Automotive Dictionary

Air Pressure

Check the pressure in your tires at a minimum of once per month and before long trips when your tires are cool. Your tires can be considered cool after the vehicle has been stopped for at least 3 hours and driven less than one mile. Adjust to the vehicle manufacturer's specified pressure while tires are cold. Never bleed or reduce air pressure when tires are hot. It is normal for pressure to build up as a result of driving. Use an accurate tire gauge to check pressure and maintain it at the level recommended by the vehicle manufacturer. Keep your tire within the manufacturer's specified pressure range. Ignoring this recommendation may result in unexpected loss of vehicle control or accidents.

Alignment of Vehicle

If you notice that your vehicle pulls to the left or right of the road while driving with the wheel in the 12 o' clock position, you may need a wheel alignment. A wheel alignment places all four wheels of the vehicle parallel, or aligned, to one another. The aligned tires are also adjusted to be perpendicular with the ground. Properly aligned wheels help with ease of steering and will distribute general wear on your tires evenly.

All Season Tires

All season tires are tires designed to be used throughout the year. These tires are designed specifically to handle mud and snow conditions. To spot an all season tire, look for the marking "MS", "M/S", "M+S", or "M&S" on the tire's sidewall. The Rubber Manufacturer's Association (RMA) defines a standard criteria for all season tires. In essence, the tire must have several large gaps over the entire length of the tread, with the gap space being at least 25% of the tread surface. The exact specification, as taken from the RMA Snow Tire Definitions for Passenger and Light Truck (LT) Tires, is as follows:

a. Extend toward the tread center at least 1/2 inch from the footprint edge, measured perpendicularly to the tread centerline.

b. A minimum cross-sectional width of 1/16 inch.

c. Edges of pockets or slots at angles between 35 and 90 degrees from the direction of travel.

2. The new tire tread contact surface void area will be a minimum of 25 percent based on mold dimensions.

General mud and snow all season tires are not required to be industry tested. For this reason, the "Severe Snow Use" tire indicator symbol is used for all season tires that have passed a RMA specification testing standard.

ABS / Anti-Lock Brake System

In traditional non-abs vehicle braking systems, applying pressure to the brakes in hazardous road conditions or emergency situations can cause the wheels to lock up and prevent the driver from steering the vehicle. These traditional systems require the driver to "pump" the brakes in order to prevent this loss of control. In emergency situations, the driver may not remember or be able to do this.

Anti-Lock brake systems electronically and precisely control brake application by automatically "pumping" the brakes (at a much faster rate than a driver can) and adjusting brake fluid levels at each of the vehicle's wheels, something the driver of traditional braking systems cannot manually accomplish. Drivers with ABS systems should NOT pump the brakes, as the system will handle this for them. A driver can tell the ABS system is working when a slight vibration can be felt on the pedal and a low buzzing heard when braking. The result of braking with an ABS system is full steering control during maximum braking conditions.

ASC / Anti-Slip Control

Vehicles in traditional systems may experience one of two tires on an axle to slip and spin without traction while the other tire does not move. In systems using anti-slip control, the power is distributed between the tires proportional to how each tire is moving. In the example, the tire that is spinning will receive less power and the stationary tire will receive more. This effect gives the vehicle more efficient use of traction.

Balancing

Tires cannot be manufactured perfectly, and therefore slight variations can cause the weight of the the tire to be uneven. At high rotational speeds of the tire, this imbalance can cause the tire to wobble and generate vibrations. Tires are tested at the manufacturer and at the time of installation to find these imperfections. To counter the wobble effect, small metal weights are applied to the wheel rim which help to give the tire a uniform weight distribution and reduce vibration.

Bead

The bead is that part of the tire that sits on the rim of the wheel and locks the tire into place. The locking effect of the bead is supported by the air pressure within the tire.

Braking distance

The distance required for braking depends on the speed of the vehicle, the condition of the road surface and the condition of the tires, particularly the tread. Keep your braking distance to a minimum by changing worn brake pads in a timely manner and replacing tires when necessary. Most tires have a tread wear indicator that will alert you to the need to replace the tire. Check these often to maintain best performance.

Camber

Camber is the the measured angle between the vertical axis of a vehicle's tire and the road. Essentially, this is a measure of how much a tire tilts into or away from the vehicle. When the tire tilts away from the vehicle, it is known as positive camber. When the tire tilts toward the vehicle, it is known as negative camber.

Camber affects the handling of a vehicle. When there is zero camber on a tire, or no tilt, the tire's tread contact with the road is maximized on flat, straight surfaces. During cornering, however, a zero camber tire will begin to lift it's outside edge off of the ground, which results in reduced handling. This affect is commonly aided with the use of negative camber.

Casing

Modern tires are made of many different materials and components. Tires are constructed in many different ways.

Casing components may include steel and/or textile cord plies, the inner liner (to make tube-less tires airtight), sidewalls, the apexes, the bead core (keeps the tire on the rim) and the bead reinforcement. There are several standard variations on this casing, or core:

Bias

Bias, or cross ply, casings consist of ply cords that angle across the width of the tire from bead to bead, generally at a 30 to 40 degree angle. A second set of metal ply cords run in the opposite angle, creating a crisscross ply effect. The benefit of this design is a smoother, quieter ride, but forfeits handling and traction at high speeds.

Radial

Radial tires have body cords that run across the tire nearly perpendicular to the beads. Radial tires have belt plies, which are laid diagonally under the tread to stabilize and strengthen the tread area and add flexibility to the sidewall. By restricting tread movement during contact with the road, the belt plies increase improve tread life, traction, and handling.

Chains

Even modern winter tires sometimes can't help when there are huge amounts of snow and steep gradients. In these situations, traction, lateral control, and reliable braking require the help of tire chains. In order to be prepared for chain use during hazardous winter conditions, it is recommended to fit chains in a "dry run". Snow chains are used only for conditions in which traction is greatly reduced and is not intended for high speeds. With some low profile tires, the reduced space between the tires and the wheel arch leaves no room to fit snow chains. Use chains with caution.

Date of Manufacture

The date of manufacture of a tire is indicated on the tire's sidewall at the end of the DOT serial number.

Tire manufacturers have adopted a standard identification system using four numbers which indicate the week and the year of manufacture. For example, the figures 0201 indicate that the tire was made in the second week of the year 2001.

Direction of Rotation

On standard tires with symmetrical tread patterns, it does not matter which way the tire is fitted on the rim and in which position it is fitted on the car. Some tire manufacturers have, however, started producing tires with specific directions of rotation in order to improve wet grip and reduce noise generation. The direction of rotation is marked on the side of the tire with an arrow. This side of the tire must be on the outside, and the tire must roll forward in the direction of the arrow for optimum tire performance. A number of tires with asymmetric tread patterns are also now available which do not have a specific direction of rotation.

DOT Serial Number

The "DOT" symbol certifies the tire manufacturer's compliance with the U.S. Department of Transportation tire safety standards. The DOT serial number is located on the lower sidewall of the tire, on one side only. Below is a description of the serial number. Starting in the year 2000, four numbers are used for the Date of Manufacture, the first two numbers identify the week and the last two numbers identify the year of manufacture. Prior to year 2000 three numbers are used for the Date of Manufacture, the first two numbers identify the week and the last number identifies the year of manufacture. To identify tires manufactured in the 90's a decade symbol (a triangle on its side) is located at the end of the DOT serial number.

For Example: DOT NJ HR 2AE2 529

DOT: Reference Symbol (certifies the tire manufactures compliance with U.S. Department of Transportation tire safety standards).

NJ: Manufactures Plant Identification Code

HR: Tire Size Code Number.

2AE2: Tire Type Code (coding for type of tire optional by manufacture).

529: Date of Manufacturer, example: 529 (52nd week of 1999) or 5200 (52nd week of 2000).

ESC / Electronic Stability Control

In traditional vehicle systems, vehicle stability is manually maintained by the driver. If the driver encounters a hazardous situation in which control of the vehicle is lost, the driver must take the appropriate actions necessary to regain control. Electronic stability control systems attempt to take these corrective actions for the driver. The system senses a loss of control in the vehicle and will apply precise braking to each of the four wheels independently in order to regain control and move the vehicle towards the driver's intended direction. In some systems, engine power is also reduced as needed.

Emergency Mobility Systems

If a standard tire is punctured and looses air pressure, a temporary spare tire must be put on in order to avoid towing the vehicle to a maintenance shop. In order to avoid the troublesome, sometimes dangerous, procedure of changing a tire on the open road, various manufacturers now offer what is known as emergency mobility systems. This system is a special tire technology that allows the tire to be driven on for a short amount of time with no air pressure to get the vehicle to a maintenance shop.

Hydroplaning

"Water skiing on the road."

Hydroplaning is an effect that occurs on wet roads, when the contact between the tire and the road is separated by a thin layer of water. Tires are designed to handle this effect with the use of grooves in the treads, allowing a place for the water to accumulate instead of between the tire and the road. The faster a vehicle travels and the less percentage of grooves on the tire's surface results in a much higher degree of hydroplaning and the potential for the vehicle to lose control and glide across the road. Although tires are designed with sophisticated tread patterns to handle this, the best method is to slow down in hazardous conditions.

Load Index, Ply Rating and Load Range

Load range is a rating system for light truck (LT) tires which indicates both ply rating and load pressure. Load range is a letter, B-F, found on the tire sidewall. Load Index is a rating system for passenger (P) tires that indicates the maximum load that the tire can carry. Load Index is a non-decimal number between 60 and 120 and is found on the tire's sidewall.

Mixing Tires

It is recommended that all four tires be of the same size, construction and speed rating. If tires of different speed rating are mounted on a vehicle, the vehicle speed capability will be limited to the lowest speed-rated tire on the vehicle. It is recommended that the lower speed-rated tires be placed on the front axle regardless which axle is driven. This should be done to prevent a potential over steer condition. Vehicle handling may also be affected. Consult the tire manufacturer for specific guidelines on your tire.

Radial Tires

Reinforced or XL (extra load) Tires

Reinforced or XL (extra load) tires are specially reinforced tires. They can carry higher loads than a tire of the same size. Reinforced tires are designated on the sidewall by the letters "RF", and extra load tires are designated with the letters "XL". Reinforced and XL tires generally require higher inflation pressure compared to standard tires of the same size.

Revolutions Per Mile (RPM)

The number of revolutions a tire makes in one mile at a given load, speed, and inflation. Sometimes measured as RPK, or revolutions per kilometer.

Rolling Resistance

Rolling resistance is the restrictive force placed on a tire by the driving surface. To put a free rolling tire into motion, there must be enough force applied to the tire to overcome the opposite force placed on the tire by rolling resistance. In order to reduce rolling resistance, manufacturers use special rubber compounds. Any reduction in the rolling resistance of the tire helps reduce fuel consumption, however, strategic use of rolling resistance can aid the driver in reduced braking. Since rolling resistance is directly affected by tire pressure, it is beneficial to check the pressure of tires regularly.

Rotation

It is recommended to rotate your tires every 6,000 to 8,000 miles, or sooner if uneven tread wear begins to appear. The purpose for regular rotation is to achieve more uniform tread wear on all tires on your vehicle. This is true for both front wheel and rear wheel drive vehicles. Full size spare tires should be included in the rotation pattern for your vehicle. Compact spares (temporary use spares) should not be included in the rotation pattern.

Refer to your Vehicle Owners Manual for recommended rotation pattern and interval for your vehicle. If tires show uneven tread wear, ask your serviceperson to check and correct any alignment or other mechanical problem before rotation.

Speed Rating

The speed ratings for a tire is indicated on the sidewall of the tire by a letter or letter and number. Each speed rating indicates the maximum speed at which the tire is designed to be safely driven. Speed ratings are based on laboratory tests which relate to performance in the road, but are not applicable if tires are under inflated, overloaded, worn out, damaged or altered.

Example: Tire Size P215/60R15 H - the H indicates a maximum permitted speed of 130 MPH.

Temporary Spare Tires

Temporary spares are designed to carry the same load as the standard size tire on your vehicle and can be applied to any position. Maintain the proper inflation pressure as shown on the sidewall of the tire, which generally requires a higher inflation pressure than a standard size tire. Refer to the information on the sidewall of the tire for proper usage & speed restrictions. With such a tire, a vehicle may be operated until it is convenient to repair or replace the disabled tire. Have your standard tire repaired or replaced as soon as possible, then return the temporary spare to the trunk to conserve its useable tread life. The temporary tire can be worn down to the tread wear indicators, just like your standard tire.

Tire Size Designation

The dimensions of a tire are detailed on the sidewall.

In the case of a P185/65R 14 tire, the figures mean the following: 185 = width of tire in mm; 65 = the ratio of the height to the width as a percentage; R = radial construction; 14 = diameter of the rim in inches.

Tire Storage

Tires should be stored in a dry, cool place, away from sunlight and sources of ozone, such as electric motors.

If you must store tires flat, (one on top of the other), make sure you don't stack too many on top of each other. Too much weight can damage the bottom tire.

Also be sure to allow air to circulate around all sides of the tires, including underneath, to prevent moisture damage.

If storing tires outdoors, protect them with an opaque waterproof covering and elevate them from the ground. Do not store tires on or over black asphalt or other heat-absorbent or reflective surfaces, such as snow-covered ground or sand. Solvents, fuels, lubricants and chemicals should be kept out of contact with tires.

Spare tire carriers on your vehicle are not intended to be used for long term tire storage. If your vehicle has a full size tire (same size and type tire recommended for use by the vehicle manufacture not temporary use spares) as a spare, it should be included in the tire rotation pattern.

Toe

Toe describes the angle at which the leading edge of a pair of tires is out of parallel. When a pair of wheels is set to point slightly toward each other, it is known as toe-in. When a pair of tires is set slightly away from each other, it is known as toe-out. Toe-in is used for increased straight line driving stability and will result in less responsive turning. Toe-out will result in slightly exaggerated movements and faster turning response. Generally, most vehicles are not performance racing vehicles, which sacrifice stability for turning (toe-out), and will want slight positive toe-in for stability. If you expect your vehicle to have neutral or slightly toe-in and you notice excessive wear, particularly on only one side (the inner or outer side) of the tire, you may be experiencing toe-in or toe-out. Your vehicle may need alignment maintenance if this is unexpected.

Tread

The tread is that part of the tire with the groove pattern which is in contact with the road. The tread is specifically design to provide traction for stopping, starting, cornering and provide long lasting wear.

Tread Depth

The measured distance from the tread surface to the bottom of the main grooves away from the Tread Wear Indicators. Usually specified in 1/32 of an inch.

TWI (tread wear indicator)

Tread wear indicators ("wear bars") are located at the base of the main grooves and are equally spaced around the tire. Always remove tires from service when they reach a remaining tread depth of two thirty-seconds of an inch (2/32"). If not corrected, wet weather accidents are more likely to happen due to skidding on bald or nearly bald tires. Also, excessively worn tires are more susceptible to damage from road hazards. Built-in tread wear indicators, or "wear bars," which look like narrow strips of smooth rubber across the tread, will appear on the tire when that point of wear is reached. When you see these wear bars, the tire is worn out and it's time to replace the tire.

UTQG (Uniform Tire Quality Grading)

Treadwear

The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. A tire graded 200 would wear twice as long on the government test course under specified test conditions as one graded 100. It is wrong to link tread wear grades with your projected tire mileage. The relative performance of tires depends upon the actual conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate.

Traction

Traction grades, from highest to lowest, are AA, A, B and C. They represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete.

Temperature

The temperature grades, from highest to lowest, are A, B and C. These represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel.

Valve

The valve, fitted in the wheel, ensures that the tire can be filled with air. The correct valve is required for the correct wheel/tire assembly, which is the job of the tire dealer. A slow loss of air pressure may be a sign of a defective valve. The valve cap should always be fitted to the valve in order to protect the valve core from dirt and moisture.

Valve Cap

The valve cap, although small, has a very important job: it protects the sensitive valve internals from dust, dirt and humidity. If valve caps are lost they should be replaced immediately in order to avoid expensive damage later.

Winter Tires

In snowy areas, many cities and counties have "snow emergency" regulations which are invoked during heavy snowfalls. Motorists are subject to fines if they block traffic and do not have winter tires on their vehicles. Check with authorities for the rules in your area.

You can avoid this by equipping your vehicle with winter tires marked "MS," "M&S," or "M + S" on the sidewall and severe snow marking. If you change to winter tires, be sure they are the same size and construction type as the other tires on the vehicle. It is recommended that winter tires be applied on all four positions, but it is acceptable to install winter tires only on the rear position of a rear wheel drive vehicle. If winter tires are installed on the front position of any vehicle, they must also be installed on the rear position. Without winter tires on the rear, vehicle handling can be adversely affected. This may result in loss of vehicle control which could cause serious injury or death. If winter tires with a lower speed rating than listed on the vehicle placard are installed for use in winter conditions, the speed capability of the vehicle is reduced to the speed rating of the winter tires.

In areas where heavy snowfalls are frequent, many drivers carry chains for use in emergencies, or have their tire dealer install studded winter tires or tires for use in severe snow conditions. Most states have time limits on the use of studs. Before installing studded tires, check the regulations in your area. If studded tires are applied to the front axle, they also must be applied to the rear axle. If you use chains, make sure they are the proper size and type for your tires, otherwise they may damage the tire sidewall and cause tire failure. Tires designed for use in severe snow conditions generally have tread patterns, structure, and materials to give superior performance. These tires are marked with the "M+S" designation plus a mountain/snowflake symbol.