This section introduces the various tire designs used on modern vehicles. It explains how tire and wheels are constructed to give safe and dependable service. This section also covers hub and wheel-bearing construction for both rear-wheel and front-wheel drives.
Most modern passenger vehicles and light trucks use tubeless tires that do NOT have a separate inner tube. The tire and wheel form an airtight unit. Many commercial and construction vehicles use inner tubes, which are soft, thin, leak-proof rubber liners that fit inside the tire and wheel assemblies. However, in the last few years tubeless tires have been introduced to commercial and construction vehicles, reducing the need for tube type tires. Tires perform the following two basic functions:
Tires must transmit driving, braking, and cornering forces to the road in all types of weather. At the same time, they should resist puncture and wear. Although there are several tire designs, the six major parts of a tire are as follows:
There are many construction and design variations in tires. A different number of plies may be used and run at different angles. Also, many different materials may be used. The three types of tires found on late model vehicles are bias-ply, belted bias, and radial.
A bias-ply tire is one of the oldest designs, and it does NOT use belts. The position of the cords in a bias-ply tire allows the body of the tire to flex easily. This design improves the cushioning action, which provides a smooth ride on rough roads. A bias-ply tire has the plies running at an angle from bead to bead (Figure 12-29). The cord angle is also reversed from ply to ply, forming a crisscross pattern. The tread is bonded directly to the top ply. A major disadvantage of a bias-ply tire is that the weakness of the plies and tread reduce traction at high speeds and increase rolling resistance.
Figure 12-29 — Bias ply tire.
A belted bias tire provides a smooth ride and good traction, and offers some reduction in rolling resistance over a bias-ply tire. The belted bias tire is a bias-ply tire with stabilizer belts added to increase tread stiffness. The belts and plies run at different angles. The belts do NOT run around to the sidewalls but lie only under the tread area. Two stabilizer belts and two or more plies are used to increase tire performance.
The radial ply tire has a very flexible sidewall, but a stiff tread (Figure 12-30). This design provides for a very stable footprint (shape and amount of tread touching the road surface) which improves safety, cornering, braking, and wear. The radial ply tire has plies running straight across from bead to bead with stabilizer belts directly beneath the tread. The belts can be made of steel, flexten, fiberglass, or other materials.
Figure 12-30 — Radial tire.
A major disadvantage of the radial ply tire is that it produces a harder ride at low speeds. The stiff tread does NOT give or flex as much on rough road surfaces.
There is important information on the sidewall of a tire. Typically, you will find Uniform Tire Quality Grading (UTQG) ratings for treadwear, traction, and temperature. You will also find the tire size, load index and speed rating, and inflation pressure. It is important that you understand these tire markings.
Tire size on the sidewall of a tire is given in a letter-number sequence. There are two common size designations (Table 14-1)—alphanumeric (conventional measuring system) and P-metric (metric measuring system). The alphanumeric tire size rating system, as shown in Table 14-1, uses letters and numbers to denote tire size in inches and load-carrying capacity in pounds. The letter G indicates the load and size relationship. The higher the letter, the larger the size and load-carrying capability of the tire. The letter R designates the radial design of the tire. The first number "78" is the aspect ratio, also known as height-to-width ratio. The last number "15" is the rim diameter in inches. The P-metric tire size identification system, as shown in Table 14-1, uses metric values and international standards. The letter P indicates a passenger vehicle (T means temporary and C means commercial). The first number "155" indicates the section width in millimeters measured from sidewall to sidewall. The second number "80" is the aspect ratio, also known as height-to-width ratio. The letter R indicates radial (B means bias belted, D means bias-ply construction).
Table 14-1 — Tire Size Designation Numbering System.
Truck tires are sometimes marked with the designation LT for "light truck" before the size.
The aspect ratio or height-to-width ratio in the tire size is the most difficult value to understand. Aspect ratio is the comparison of the height of a tire (bead to tread) to the width of a tire (sidewall to sidewall). It is height divided by width. An 80-series tire, for example, has a section height that is 80 percent of the section width.
As the aspect ratio becomes smaller, the tire becomes squatted (wider and shorter). A 60-series tire would be "short" and "fat," whereas an 80-series tire would be "narrower" and "taller."
The term load index, or load range, is used to identify a given size tire with its load and inflation limits when used in a specific type of service. The load index of a tire and proper inflation pressure determines how much of a load the tire can carry safely.
A letter identifies the load index for most light trucks, these letters being B, C, D, or E. A tire with a B load rate is restricted to a load specified at 32 psi. Where a greater load- carrying ability is required, load rate C, D, or even E tires are used.
Table 14-2 — Load Index Chart for a Passenger Vehicle.
Passenger vehicle tires come with a service description added to the end of the tire size. These service descriptions contain a number, which is the load index, and a letter,
which indicates the speed rating. The load index represents the maximum load each tire is designed to support (Table 14-2). Because the maximum tire load capacity is branded on the sidewall of the tire, the load rate is used as a quick reference. Speed ratings signify the safe top speed of a tire under perfect conditions (Table 14-3).
Table 14-3 — Speed Rating Chart for a Passenger Vehicle.
The maximum inflation pressure, printed on the sidewall of a tire, is the highest air pressure that should be induced into the tire. The tire pressure is a “cold” pressure and should be checked in the morning before operating the vehicle.
In most parts of the world, fall and early winter months are the most critical times to check inflation pressures because the days are getting colder. And since air is a gas, it contracts when cooled. For every 10°F change in ambient temperature, the inflation pressure of a tire will change by 1 psi. It will go down with lower temperatures and up with higher temperatures. The typical difference between summer and winter temperatures is about 50°F, which results in a loss of 5 psi and will sacrifice handling, traction, durability, and safety.
The Department of Transportation requires each manufacturer to grade its tires under the UTQG labeling system and establishes ratings for treadwear, traction, and temperature resistance (Figure 12-31).
Figure 12-31 — Uniform tire quality grade system ratings on the sidewall of the tire.
These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by the manufacturers following field-testing and are most accurate when comparing tires of the same brand.
Uniform Tire Quality Grade ratings are NOT required on winter, light truck, and commercial tires.
Tubes (inner tubes) are circular rubber containers that fit inside the tire and hold the air that supports the vehicle. Though it is strong enough to hold only a few pounds of air when not confined, the tube bears extremely high pressures when enclosed in a tire and wheel assembly. Because the tube is made of comparatively soft rubber to fulfill its function, it is easily chafed, pinched, punctured, or otherwise damaged. Tubes generally are made of a synthetic rubber that has air-retention properties superior to natural rubber. There are two types of synthetic rubber tubes: butyl and GR-S. A butyl type tube is identified by a blue stripe, and GR-S has a red stripe. Other than the standard tube, there are three special types of tubes: radial tire, puncture sealing, and safety.
If a tube tire has been punctured but has no other damage, it can be repaired with a patch. Remove the tube from the tire to find the leak. Inflate the tube and then submerge it in water. Bubbles will appear where there is a leak. Mark the spot. Then deflate and dry the tube.
There are two methods to patch a tube leak: the cold-patch method and the hot-patch method. With the cold-patch method (also known as chemical vulcanizing), first make sure the area is clean, dry, and free of grease and oil. Scuff the area around the leak. Then cover the area with vulcanizing cement. Let the cement dry until tacky. Press the patch into place. Roll it from the center out with a "stitching tool" or the edge of the patch kit can.
With the hot-patch method, prepare the tube in the same way as for the cold patch. Put the hot patch into place and clamp it. Then, with a match, light the fuel on the back of the patch. As the fuel burns, the heat vulcanizes the patch to the tube. After the patch has cooled, inflate the tube and recheck for leaks by submerging the tube in water.
Another kind of hot patch uses a vulcanizing hot plate. The hot plate supplies the heat required to bond the patch to the tube.
Wheels must have enough strength to carry the weight of the vehicle and withstand a wide range of speed and road conditions. Automobiles and light trucks are equipped with a single piece wheel. Larger vehicles have a lock ring (side ring) that allows for the easy removal of the tire from the wheel and, when in place, it provides a seat for one side of the inflated tire.
A standard wheel consists of the RIM (outer lip that contacts the bead) and the SPIDER (center section that bolts to the vehicle hub). Normally the spider is welded to the rim.
Common wheel designs are as follows:
The drop center wheel is made in one piece and is commonly used on passenger vehicles because it allows for easier installation and removal of the tire (Figure 12-32). Bead seats are tapered to match a corresponding taper on the beads of the tire.
Figure 12-32 — Drop center wheel.
Figure 12-33 — Semidrop center and a split wheel.
The semidrop center wheel has a shallow well, tapered-head seat to fit the taper of the beads of the tire (Figure 12-33). It also has a demountable flange or side ring which fits into a gutter on the outside of the rim, holding the tire in place.
A safety wheel is similar to the drop center wheel (Figure 12-34). The major difference is that the safety wheel has a slight hump at the edge of the bead ledge that holds the bead in place when the tire goes flat.
Figure 12-34 — Safety wheel.
A split wheel (rim) has a removable bead seat on one side of the rim (Figure 12-33). The seat is split to allow for its removal so tires can be easily changed. Some bead seats also require the use of a lock ring to retain the seat. These wheels are used on large commercial and military vehicles.
Lug nuts hold the wheel and tire assembly on the vehicle. They fasten onto special studs. The inner face of the lug nut is tapered to help center the wheel on the hub. Lug studs are special studs that accept the lug nuts. The studs are pressed through the back of the hub or axle flange. A few vehicles use lug bolts instead of nuts. The bolts screw into threaded holes in the hub or axle flange.
Normally, the lug nuts and studs have right-hand threads (turn clockwise to tighten). When left-hand threads are used, the nut or stud will be marker with an "L." Metric threads will be identified with the letter M or the word Metric.
Wheel bearings allow the wheel and tire assembly to turn freely around the spindle, in the steering knuckle, or in the bearing support. Wheel bearings are lubricated with heavy, high-temperature grease. This allows the bearing to operate with very little friction and wear.
The two basic wheel-bearing configurations are tapered roller or ball bearing types. The basic parts of a wheel bearing are as follows:
There are two types of wheel bearing and hub assemblies: nondriving and driving. For example, the front wheels on a rear-wheel drive vehicle are nondriving.
Figure 12-35 — View of a nondriving wheel bearing and hub assembly.
The components of a nondriving wheel bearing and hub assembly (Figure 12-35) include the following:
Spindle (a stationary shaft extending outward from the steering knuckle or suspension system to which the following components are attached).
Since a nondriving wheel bearing and hub assembly does NOT transfer driving power, the spindle is stationary. The spindle simply extends outward and provides a mounting surface for the wheel bearings, hub, and wheel. With the vehicle moving, the wheel and hub spin on the wheel bearings and spindle. The hub simply freewheels.
Figure 12-36 — View of a driving wheel bearing and hub assembly.
The components of a driving wheel bearing and hub assembly (Figure 12-36) includes the following:
The driving wheel bearing and hub assembly has bearings mounted in a stationary steering knuckle or bearing support. The drive axle fits through the center of the bearings. The hub is splined to the axle shaft. Instead of a stationary spindle, the axle shaft spins inside the stationary support. With the hub splined to the axle shaft, power is transferred to the wheels.
Wheel bearings are normally filled with grease. If this grease dries out, the bearing will fail. Some wheel bearings can be disassembled and packed (filled) with grease, while others are sealed units that require replacement when worn. When performing tire- related service, check the wheel bearings for play and wear.
For procedures on checking, removing, and replacing wheel bearings, refer to the manufacturer’s service manual.
Leaks from a tubeless tire are located by filling the tire with air and then placing the tire in a drum full of water. Bubbles will show the location of any leaks. If a drum of water is not available, coat the tire with soapy water. Soap bubbles will show the location of the leak.
It has been common practice to attempt the repair of some punctures without dismounting the tire through the use of a rubber plug. However, this practice is no longer recommended, because of serious safety concerns.
Using a plug to attempt tire repair without dismounting is effective only 80 percent of the time. The remaining 20 percent of such repairs will result in tire failure, which may take the form of a dangerous sudden deflation (blowout).
The safe and correct procedure for tire repair is to always remove the tire from the wheel and make the repairs from the inside of the tire. After the tire has been dismounted, it should be thoroughly inspected. During this inspection, check the inside surface carefully, to locate the puncture and determine the nature and extent of the damage.
The Rubber Manufacturers of America lists two requirements for correctly repairing a puncture: the repair MUST fill the injury to the tire and the repair MUST soundly patch the inner liner. Various products are available for repairing the puncture to the tire, including plugs and liquid sealants.
Figure 12-37 — Repairing a tire puncture.
Before replacing a tubeless tire, examine the rim carefully for dents, roughness, and rust; any defects may impair or break the air seal. Straighten out any dent with a hammer, and use steel wool or a wire brush to clean out any rust or grit in the bead seat area. After cleaning, paint any bare metal spots where the tire bead seats so that the tire is easier to remove later. If the rim is badly damaged, replace it with a new one. The procedure for repairing a tubeless tire (Figure 12-37) is as follows:
Serious injury can result using your bare hand to feel for obstructions; use a rag to feel inside the tire.
Each patch or plug kit should contain specific instructions.
A few basic safety rules for repairing a tubeless tire are as follows:
Preventive maintenance of tires and wheels involves periodic inspections, checking inflation pressure, wheel balancing, and rotation. Wheel bearings are periodically lubricated and checked for wear.
Rotating the tires will preserve balanced handling and traction of the tires and even out tire wear. Manufacturers recommend that tires be rotated every 6 months or 6,000 miles (whichever comes first), even if they do not show signs of wear. Tire rotation when done at the recommended times helps even out tire wear by allowing each tire to serve in as many of the wheel positions of the vehicle as possible.
Remember that tire rotation CANNOT correct problems due to worn mechanical parts or incorrect inflation pressures.
While every vehicle is equipped with four tires, usually tires on the front need to accomplish very different tasks than the rear tires. Each wheel position can cause different wear rates and different types of tire wear. It is to your advantage that all four tires wear together because wear reduces tread depth of a tire, and uniform wear allows tires to respond to the operator's input more quickly, maintains the handling, and helps increase the cornering traction of a tire. Figure 12-38 shows common tire rotation diagrams. A description of each is as follows:
Figure 12-38 — Tire rotations.
On vehicles that have non-directional tread patterns, rotate the tires in a forward cross pattern; you can include the spare tire as well.
If the vehicle has directional tires, rotate these tires from front to back only and vice versa.
If the vehicle has non-directional tires that are a different size from front to rear, rotate these tires from side to side only.
When your tires wear out together, you can get a new set of tires without being forced to change tires in pairs. You will also be able to maintain the original handling balance of the vehicle.
Improper wheel balance is the most common cause of tire vibration. Often a tire will appear to be round and true when rotated slowly. However, when one side is heavier than the other, centrifugal force tries to throw the heavy area outward during operation. To obtain maximum tire wear and a comfortable ride, you should balance the wheels. The two types of tire imbalance are as follows:
Figure 12-39 — Dynamic imbalance.
Figure 12-40 — Static imbalance.
To static balance a wheel and tire assembly, add wheel weights opposite the heavy area of the wheel. If a large amount of weight is needed, add half to the outside and the other half to the inside of the wheel. This will keep the dynamic balance of the tire. However, when dynamically balancing a wheel and tire assembly, you must add the weights exactly where needed (Figure 12-41).
Figure 12-41 — Where to add weights to a wheel.
A wheel-balancing machine is used to determine which part of a wheel assembly is heavy. The three types of balancing machines are as follows:
Figure 12-42 — Bubble balancer.
Figure 12-43 — On the vehicle balancer.
Tire problems usually show up as vibrations, abnormal wear patterns, abnormal noises, steering wheel pull, and other similar symptoms. In some cases, you may need to operate the vehicle to verify the problem. Make sure that symptoms are NOT being caused by steering, suspension, or front-wheel alignment problems.
When inspecting tires, you should look closely at the outer sidewall, tread area, and inner sidewall for bulges, splits, cracks, chunking, cupping, and other abnormal wear or damage. If problems are found before repairing or replacing the tire, determine what caused the failure.
Tire impact damage or road damage includes tears, punctures, cuts, and other physical injuries. Depending upon the severity of the damage, the tire must either be repaired or replaced.
Figure 12-44 — Tire wear patterns.
Tire wear patterns can be studied to determine the cause of abnormal tread wear.
By inspecting the tread wear, you can determine what parts should be serviced,
repaired, or replaced. Common tread wear patterns are as follows (Figure 12
The correct tire inflation pressure is important to the service life of the tire. Proper inflation is required to ensure that the tread of the tire fully contacts the road surface. This condition allows for even wear across the tread, therefore resulting in increased tire life and improved handling and safety. Tire over inflation causes the center area of the tread to wear quickly. The high pressure causes the body of the tire to stretch outward, pushing the center of the tread against the road surface. This action lifts the outer edges of the tread off the road. An overinflated tire produces a rough or hard ride. It is also more prone to impact damage.
Tire under inflation is a very common and destructive problem. This condition wears the outer edges of the tread (shoulders) because low pressure allows the sidewalls of the tire to flex, which builds up heat during operation. The center of the tread flexes upward and does not touch the surface of the road. Under inflation will cause rapid tread wear, loss of fuel economy, and possibly ply separation (plies tear away from each other).
Uneven tire inflation pressure can cause steering wheel pull. For example, when a vehicle that has the left front tire underinflated and the right front tire properly inflated, the vehicle has a tendency to pull to the left. The low air pressure in the left tire has more rolling resistance. This action tends to pull the steering wheel away from the normally inflated tire.
When one of the front tires is vibrating, it can be felt in the steering wheel. When one of the rear tires is vibrating, the vibration can be felt in the center and rear of the vehicle. Tire vibration can be attributed to several problems, such as out-of-balance condition, ply separation, tire run out, a bent wheel, or tie cupping.
Tire noise usually shows up as a whine due to abnormal tread wear or a thumping sound caused by ply separation. Tire replacement is required to correct these problems.
Wheel-bearing noise is produced by dry, worn wheel bearings. The bearing will make a steady humming type sound. This is due to the rollers or balls being damaged from lack of lubrication and being no longer smooth. To check for a worn wheel bearing, raise and secure the vehicle, and rotate the tire by hand. Feel and listen carefully for bearing roughness. Also, wiggle the tire back and forth to check for bearing looseness. It may be necessary to disassemble the wheel bearing to verify the problem.
5. What type of tire has the plies running at an angle from bead to bead?