Tire Care and Maintenance

Tire Inflation
Tire Pressure Loss
Temperature Effects
Tire Repair
Tire Alignment
Tire Balancing
Troubleshooting Vibration
Rotating Tires
Mixing Tires

Inflation

A tire is a pneumatic system which supports a vehicle's load. It does this by using a compressed gas (usually air) inside to create tension in the carcass plies. It is important to realize that a tire carcass has a high tension strength, but has little or no compression strength. It is the air pressure that creates tension in the carcass and allows the tire to function as a load-carrying device. That's why inflation is so important. In an unloaded tire, the cords pull equally on the bead wire all around the tire. When a tire is loaded, the tension in the cords between the rim and the ground is relieved by pressure from the ground. The tension in other cords is not changed. Therefore, the cords opposite the ground pull upwards on the bead. This is the mechanism that transmits the pressure from the ground to the rim.

However, a tire's job is more than to hold a load. It must transmit handling (acceleration, braking, cornering) to the road. Cornering forces are transmitted to the rim in a similar manner to load. Acceleration and braking forces rely on the friction between the rim and the bead. Inflation pressure also supplies the clamping force which creates this friction.

A tire also acts as a spring between the rim and the road. This spring characteristic is very important to the vehicle's ride. Too high an inflation pressure causes the tire to transmit shock loads to the suspension and reduces a tire's ability to withstand road impacts. Too low an inflation pressure reduces a tire's ability to support the vehicle's load and transmit cornering, braking, and acceleration forces. Finding the optimum inflation pressure requires extensive engineering efforts on the part of tire and vehicle manufacturers.

Underinflation can cause many tire related problems. Since a tire's load capacity is largely determined by its inflation pressure, underinflation results in an overloaded tire. An underinflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and excessive shoulder wear. High deflection also causes excessive heat buildup leading to catastrophic tire failure.

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Proper Inflation

The correct profile for full contact with the road which results in optimum tire performance. Because radial ply tires have a characteristic bulge in the sidewall even when properly inflated, it is impossible to visually determine the degree of inflation. Use a reliable air guage to check inflation pressure.

Under Inflation

Correct inflation is especially significant to the endurance and performance of radial performance tires. For example, because of a performance radial's aspect ratio and design, it may not be possible to look at a radial tire and actually see underinflation of 5 psi. However, underinflation of 5 psi can reduce a performance tire's tread life by 25%.
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Pressure Loss

While a tire is carefully designed to contain air, some leaks out. Air leakage can occur over time through the tire itself, through the valve core and at the rim. Typically, leakage results in a loss of about 1 psi inflation per month. At that rate it won't take long for a tire to become excessively under inflated.

Undetected punctures, bruise damage, cuts, and rim/ valve damage may produce more rapid air loss. Under inflated tires in general and performance radials in particular, may not convey a message of under inflation to the driver until heat failure or rim dislodgment occurs. Therefore, cold inflation pressures must be checked as often as possible.

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Temperature Effects

Air pressure is affected by temperature. The air under pressure in a tire is no exception. Typically, an inflation pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change. Higher temperature means increased pressure. For example, if a tire is inflated to 35 psi on an 80-degree July day, it could lose enough air to have an inflation pressure of 23 psi on a 20-degree day the following January. This represents a loss of 6 psi over six months and an additional loss of 6 psi due to the 60 degree temperature change. At 23 psi, this tire is severely under inflated.

NOTE: For safety and vehicle performance, you should recommend to your customers that they check tire inflation pressure at least once each month and as often as possible, when the tires are cold (ambient air temperature or if the vehicle has not been driven for several hours).

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Tire Repair

Any puncture or injury to a tire's tread area obviously affects performance and safety. Proper repair is critical. The puncture must be repaired on both the inside and the outside of the tire. One reason is the differences in the air retention abilities of the different layers of rubber used in a tire's construction. The thin layer of rubber on the inside of a tire is halobutyl rubber, which is designed to hold air. While the tread area is thick and durable, it is designed primarily for traction. Since all parts of a tire are engineered to function as a single unit, any repair must take that into consideration.

A tire patch must be firmly adhered to the inner liner. If a patch is not used or is incorrectly applied, air under pressure can leak through the puncture into the body of the tire causing separation. Also a plug must fill the in jury or puncture and seal the tread surface. Winter driving is especially corrosive. If road salt, debris or moisture penetrates a tire, rust in the steel belts can result, leading to separation. Therefore, a plug must be in place to seal the tire from the outside as well.

Nails and other objects may protrude into a tire causing internal damage that is not visible or apparent from the outside. A tire repair can be properly made only if the tire is removed from the rim, a thorough internal inspectio n is carried out, and repair is made from the inside out.

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Alignment

Tires should be checked monthly for signs of irregular wear in both tread and shoulder areas. Irregular wear indicates the need for a wheel alignment or suspension repairs. Also, when a replacement set of tires is fitted to a car , a wheel alignment is mandatory. Cars with four wheel independent suspension systems or cars with four wheel steering require four wheel alignment.

The three elements involved in wheel alignment are angles referred to as caster, camber and toe. Each affects the vehicle's performance.

Caster is the angle between a vertical line and a line drawn through the center of the ball joints (steering axis) when the vehicle is viewed from the side.

Positive Caster

Negative Caster

Caster is called positive when the steering axis tilts toward the rear of the vehicle at the upper steering pivot. Positive caster helps in maintaining directional control of a vehicle, tends to return the vehicle to a straight a head position when cornering, and helps compensate for the crown on a road. Most cars are engineered with positive caster.

Improper caster can cause hard steering, increased road shock, reduced straight line stability and cause the vehicle pull to one side or the other.

Camber is the angle between a perpendicular line and a line drawn through the centerline of the tire when the vehicle is viewed from the front.

Negative and Positive Camber

Camber is called positive when the tops of the tires tilt away from each other. Camber helps distribute the vehicle's weight across the tire contact patch during cornering for reduced tread wear and is used to fine tune a vehicle 's handling characteristics.

Incorrect camber causes uneven tire wear, poor steering and vehicle stability, and vehicle pull.

Toe refers to the angle between a line through the center of a vehicle and lines drawn through the centerlines of the tires when the vehicle is viewed from above.

Toe-In and Toe-Out

When the tires are closer together at the front than at the rear, the condition is called toe-in. When the tires are closer together at the rear than at the front, the condition is toe-out.

The toe adjustment is the most critical to tire wear. Incorrect toe results in rapid and uneven wear.

When a manufacturer designs a car, engineers select these elements (angles) and establish them as specifications for the best overall vehicle handling performance. Failure to properly align a car's wheels can result in excessive tire wear, excessive fuel consumption and unsafe handling.

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Balancing

Tire/wheel balance is important for vehicle ride and stability and for customer satisfaction as well. Tires are manufactured to close tolerances for roundness and shape, but as a tire wears, the tire's mass may become unevenly distributed, and the original rim/ tire balance may be affected.

A tire both rolls and steers. To provide smooth running, a tire's mass must be evenly distributed around both the rolling axis and its centerline. A heavy spot in the center of a tread can cause bounce. This is referred to as static imbalance.

An improperly mounted tire or a heavy spot in a sidewall can cause wobble, or side-to-side shake. This is called dynamic imbalance. Any such movement transmits itself throughout the vehicle. Wheel imbalance causes forces that res ult in vibration through the vehicle's steering, suspension and body. Imbalance is the cause of the majority of vibration complaints. Rims and poorly matched rim/tire combinations are frequent contributing factors.

Static and Dynamic Imbalance

There are two approaches to wheel balancing: on-car balancing and off-car balancing. Various types of balancing equipment are available, including static and dynamic balancers for off-car work and mechanical and electronic balancers for on-car work.

Off-car dynamic balancing is recommended because it balances the wheel/tire unit very accurately. However it cannot compensate for imbalance in the brake/hub assembly. Some vehicles may have imbalanced brakes. In this case, the brake/hub must be removed and balanced separately. This gives the optimum balance and allows for future tire rotation.

On-car electronic balancing includes the brake/hub assembly and the wheel/tire unit, making possible an accurate balance. It is difficult to perform a dynamic balance using this method, but it does balance the entire rotating mas s as a unit. The wheel must be marked so it can be reinstalled in the same location (both position on the car as well as position on the hub) after future service work.

Wheel weights added to the inside or outside of the rim correct imbalance as indicated by balancing equipment. While balancing may seem a relatively simple operation, care must be taken to eliminate imbalance, since even a minor imbalance contributes to wheel wobble and poor ride and handling characteristics.

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Vibration Troubleshooting

There are many rotating items in a car's drive train which can cause vibrations. Each part has its own range of natural frequencies. If a natural frequency is excited, the part causes the greatest disturbance as it vibrates. Engine vibrations occur at the highest frequency, many hundreds of vibrations per second, and cause a buzzing or humming sound. These vibrations can be felt in the floor or dashboard. Tire and wheel related vibrations occur at a much lower frequency, typically 10 to 15 times a second. This results in the "shake" most of us have experienced. Even a small amount of wheel imbalance can translate into vibration problems.

Tire-induced vehicle shake is most often caused by:
(1) a tire poorly seated on the rim;
(2) a heavy spot in the tire,
(3) a stiff spot in the tire, or
(4) an out of round tire.
The most critical speed for tire-induced shake is from 55 to 65 mph. At this speed, a tire rotates at 10 to 15 times per second. This is the frequency that corresponds to the natural frequency of a car suspension for virtually all cars, making shake a potential problem for all drivers.

A heavy spot in a tire or tire/rim combination causes a radial force which bounces the tire up and down (static imbalance) once per tire revolution. This force increases as the cars speed increases. It is only at higher speeds, faster than 40 mph, that this weight induced bounce becomes noticeable. An out of round tire or wheel causes a once per tire revolution up and down force that is independent of speed If the vibration is not evident at low speeds but begins at higher speeds , imbalance is the likely cause. If a bounce is felt at very low speeds, the likely cause is an out of round tire or wheel. Sometimes a low speed bounce is caused by "flat spotting". Make sure the tire is warmed up from several miles of driving to elimina te flat spotting as a source of low speed bounce.

While the up and down shake or bounce results from a heavy spot in the tread, the side-to-side shake or wobble (dynamic imbalance) can result from poor bead seating or a heavy spot in a sidewall. If both front tires are affected, the vibration will be added together when both tires are in phase, and may go away completely after a corner, which causes the outside tire to roll more than the inside tire. This tire repositioning can cause the vibrations to cancel each other. Soon, however, the tires will be in phase again, and the vibrations will recur. Proper balancing can correct vibrations resulting from heavy spots in tires.

An out-of-round (runout) tire/wheel, or a stiff spot in a tire can cause vibration even in a perfectly balanced tire. In these situations, forces create vibrations that are present regardless of vehicle speed. Wheels don't have force variation, but they all have some runout. Likewise, every tire has some force variation. Each of these variations, by itself, would not cause a problem but there will be a vibration problem if the stiff spot or high point of a tire happens to be plac ed at the high spot on the rim during mounting. It is easy to avoid this condition. New tires and rims are marked with dots or similar devices. When these dots are matched, the high spot on the tire is lined up with the low spot on the wheel, and vibration is minimized (see Match Mounting).

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Red Dot - To be used in Match Mounting

If wheels and tires are not marked, and the vehicle exhibits runout vibration, the tire should be rotated one half turn (180 degrees) on the wheel. This will generally reduce the vibration.

Worn shocks or suspension will allow a minor vibration to magnify until it is felt as a major one.

Harmonic Marking/Match Mounting

For the smoothest ride and least vibration, the tire must be carefully matched to the wheel. Even though new wheels are marked, when mounting new tires on a used wheel, the original marks may not be found. Even if they were found , the wheel may have shifted around and must be re-measured.

Rim runout can be measured using a dial indicator. Measure and mark the lowest spot of the inside flange and the outside flange. Midway between these two marks is the average low spot of the rim. Mark this spot on the valve stem side of the rim.

Tire runout is not so simply measured. Tire runout includes both measurable runout, as well as the stiff spot in the sidewall. Interaction between the visible runout and the stiff spot make it impossible to find a tire's true high spot virtually without specialized equipment. This specialized equipment, loads a tire is against a road wheel which simulates a highway surface. The tire is rotated against the revolving road wheel, and the radial force variation of the tire acting aga inst the road wheel is measured. This force variation is analyzed and the high spot of the first harmonic is marked on the tire.

Now that both the rim and tire have marks, it is easy to mount the tire so that the harmonic mark on the tire is lined up with the average low spot of the rim. This is called match mounting and cancels the tire runout and wheel runout, providing the smoothest possible ride.

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Tire Rotation

Tire rotation is important for even tread wear and long tread life. As a tire is driven on a road surface, it begins to wear. Tire wear rarely takes place uniformly on tires, because each tire is mounted at a different position on a vehicle.

Front and rear, drive and non-drive tires exhibit different wear pattems. Front tires tend to wear more rapidly in the shoulder area, because of steering/cornering forces. Drive tires wear more rapidly in the tread center because of drive traction forces. On front-wheel-drive cars, front tires wear much more rapidly than rear tires. Personal driving habits and vehicle performance characteristics also cause tires to wear differently. Rotating tires at frequent intervals (at 6,000 miles or less) tends to equalize tire wear and minimize the progress of irregular wear.

Tires are considered to be legally worn out and must be replaced when the tread depth across two adjacent grooves is 2/32". Look for the 2/32" tread wear indicator (TWI) bars distributed around the tire circumference at the base of the grooves. Drive tires with worn out centers or steering tires with worn out shoulders should be replaced, even though there may be useful tread remaining in less worn areas.

To get maximum life from a set of tires, they must be rotated. Vehicles are engineered to operate with a matched set of tires. Even those vehicles fitted with different size tires front and rear require a matched set on each axle . Handling capabilities, including cornering and braking traction depend on matched tires. If tires are not properly matched or equally worn, vehicle performance may be compromised.

Tires should be rotated at frequent and regular intervals. Tires can be rotated conveniently during a regular oil change, which for most vehicles occurs every 5,000 to 7,500 miles. Dunlop recommends rotation every 6,000 miles or less for optimum benefit.

There are many tire rotation patterns. Any routine pattern is better than no rotation at all. The important factor in a tire rotation pattern is that all tires eventually are placed at all applicable wheel positions. If a vehicle is equipped with a full size spare, the spare should be included in the rotation pattern.

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Mixing Tires

Broadly speaking, tires should not be mixed. It is preferable that all tires on a vehicle should be the same brand, performance rating, size, and construction, unless the vehicle was engineered for front and rear tires of different sizes.

If there is no choice but to mix tires:

Match the tire sizes on an axle in pairs (except when it's necessary to use a temporary spare).

If two radial tires and two non-radial tires must be fitted to a vehicle, always put the radials on the rear.

Always match tires sizes and constructions on 4 wheel-drive vehicles.

Snow tires can be fitted in pairs on rear wheel drive vehicles, but on front wheel drive vehicles tires should be fitted on all four positions.

Safety Note:
Failure to fit four (4) tires of the same size/construction, type and required performance rating to a performance vehicle may result in seriously deteriorated handling and control. M8S designated tires must be fitted in sets of four to performance vehicles.
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