same position because the tire and rim have been balanced together. Make a chalk mark on the tire across from the tire valve assembly in the rim. When you remount the tire, line the mark up with the valve.

The next step in dismounting is to deflate the tire. Remove the tire valve cap. The air is removed from the tire by unscrewing the tire valve core with a valve core tool. Insert a tire valve tool in the valve assembly and unscrew the valve core.

Next, unseat the bead from both sides of the rim. Some machines do this with a unit on the side of the tire changer. Others do this when the tire is mounted on the machine. Follow the directions for the type of machine you are using.

CAUTION: The arms and equipment on the tire changer grip with great force. Be extremely careful not to get your hand or fingers caught in the equipment.

The tire will have to be installed on the machine to use the arms to dismount the tire from the rim. Many machines grip the rim with air- or screw-operated cones. The cones fit into the center hole of the rim. There may be several sizes of cones for different sizes of rims. Check the instructions with your tire changer.

WARNING: Special adapters are often necessary to mount aluminum, magnesium, or chromed wheels. The use of an incorrect adapter can cause the machine to break or damage these rims. Follow the directions supplied with the tire changer.