Tire Terms

Your Tire Dictionary

Here’s a comprehensive list of tire and vehicle related words so you can speak fluently about tires. Interested in tire trivia? Check out these fun tire facts.

ACCELERATOR - A chemical, which speeds the vulcanization of rubber; used in tire compounds to reduce curing time.

ACTIVATOR - A rubber compound  chemical used to help initiate the vulcanization process.

ADJUSTMENT - A prescribed allowance given  to a customer toward the replacement of product pursuant to the warranty.

AGING - Deterioration of physical and chemical properties of rubber by oxidation over a period of time.

AIR PRESSURE - Force exerted by air within a tire, expressed in pounds per square inch or kilopascals (kPa).

ALIGNMENT - Angles of the tire and suspension axis relative to each other and the ground: camber, caster, toe. Also, the adjustment of components to bring them into a predetermined position for the most efficient operation of wheel and vehicle for proper even tire wear.

ALPHA-NUMERIC - A load-based tire sizing system containing the load capacity, expressed as a letter (i.e.: A, B, C, etc.) type of tire construction, aspect ratio, and the rim diameter in inches.

ANTIOXIDANT - A chemical, which when added to a rubber compound, prevents surface oxidation; used in tire tread and sidewall compounds to prevent weather checking and cracking.

ANTI-ROLL BAR - A steel bar, linking the left and right side of a suspension. It comes into play during cornering. As the car leans in a turn, the anti-roll bar resists this leaning by transferring more weight to the outside tire. It provides a means to achieve good handling from stiff roll resistance while maintaining the tire’s contact patch.

ASPECT RATIO - The dimensional relationship between tire section height and section width; section height divided by section width.

AUTOCROSS - Type of auto competition where cars traverse a twisting course marked by pylons. Sometimes referred to as a Gymkhana or solo.

AXLE - A cross support of a vehicle on which its road wheels turn.

BACKSPACING - The measurement from the back of the bolt pad to the back edge of the rim; used to calculate offset and determine where the back of the bolt pad is located in relation to the rim width, sometimes referred to as rear spacing.

BALANCE - A uniform mass distribution of a tire and wheel assembly about its axis of rotation.

BALE RUBBER - The form in which solid rubber is shipped to tire manufacturers.

BALL JOINT - A ball-and-socket connection that lets a steering knuckle move in several directions  at the same time.

BANBURY MIXER - An enclosed mixing machine for the production of rubber materials

BEAD - The area of the mounted tire which seats against the wheel

BEAD FILLER - A rubber extrusion in the bead area of a tire; used to permit a smooth contour    of casing plies around the bead and to the lower sidewall. Also used in enlarged form to stiffen the lower sidewall of a tire.

BEAD SEAT - The position where the tire rests and seals on the inside of the rim.

BELT - An assembly of fabrics and/or wire used to reinforce a tire’s tread area. In radial tires, it also constrains the outside diameter against inflation pressure and centrifugal force.

BELT EDGE WEDGE OR INSERT - An extrusion of rubber placed under the edges of a belt; used in radial tires to improve durability.

BLADDER - A rubber bag used inside a tire during the molding and curing process; contains hot water or steam which presses the inside of the tire into the mold.

BLOCK - Part of a tire tread pattern created by lateral (side-to-side) grooves.

BODY - Tire structure except for tread and sidewall rubber.

BOLT CIRCLE - Often referred to as the wheel bolt pattern; the diameter of an imaginary circle drawn through the center of each lug hole.

BRAKING TORQUE - Torque applied by a brake to a tire/wheel assembly,  which  slows  or  stops the vehicle.

BREAKAWAY - The point at which tire cornering traction is lost.

BUMP STEER - A steering effect resulting from toe or camber changes as the suspension moves up and down.

CONICITY - Quality or state of being conical. (Cause of radial tire pull.)

CONTACT PATCH – The part of the tire in contact with the road surface. See Footprint.

CONTROL ARM - A device used to connect the unsprung position of a suspension to the sprung chassis, which allows suspension travel.

CORD - Fabric or steel wire strands forming plies and belts in tires.

CORD ANGLE - The degree at which the plies or belts cross the centerline of any given tire.

CORNERING FORCE - The force that turns a vehicle around a corner. The opposite of lateral or centrifugal force.

CROSS PATTERN - The sequential torquing of the lug nuts in a pattern across from one another.

CROSS-SECTION WIDTH - External sidewall- to-sidewall measurement of inflated tire, exclusive of ornamental ribs and lettering. Sometimes called section width.

CROWN - The region between the shoulders of the tire.

CROWNED ROAD - A road design with a slope or pitch from its center to the curb or shoulder in order to facilitate water drainage.

CURB GUARD - A rubber protrusion running circumferentially around some tires just above the whitewall to prevent curb scuffing on the whitewall area of a tire.

CURB WEIGHT - The total weight of a vehicle with no passengers and a full tank of gas.

CURE - To vulcanize; also time and temperature conditions used to vulcanize a tire.

C.U.V.  - Crossover Utility Vehicle.    

DEFLECTION – The difference between a tire’s unloaded or free radius and the loaded radius.

DESIGN RIM - A rim with a specified width; used to determine basic tire dimensions.

DIRECTIONAL STABILITY - The ability of a car to travel in a straight line with a minimum of driver control.

DOG TRACKING - A condition where the rear wheels do not follow the path of the front wheels.

D.O.T. - A tire branding symbol, which  denotes the tire meets requirements of the Department of Transportation.

DOUBLE A-ARM - A suspension system which uses two “A”-shaped links or arms of unequal length to attach the upright supporting the wheels to the frame.

DRIVE WHEEL APPLICATION - Describes tires, which are designed specifically for optimum performance on drive wheels.

DUAL COMPOUND TREAD - A tire tread with two rubber compounds.

DUALS - Two tire and wheel assemblies, mounted on one side of an axle.

DUROMETER - A measure of the hardness of a rubber compound; its resistance to penetration of a spring-loaded blunt needle.

DYNAMIC BALANCE – Balance  in  Motion. The balance of a tire and wheel while it is rotating. A condition in which a tire and wheel  assembly  has weight distributed equally on both sides of the wheel’s axis of rotation.

ETRTO - European Tyre and Rim Technical Organization.

EURO METRIC TIRE SIZE SYSTEM - A tire sizing system expressing the cross section in millimeters, aspect ratio, tire construction, rim diameter in inches, load index and speed symbol (e.g.: 185/70R14 88T).

EXTRA LOAD - A P-Metric tire with a maximum inflation of 41 psi. or 50 psi. For Euro-Metric tires, the term ‘reinforced’ means the same as “Extra Load”. This higher pressure than a “Light Load” and “Standard Load” tire permits a greater load capacity.

EXTRUSION - The process of forcing a material through an orifice to obtain a length of material; used to fabricate tire components.    

FABRIC - An array of parallel cords used in tire construction.

FLOTATION TIRE - A tire designed to minimize soil penetration and compaction.

FLUSH FIT - A condition where the mounting pad of the wheel goes into place freely and without obstruction against the hub-mounting surface.

FOLLOWER JOINT - A non-load-carrying ball joint that maintains the position of the steering knuckle and provides the correct steering axis inclination angle; sometimes called pilot joint or friction joint.

FOOTPRINT - The mark left by a loaded tire’s tread as it meets the road surface. See Contact patch.

FORE AND AFT WEIGHT TRANSFER - A load factor where weight is transferred from the front tires to the rear tires during acceleration and from the rear to the front tires during braking.

“g” - A unit of acceleration. “g” is a symbol that represents the force of gravity. Acceleration of one g equals 32 feet per second.

GREEN TIRE - A tire which has not been vulcanized or cured.

GROOVES - Circumferential channels between the tread ribs of a tire.

GROSS AXLE WEIGHT RATING (GAWR) - The maximum weight that the front or rear axle can carry. The front and rear gross axle weights must not exceed the front and rear GAWR’s.

GROSS COMBINATION WEIGHT RATING (GCWR) - The total weight of the loaded tow vehicle and the loaded trailer.

GROSS VEHICLE WEIGHT (GVW) - The total

weight of the vehicle, including passengers, fuel, cargo, and attachments.

GROSS VEHICLE WEIGHT RATING (GVWR) - The maximum permissible loaded weight of the vehicle and takes into account the capabilities of the engine, transmission, frame, spring, brakes, axles, and tires. The GVW must not exceed the GVWR.    

HIGH FLOTATION SIZING SYSTEM FOR LIGHT TRUCKS - A system using overall diameter in inches, section width in inches, type of tire construction, and rim diameter in inches (e.g.: 33x12.50R15LT).

HIGH PRESSURE DIE CAST - A wheel manufacturing process using aluminum alloys in special high-pressure die casting machines.

HUB CENTRIC - A situation where the center bore hole of a wheel is made  to match  up with  the hub diameter of the vehicle; the wheel is then centered by the center hole, rather than the lug nuts.

 

IDLER ARM - A device  attached  to the frame  of the car, which duplicates the movement of the Pitman arm and keeps the center link aligned.

IMBALANCE - A non-uniform distribution of mass in a tire and wheel assembly about its axis of rotation causing bounce (static imbalance) or shake (dynamic imbalance).

INDEPENDENT SUSPENSION - A suspension system in which the front or rear pair of wheels of a car are independently connected to the frame or underbody. In this system, deflection of the wheel on one side has a minimal affect on the wheel on the other side.

INERTIA - The tendency of any mass at rest to stay motionless, or any mass which is moving to remain moving.

INNERLINER - The layers of low permeability rubber, which are laminated to the inside of a tubeless tire to insure the air retention quality  of the tire body.

KILOPASCALS (kPa) - Unit of air pressure; in metric terms it takes 6.895 kPa to equal 1 p.s.i.

LATERAL RUNOUT - Wobble or the side-to-side motion of a rotating wheel or tire/wheel assembly.

LATERAL WEIGHT TRANSFER - A load factor in cornering where weight is transferred from the inside tires to the outside tires

LEAF SPRINGS - A series of steel leaves  used on suspension systems that are bolted together in the middle. Under compression, they flatten and expand in length, then rebound to their original arched shape.

LIFT POINTS - Those contact points on the chassis of  a  vehicle  used  to  hoist  the  vehicle for servicing; to prevent serious problems and permanent damage, owners’ manuals should always be checked for proper lift point locations.

LIGHT LOAD - A P-Metric tire with a maximum inflation pressure of 35, 44, or 51 psi.

LINEARITY - The ability of a vehicle to respond linearly to the driver’s steering input at low cornering levels.

LOAD-CARRYING BALL JOINT - A ball joint that supports the weight of a vehicle.

LOAD CARRYING CAPACITY - The load a particular size tire can carry at a given inflation pressure under certain driving conditions, as established by the Tire and Rim Association.

LOAD INDEX - A numerical code  associated with the maximum load a tire can carry at the speed indicated by its Speed Symbol under specified service conditions up to 130 mph. For speeds in excess of 130 mph, actual load  on the tire  shall  be reduced in accordance with Tire and Rim Association guidelines.

LOADING - The amount of weight put on tires. Increased load can increase cornering force.

LOAD RANGE - Replaces the former ply rating term and identifies load and inflation limits.

LOAD RATING - The weight that a wheel is designed to support in normal service.

LT-METRIC - A light truck sizing system expressing the section width in millimeters, aspect ratio, type of construction, and rim diameter in inches (e.g.: LT235/85R16 120/116Q E).

LUG CENTRIC - The centering of a wheel by matching it up with the attachment points (i.e. studs), rather than by the center bore hole of the wheel.

M + S, M/S, or M & S—A tire sidewall designation indicating that the tire meets the RMA/RAC definition of an all-season tire.

MACPHERSON STRUT - A front suspension assembly that combines the functions of the shock absorber, the upper steering pivot, and the wheel spindle in a single unit.

MATCH MOUNTING - A mounting procedure that matches the high point of a tire with the low point of its wheel. A dot or mark on the tire is matched with a dot, a sticker, or the valve hole on the wheel.

MOUNTAIN SNOWFLAKE PICTOGRAPH Winter passenger and light truck tire design identification. Tires with this identification provide improved snow performance over tires meeting the existing RMA/RAC all-season tire definition.

MOUNTING PAD - The surface area of the back of the wheel’s center that contacts the brake drum or hub flange of the vehicle.

NEGATIVE CAMBER - A condition where the top of the tire is leaning inward from the tire’s vertical centerline, as viewed from the top.

NEGATIVE CASTER - A setting where the steering axis is inclined forward at the top as viewed from the side; a condition which tends to cause instability.

NEGATIVE OFFSET - When the back of  the bolt pad is closer to the inside of the wheel; when the mounting face is inboard of the rim centerline; extends assembly away from vehicle resulting in wider tracking.

NHTSA - National Highway Traffic Safety Administration.

NOMENCLATURE - Systematic naming of tire sizing systems.

NOMINAL – A designated or theoretical size that may vary from the actual.

NUMERIC SYSTEM - A tire sizing system expressing tire cross-section width and rim diameter in inches (e.g.: 7.35-14).

OFFSET - The distance from the centerline of the wheel to the mounting face of the wheel.

ORIGINAL EQUIPMENT (OE) - Refers to tires sold to automobile manufacturers for equipping their new vehicles.

OUT-OF-ROUND - A wheel or tire condition in which the wheel or tire is not round.

OVERALL DIAMETER - The maximum height of a tire when mounted on a wheel and inflated to rated pressure.

OVERINFLATION - The inflation of a tire above the car manufacturer’s recommended pressure negative byproducts are rough ride, bruise damage, and suspension system strain.

OVERSTEER - A cornering  condition  where rear tires lose traction before the front tires; the tendency of a car to turn more sharply than the driver intends while negotiating a turn.

OXIDATION - Reaction of a material with oxygen, usually resulting in degradation of the material.

PERMEATION – The process where air molecules migrate through the sidewalls of the tire. Tires lose air normally through the process of permeation.

PITCH - The length from a point on one tread block to the same point on the next tread block. Pitch is varied around a tire to minimize noise.

PITMAN ARM - A device in a recirculating ball steering system that converts  circular motion into  a back-and-forth motion through its connection to the system’s center link or relay rod.

PLASTICIZER - A rubber compound chemical; used to make or keep rubber soft and flexible.

PLIES - The layers of material that make up the cord body and belts of a tire.

PLOWING - Loss of traction of front tires during cornering. also known as understeer or “pushing ”.

PLUS 1/PLUS 2 CONCEPT - A concept to improve handling and performance through the mounting of tires with wider section widths and lower section heights to rims of 1, 2 and sometimes even 3 inches greater diameter. Correct Plus 1 or Plus 2 fitments maintain the same tire diameter as the original tires.

P-METRIC SYSTEM - A tire sizing system using the section width in millimeters, aspect ratio, type of tire construction, rim diameter in inches, load index and speed symbol (e.g.: P225/70R15 100S).

POLYESTER - A strong and lightweight synthetic cord material used in casing construction.

POLYMER - A  chemical  compound  made  up  of a large number of identical components linked together like a chain.

POSITIVE CAMBER - A condition where the top of a tire is leaning outward from the tire’s vertical centerline, as viewed from the top.

POSITIVE CASTER - A setting where the steering axis is inclined rearward at the top, as viewed from the side; makes possible the self-centering force that tends to return the wheel to the direction the vehicle is traveling.

POSITIVE OFFSET - When the back of the bolt pad is closer to the outside of the wheel; when the mounting face is outboard of the rim centerline, draws assembly towards the vehicle resulting in narrower tracking.

PSI - Pounds per Square Inch.

PULL - The tendency of a vehicle to veer to one side when driving straight ahead.

PYRAMID BELT - A belt design in which the upper layer is narrower than the lower layer.

PYROMETER - A thermocouple device used for measuring tread temperatures in tires.

RACK-AND-PINION SYSTEM - The steering system in which a gear at the end of the steering column meshes with a rack of steel teeth; the rack is then connected to the steering arms.

RADIAL PLAY - The side-to-side  movement  of a ball joint.

RADIAL RUNOUT - A measurement of out-of-roundness; by rotating the inflated tire and measuring how far the tread surface varies (up and down) from a true circle.

RADIAL TIRE - A tire built with casing plies that cross the crown at an angle of 90 degrees.

RAYON - A synthetic cord material used in casing and belt construction; provides high dynamic strength and good rubber adhesion.

REAR SPACING - See Backspacing.

RECIRCULATING BALL SYSTEM - The conventional type of steering system in which steering motion is transferred through a gearbox.

RETURNABILITY - The ability of a vehicle to return to a straight-ahead attitude after removal of steering input.

REVOLUTIONS PER MILE (RPM) - The number of revolutions a tire makes in a mile at a given load, inflation, and speed.

RIBS - The rubber elements at the  tire  tread which contact the ground, oriented in a generally circumferential direction.

RIDE HEIGHT - The distance between the frame of the vehicle and the road.

RIM DIAMETER - The diameter of the bead seat, not the diameter of the rim edge.

RIM DROP - The area of the wheel’s rim having the smallest diameter.

RIM FLANGE - The outermost edge of a wheel’s rim to which clip-on weights are attached.

RIM WIDTH - The measurement inside of the rim flanges; i.e. from inside the flange on one side to inside the flange on the other side.

RMA - Rubber Manufacturers Association.

ROLLING RESISTANCE - The force required to roll a loaded tire.

ROTATION - The pattern of movement of tires to different positions on a vehicle to compensate for irregular or unequal tire wear.

RUBBER-TO-VOID-RATIO - The ratio between the rubber area and the groove area in a tire footprint.

RUNOUT GAUGE - A device used to check radial and lateral runout.    

SAFETY HUMP - The raised area circling the rim of the wheel and located slightly inward from the bead seat; to keep the tire from slipping into the rim well, if accidentally deflated.

SECTION HEIGHT - The distance from the bottom of the bead to the top of the tread.

SECTION WIDTH - The distance from sidewall to sidewall, exclusive of any raised lettering.

SELF-ALIGNING TORQUE - The force which causes a tire/wheel assembly to return to its straight- ahead position after a turn.

SEMI-TRAILING ARM - A rear suspension system that uses a large A-arm for each wheel; its pivot axis is set between the 0 angle of a swing axle and the 90 angle of a trailing arm to a line running straight across the car.

SERIES - This is the part of the size  designation in tires, which gives the ratio of the height of a tire (from the rim to the top of the tread) to the width of the tire (from sidewall to sidewall). It is also referred to as the aspect ratio of a tire.

SERVICE DESCRIPTION (LOAD INDEX/SPEED SYMBOL) – The Service Description consists of a Load Index and a  Speed  Symbol. The Load Index is a numerical code that specifies the maximum load a tire can carry at the speed indicated by its Speed Symbol, at maximum inflation pressure.

SHIMMY - A rapid oscillation or wobble of a wheel and tire assembly about the steering axis.

SHOCK ABSORBER - A  “damper”  between the frame of the car and the suspension; used to cushion road bumps and bounces and keep the tire in contact with the road.

SHOULDER –The outer edges of a tire’s tread where it joins the sidewall.

SHOULDER GAUGE  -  The  total  thickness  of a tire in the shoulder area. This is invariably the thickest part of the tire and this gauge directly affects the running temperature of a tire.

SIDEWALL - The side of a tire between the tread shoulder and the rim bead.

SIDEWALL ROLLOVER –The condition that occurs during hard cornering when a tire sidewall rubs the road surface.

SIPES - Small, narrow slots molded into the ribs of the tread design which increase the traction edges of the tire and increase the traction ability of the tire on wet pavement.

SKID RESISTANCE - Maintenance of grip on the road and resistance to slide or slip, either longitudinally or laterally.

SLIP - The change in distance traveled per tire revolution due to driving or braking conditions; expressed as a percentage of the distance traveled under a free rolling condition.

SLIP ANGLE - The difference between the direction a tire is traveling and the direction it is pointing.

SLOTS - Grooves generally positioned in the ribs and shoulder areas of some tires, which aid in wet pavement traction.

SPEED RATING (SPEED SYMBOL) – An alphabetical system describing a tire’s capability to travel at established and predetermined speeds and gauged by controlled indoor laboratory testing. An indication of the tire’s performance.

SPINDLE (OR KNUCKLE) - A device connected to a vehicle’s control arm by a ball joint; it is moved by the steering arm to change the direction of the wheels and to transmit braking torque.

SPRUNG WEIGHT - The total weight of a vehicle that is supported by the suspension system.

SQUIRM - The footprint distortion of a rolling tire.

STABILITY - The ability of tires to maintain direction of a vehicle on curves without causing excessive body sway.

STACKED BELT - A belt design in which both layers are of equal width.

STANDARD LOAD - A P-Metric tire with a maximum inflation pressure of 35, 44, or 51 psi.

STAR PATTERN - The proper method for sequential torquing of lug nuts in a 5-lug bolt circle.

STATIC - Having no motion.

STATIC BALANCE - Balance at rest. A condition in which a tire and wheel assembly has equal weight around the wheel’s axis of rotation.

STATIC LOADED RADIUS - The measurement from the middle of the axle to the road surface; measured with the tire inflated to required pressure and carrying the rated load.

STEEL BELT - A belt material used in tires. Its high stiffness provides good handling and low tread wear.

STEERING AXIS - An imaginary line drawn through the center of the steering pivots. The axis about which the wheel pivots when turned.

STEERING RESPONSE - Reaction time between driver input at the steering wheel and the directional change of the vehicle.

STEERING SYSTEM - A major control mechanism that multiplies driver input on the steering wheel into the motion of turning a vehicle’s front wheels.

SULFUR - A chemical element used in the vulcanization process.

SUSPENSION - A system of devices supporting the upper part (body and chassis) of a vehicle  on its axles.

SUSPENSION PACKAGE - A specialized kit of suspension components designed specifically for individual makes and models of vehicles; usually sold to improve performance.

SWING AXLE - A rear suspension system comprised on half shafts with  universal  joints  only at their inward ends on either side of the differential.

SWING OUT - The tendency of the rear  tires  of a vehicle to break away during sudden steering maneuvers.

SYNTHETIC RUBBER - Rubber made from chemicals as a substitute for natural rubber; properties can be tailored for specific needs.

 

TENSILE STRENGTH - The maximum tensile force per cross-section area that a material can withstand before it breaks.

TIE RODS - Devices that join the center link of a steering system to the steering arms that connects to the wheels.

TOE - The difference in distance between the front and the rear of a pair of tires mounted on a common axle.

TOE-IN - A condition where the fronts of two tires on the same axle are closer together than at the rear.

TOE-OUT - A condition where the fronts of two tires on the same axle are farther apart than at the rear.

TOE-OUT-TURNS (ACKERMAN ANGLE) - The difference between the turning angle of the inside wheel and the outside wheel during a turn to the left or right; toe-out turns is not adjustable and is designed into the steering linkage system by the manufacturer.

TORQUE - The product of a force applied through a lever arm to produce a rotating or turning motion.

TORQUE RATING - The proper torque, expressed in foot-pounds, for tightening lug nuts of various diameters.

TORQUING - The securing of the tire/wheel assembly to the vehicle by the tightening of the wheel’s lug nuts to the studs of the vehicle’s hub; in the case of specialty wheels, torquing should always be done with a manual torque wrench, containing an insert socket of plastic or Teflon.

TORSION BAR - A “straight’’ spring that twists under loads; the natural resistance to this twisting provides the spring action.

TPC - A Tire Performance Criteria marking, appearing on radial tires, which denotes that a tire meets performance specifications set by General Motors, for original equipment use.

TPMS - TIRE PRESSURE MONITORING SYSTEM - Device mandated by the NHTSA (USA) to help motorists maintain proper tire air pressure.

TRACK - The distance between the front tires on the front axle and the rear tires on the rear axle.

TRACKING - The difference in distance between each of the rear wheels and the centerline of the vehicle.

TRAILING ARM - A rear suspension system consisting of an arm whose pivot axis is exactly across the vehicle or perpendicular to the direction of travel.

TREAD - The portion of a tire which contacts the road surface.

TREAD DEPTH - The distance from the tread surface to the bottom of the grooves

TREAD RADIUS - The radius of curvature of the tread arc across the tread.

TREAD SHAVING - The shaving  of tread  from a tire with a blade (usually to half of original tread depth) to reduce tread squirm and tearing in racing applications.

TREAD WEAR INDICATORS - A raised area  in the tread grooves which becomes even with the tread surface when the tire is worn to 2/32” tread depth; used to define the legal wear-out point in a tire’s life.

UNDERINFLATION - A condition where  a tire is inflated below the carmaker’s recommended pressure.

UNDERSTEER - A cornering situation  where  the front tires generate more slip angle than  the rear tires; the tendency of a car to turn less sharply than the driver intends and compensated for by the addition of more steering input.

UNDERTREAD - The portion of the tread compound between the bottom of the tread grooves and the top of the uppermost ply belt.

UNDULATION - A slight indentation or wavy appearance on the sidewall surface of an inflated radial tire. Radial body ply cords run  straight across the tire from bead  to bead  and the joining of the ply material in the sidewall area may sometimes cause this condition. Undulation is a common characteristic of radial tires and will not affect the performance of the tire.  Refer to the RMA “Tire Information Service Bulletin”, Volume 21 / Number 1, dated December 1984, for more information.

UNIFORMITY - A term describing the amount of radial and lateral force variation in a tire.

UNSPRUNG WEIGHT - The total weight of a vehicle not supported by the suspension system; example: tires and wheels.

UTQG - Uniform Tire Quality Grade labeling; a performance measurement of a tire, based upon its test results in three categories: treadwear, traction, and temperature resistance.

VULCANIZATION - The linking together, under heat and pressure, of rubber compound polymers, which gives rubber increased strength and elasticity.

WANDER  -  The  tendency  of  a  vehicle  to veer or drift to either side from a straight path.

WHEEL BASE - The distance between the center of the front wheels and the center of the rear wheels.

WHEEL BEARING - Most commonly, a tapered roller bearing consisting of tapered rollers, matching races and a cage. Mounted on a hub, they permit the spindle to rotate freely with minimum friction.

WHEEL PLAY - The up-and-down movement of a ball joint.

WHEEL WEIGHT - Weights that are either clipped, taped, or self-adhered to the inside or outside of the wheel in order to balance the tire/ wheel assembly.

ZERO OFFSET - A condition where the centerline of the wheel coincides exactly with the mounting face.

ZERO TOE - A condition in which two tires on the same axle are exactly parallel.