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Glossary Of Terms

Term used for tire sizes 16.00 and larger in nominal cross section. Also referred to as earthmover or off-the-road tires (O.T.R.).
Deterioration of rubber properties by oxidation over a period of time.
Wheel alignment is the mechanics of keeping all interrelated parts which affect steering in proper adjustment. There are five steering angles involved:
(a) Camber
The inward or outward tilt of the wheel at the top from the vertical. Positive tilts outward and negative tilts inward. It is measured in degrees.
(b) Caster
The forward (negative), or rearward (positive) tilt of the kingpin at the top from the vertical, when referring to trucks. In cars having independent front suspension, the upper ball joint is set forward or rearward in relation to the lower ball joint. Caster is measured in degrees. See 'trail distance'.
(c) Toe-in/Toe-out
The front tires of a vehicle are adjusted closer together in the front than at the back for toe-in. The opposite setting is toe-out. It is measured in fractions of an inch.
(d) Kingpin inclination/steering axis inclination
The inward tilt (side to side) of the kingpin or spindle support arm, at the top, from the vertical. Thus a line drawn down through the centre plane of the tire and a line through the kingpin, or ball joints, would come closer together at the ground than at the top. See 'centre distance'.
(e) Turning radius (Toe-out on turns)
The front tires assume a toe-out relationship to one another when making a turn. This makes allowance for the fact that they are forming different size circles around a common centre. (Sometimes called the Ackerman System).
Ambient temperature
The surrounding air temperature.
Angular vibration
Side to side movement or shimmy of a tire. See 'BALANCE, DYNAMIC.'
Anti-skid system
Computer Brake Control. A system installed on many vehicles to prevent skids caused by wheel lock-up. It allows for automatic adjustments in braking pressure for maximum braking force without loss of steering control.
See 'hydroplaning'.
Aspect ration
The ratio of 'section height' to 'section width'. Section height divided by section width of a tire expressed as 78 series, 70 series, etc. The section height is 70% of the section width when referring to 70 series. These are low profile tires, and may be called ultra low profile.
A pointed or flat tool used to probe nail holes and injuries.
Axle live
Driven axle.
Axle, ratio
Balance patch
A factory installed patch used to bring a tire within quality control balance tolerances before distribution. It is placed inside the tire casing and looks much like a nail hole repair patch.
Tire balance
A tire properly weighted so that it rotates without causing bounce or vibration:
A tire that is not weighted properly can cause vibration. The condition is exaggerated by centrifugal force which increases as speed is increased and by the distance the heave spot is from the wheel axis. (See 'ounce inches'.)
Static balance - (kinetic)
A tire that has a heavy spot at any point around its circumference. The heaviest point will come to rest at the bottom when the tire is rotated on a free bearing. On a vehicle, the tire will tramp or bounce.
Dynamic unbalance
Exists only when the wheel is rotating and is caused when a heavy spot in the tire is not in the centre plane of the wheel, or when a heavy spot is not in the same plane as the static counter-balance weight. On a vehicle the tire would wobble from side to side.
Balancing of tire using a machine capable of balancing tire, wheel and brake assembly while the tire is properly mounted on the vehicle.
The tire assembly is removed from the car and mounted on a balancing machine.
Balance weight
The most common are lead blocks of varying sizes marked by weight in ounces (or fractions of an ounce) and having metal dips for attaching to the rim flange
Ball joints
Upper and lower ball joints, attached to upper and lower A-frames and spindle support arm in vehicles having independent front suspension, provide for pivoting of front wheels for steering.
The addition of weight, liquid or dry, inside the tire to act as a counterbalance, to increase traction, reduce wheel spin and dampen out bounce:
Usually water mixed with calcium chloride to prevent freezing and increase specific gravity.
A fine powder of barium sulphate. Sometimes referred to incorrectly as lead ballast because of one brand name, "Ledballast".
Bead expander
Any of several devices used in the mounting of tubeless tires to prevent inflation air from escaping and bring the tire beads against the tapered bead seat area (rim). Also see 'spreader tire'.
Bead movement (bead rocking)
Movement of bead on rim caused by improper inflation, excessive loading, improper design, improper seating or improper rim or tire size.
The parts of the tire that seat on the rim. Bead heel, sole and toe form a foot-like shape.
Bead points
A feathered rubber extension of the bead toe used where a flap is not required; protects the tube from chafing between bead toes and rim base.
Bead seat diameter
The measurement of tire diameter, at the bead heel, where it seats on the rim. It is marked on the tire sidewall following 'section width'.
Bead wires
Steel wires wound circumferentially and placed in the beads, and by their tension prevent the beads from lifting over the rim flanges.
Refers to a tire having a stabilizing belt, of two or more plies of steel, fibreglass, etc., running circumferentially around the tire between the carcass and the tread rubber. The carcass can be either radial or bias ply.
Bias belted
A tire consisting of a conventional bias ply carcass with the addition of a belt of rayon, nylon, fibreglass, or steel running circumferentially in the tread area. See 'BELTED'.
Bias ply-conventional bias-ply
A tire having two or more carcass plies arranged in a criss-cross manner and diagonally to the beads and travels approximately 1/3 the distance around the circumference before attaching to the other bead. Each cord in the next ply is arranged in the same manner, but in the opposite direction.
The harmful practice of reducing the pressure in tires that have been run, warmed up and thus increased from their starting pressure.
Blemish - (blem)
A tire having a slight imperfection in appearance that will not affect tire life or safety.
A localized bubble on the surface of a tire, normally caused by a separation between plies or between surface rubber and a ply.
Instantaneous rupturing of tire body, causing complete loss of air pressure.
A simple protective device (sometimes a piece of tube, tire, etc., cut to size) placed between the casing and tube. It offers temporary protection for the tube against pinching by an injury in the casing. It is not a repair of the injury and is unsafe to use.
Bottom dumps
Trailers that unload through bottom gates.
Brake fade
The loss of effectiveness of braking, usually after becoming hot from continual or repeated use in a short period of time.
A braking system, effective in forward or reverse direction, consisting of a revolving steel disc (rotor) and brake pads which make contract with this disc under hydraulic pressure. It is resistant to fading in severe heat and wet conditions.
Single leading brake shoe type
The use of a single hydraulic plunger and a common pivot point expands two shoes against the drum with equal pressure whether braking from forward or reverse direction. Leading brake shoe usually termed 'primary shoe', trailing shoe termed 'secondary shoe'.
Double leading brake shoe type
The use of two hydraulic plungers and separate pivots allows for more effective braking in the forward direction, though not very effective in reverse.
Brand number
An identification burned into sidewall rubber of a truck tire by some customers. Often it is coded to show company's initials, mounting date, etc.
The loss of traction as in turning with such force that the tire suddenly loses all contact with the ground. A tire having greater tread stability than a Bias ply (bias-belted or radial) can resist breakaway longer.
Bridge (LEMNA)
A metal support easily installed in the valve slot of a rim without dismounting the tire. It prevents the flap and tube from bulging through the slot under high pressure and high heat conditions.
A protective thin metal washer sometimes installed at the valve stem between the flap and rim base to prevent tube and flap from protruding through the valve slot due to high pressure and high temperature.
An injury to a tire, weakening, breaking, or separating cords in the carcass without cutting or tearing of surface rubber.
Burred wheel
Wheel with rough slivers or projections of metal around rim edges.
Heavy rubber reinforcements in the tire shoulder. It provides support at the edge of the tread, limits shoulder area flexing and protects the shoulder of off-road tires.
A non-porous synthetic rubber used to manufacture most tubes and as an air proof liner for tubeless tires.
Driver's compartment of a truck.
Calcium chloride
Chemical added to prevent freezing of water ballast in farm tires and EM tires.
A thin layer of rubber inside the tire casing covering the carcass cords, protecting them from moisture and giving protection to the tube against chafing by the cord-body. In tubeless tires, calendering consists of a layer of air proof rubber (Butyl).
See 'alignment'.
Camber axle
An axle beam formed in a slight arc, curving upward at the centre to allow the tires to tilt outward at the top. It compensates for normal axle sag under load.
See 'retreading'.
Carbon black
A by-product of the petroleum industry used as a pigment and to give body in the manufacture of rubber products, both natural and synthetic. Carbon is the black residue from burning petroleum.
The cord body (ply/plies). The foundation structure of a tire that provides strength to hold air pressure within the casing.
Carrying capacity
The maximum load that should be permitted on a given tire, wheel, or rim, etc., as recommended by the manufacturer's data book.
The tire without tube, flap, or tim.
Casing factor
That portion of the load supported by casing stiffness instead of air pressure.
See 'alignment'.
An adhesive rubber compound dissolved in solvent used to provide building tack and cured adhesion. May be brushed or sprayed on the buffed surface.
Centre of gravity. A point within the length, width, and height of a vehicle about which all the vehicle weight is balanced.
Chain drive
Rear axle driven by means of a chain belt.
Removing new original equipment tires in trade for a different make, size or type.
See 'groove'.
The basic truck frame, including suspension, steering, driving and power components, excluding cab or body.
Chassis, cab
Truck chassis including driver compartment.
Chassis, cowl
Truck chassis with front fenders and hood complete with instrument panel. For use with custom built body and cab.
Check valve
A one way valve used to prevent pressure loss.
Minute cracking in surface of rubber caused by aging and oxidation.
Chemical cure
Vulcanization at room temperature or above, activated by chemical agents without the application of heat from an outside source.
Flaking or tearing away small bits of tread rubber.
Tearing or breaking away pieces of tread rubber
Circumferential break
A tire injury running parallel to the bead.
Circumferential crack
A crack in a tire running parallel to the beads. Usually consists of cracks in the grooves of the tread
Between dual - See 'dual spacing'.
Vehicle to tire: Minimum -
A. To a fixed part: 5/8' (15mm)
B. To a moveable part: 1' (25mm)
The distance between the top of the tread and some part of the vehicle closest above it, after subtracting the axle stop clearance and any increase in tread depth from the existing tire.
The distance between the tread and the closest point forward or rearward, reduced by increase in tread depth and rearward movement of the axle under load (1/3 the distance between shackle pin centres).
he distance between the tire sidewall and the nearest point on the vehicle, reduced by any increase in rim offset and 1/2 any increase in tire section from the existing tire.
Front wheel clearance
The distance between the tire and the closest point on the vehicle laterally, longitudinally and vertically, checked to lock and all intermediate points.
Coefficient of friction
The horizontal force required to move a body (on a relatively smooth level surface) divided by the weight of the body. The coefficient of rolling friction is the maximum retarding force (that can be applied to a rolling body on a relatively smooth level surface without causing a cessation of rolling) divided by the weight at the contact surface.
Coil spring
Spring stock coiled in cylindrical form.
Commercial tires
Generally indicating truck and industrial tires.
Conventional tire
See 'bias-ply'.
Shipping system based upon large cargo-carrying containers that can be easily interchanged between trucks, trains, and ships without rehandling of contents.
Contract carrier
An operation transporting under contract with an individual shipper.
A strand of fabric material or steel cable used in the ply of a tire.
Cost per mile
The actual cost of a tire after considering all costs (price of tire, price of retreads, repairs, services, etc.) less any credits (warranty, etc.), divided by the total number of miles it has run. True cost per mile can be figured only after the tire is no longer serviceable and should include costs to the user incurred as a result of downtime.
Cracking tread or groove
Splitting in grooves caused by excessive strain.
Cross section
See 'section width'.
Cross member
Structural shape tying in side rails of frame.
The tread area of a tire.
Crown radius
The measurement of the curvature of a tire tread between the shoulders of the tire. Expressed as a percentage, it indicates the relative flatness of the tire tread area.
Crown width
The distance shoulder to shoulder measured along the buffed contour.
Cure time
The time required at a reference temperature for a compound to reach optimum physical properties.
Process of heating or otherwise treating a rubber or plastic compound to convert it from a thermoplastic or fluid material into the solid, relatively heat-sensitive state desired in the commercial product. When heating is employed, the process is called vulcanization.
Cushion gum
A soft, tack rubber compound used in retreading and repair to facilitate bonding between different rubber compounds and between plies, etc.
Dead axle
The non-powered axle of a tandem rear mounting in which the other axle is powered.
Deflection rate
Generally refers to rate of spring deflection to inches per 1000# load.
Demountable rim
See 'rim and wheel terms'.
An arrangement of gears at the centre of a drive axle that allows the wheels on one side to go faster (or slower) than wheels on the other side, as in rounding curves.
Directional tread
An arrangement of bars, grooves and ribs in any manner that gives most effective traction when the tire revolves in only one direction.
Disc wheel
The combination of a rim and a metal disc riveted or welded together. The disc is usually offset from the centreline of the rim to allow for dual tire mounting and to provide sufficient clearance between duals. Disc wheels are attached to the hub with either single nuts or double cap nuts.
Offset. See 'rim and wheel measurements'.
Department of Transportation, a federal agency.
DOT number
The symbol DOT means the tire meets or exceeds Department of Transportation safety standards. Following DOT are a maximum of eleven numbers.
Example: DOT FT TW A2NX 092:
DOT-Meets or exceeds standards.
FT-Identifies manufacturing plant
TW-Code for tire size.
A2N or A2NX-3 or 4 digits optional with manufacturer to identify characteristics of the tire.
092-Week of mfg., in this case, 9th week of 1972.
Prior to May 22, 1971 only the manufacturer's plant was identified by three numbers -- (Example DOT 129). Retreaded tires must also have a new serial number and can be determined by the letter "R" following DOT letters.
Dowel pin
A pin aligning or securing two parts to prevent movement between them.
The vehicle operating time lost due to maintenance difficulties, tire damages, etc.
Drag link
Connection between Pitman Arm and Steering Ball (Steering Arm) on front axle of a vehicle.
See 'tire deviation angle'.
Drive train
Components used to propel vehicle.
Drive axle tires (slang usage).
Drivers, rim
Riveted or welded metal blocks or extrusions on both sides of the valve slot to prevent valve damage due to rim slippage on cast spokes.
Drop centre rims
See 'rim types'.
Dual axles
See 'tandem axles'.
Dual bead tires
Heavy service and large truck tires using two or more sets of bead wires in each bead rather than one.
Dual mountings
Tandem axles, both powered directly by the engine. (Slang term, twin screw.)
Dual spacing
A measurement in inches from the centre of the tread of one tire, to the centre tread of the other tire in dual, which provides clearance between duals for air circulation.
Dump body
Metal body generally hinged at rear and dumped by hydraulic means. Size generally given in cubic yard water level capacity.
A device to indicate the hardness of rubber.
Dust cover
A soft, flexible valve cap to protect valve assembly from dust while in shipment and storage. It is not capable of sealing the air pressure and should not be used in service.
Dust shield
Sheet metal disc or plate placed on brake assembly to keep debris from brake assembly.
Electric or hydraulic instrument used in determining power output of engine, or friction in chassis components.
Dynamic balance
See 'balance'.
A term which includes natural rubber and the many synthetic materials that possess rubber-like properties.
End dump
A term used to describe various dump trucks or trailers that tilt to unload at the rear.
Engine break
(Energy retarder)
A system that allows for slowing of a vehicle that is independent of the conventional braking systems. A driver would normally down-shift to slow his descent of a hill, using engine compression. The 'engine brake increases the effectiveness of this retarding force.
A machine that shapes a rubber compound into a usable form. Uncured rubber is heated, to soften, and forced through dies having the desired shape and dimensions.
Fabric fatigue
Fabric degradation and resultant tire cord breakdown due to repeated flexing and heat.
Flange seal
An "L" shaped rubber seal used in some earthmover tubeless tire mountings. It prevents air loss between the tire beads and the: (1) flange and rim base, and (2) flange and loose taper seat. Also see 'O ring'.
A rubber protector used in tube type truck tires to prevent injury to the tube by the bead toes and at the valve slot of the rim.
Flat spot
Irregular wear in an isolated spot or spots around the tire tread.
Flipper strip
See 'ply turn up'.
Ability to float generally referring to large low pressure tires. (The ability of a tire, to pass over soft surfaces without sinking in.)
Foot pound
One pound of force at the end of a one hot last lever; a measurement of torque i.e., 3ft. bar x 150 Ibs.= 450 foot pounds).
Fork lift (lift truck)
A machine having metal arms extended in front for picking up and moving pallets or skids.
The backbone structure around which the vehicle is assembled.
Free radius
Measurement in inches from the wheel axle centreline to the top of the tire when inflated. (The radius that is not under load.)
Free rolling wheel
A non-powered wheel position on a vehicle, such as steering axle, tag axle or trailer wheel positions.




Gross combination weight (GCW)
The weight of a truck and trailer combination and its entire contents.
Gross train weight (GTW)
Gross train weight. Same as GCW.
Gross vehicle weight (GVW)
Total weight of fully equipped truck and payload.
Gear ratio
The number of revolutions of a driving gear required to turn a driven gear through one full revolution. For a pair of gears the ratio is found by dividing the number of teeth on the driven gear by the number of teeth on the driving gear. Changing tire size will change the effective gear ratio. An increase in loaded radius will increase ratio, increase speed and reduce power.
Glad hands
A separable mechanical connector used to join air line hoses when combination vehicles are coupled together.
Green tree
The complete rubber/fabric/steel just before being cured.
The channel between ribs in the tread of a tire.
The cutting of a tread design into tread rubber where a design does not already exist. Also altering an original design, i.e. cross-grooving to increase traction.
Groove cracks
Splitting or cracking of the rubber (under tread) at the base of the grooves, between tread ribs. Primarily caused by growth in textile casings.
Growth, tire
The stretching of textile tire cord materials due to heat and loss of strength, resulting in the casing increasing in size.
Gutter, bevel
See rim terms.
Heel & toe wear
Uneven wear of tread blocks on a tire. The trailing edge of the block often tends to wear at a faster rate that the leading edge.
Helper spring
Additional spring device permitting greater load on axle
The base having studs protruding from its face upon which the wheel is mounted to the vehicle.
Hub Odometer
A register showing miles travelled by vehicle mounted directly on the axle hub. It is popular to record mileage for leasing of vehicles or tires particularly on trailers, since there is no other odometer present.
Hydrostatically inflate
To inflate with water in place of air. (Also see liquid ballast).
Loss of road contact due to the build-up of water between tire tread and road surface. It occurs when factors, including speed, water depth, tread depth and inflation pressure, so combine that lifting force is exerted under the tire.
The energy lost and not returned, when tire materials are subjected to stress in any direction. Lost energy is converted to heat through molecular interaction, and since rubber has poor thermal conductivity, internal temperatures of a tire can build up rapidly under repeated flexing.
Impact break
Rupture to a tire resulting from shock of striking a chuck-hole, rock curb, etc. and not caused by cutting.
Included angle
The sum of the camber and kingpin inclination angles. This angle is designed into the steering knuckle and must remain constant
Independent suspension
One wheel position can move vertically without imposing any corresponding movement on the other wheel on an axle. The straight axle had been eliminated and replaced with upper and lower A-Frames (control arms) that pivot at the individual wheel position.
Industrial tires
Heavy duty tires, for use on forklifts, lowbed trailers, etc.
Industrial solid-A non-pneumatic tire, used most often on forklifts where flats are a constant problem.
The tendency of a body at rest to remain at rest and a body in motion to persist in motion.
Inner cap nut
See dual mountings.
Inner tube
A tire shaped air chamber, containing a valve and placed inside the tire casing. It seals in the air which supports the load, but is not capable of supporting the load without the strength of a tire and rim surrounding it.
Inter-axle differential
Gear device equally dividing power between axles and compensates for unequal tire diameters.
Mechanical, hydraulic, or pneumatic lifting device to raise chassis for repairs or tire changes.
The trailer over-turns the tractor, pushing the tractor drive wheels sideways, forcing the combination into a V-shaped attitude, resulting in a complete loss of control with no possibility of recovery.
Kinetic balance
See balance, static.
Kissing between duals
The intermittent contact of tires in dual as they flex; caused by inadequate dual spacing or by overload.
Laden weight
See gross vehicle weight (GVW).
The load; the freight in a vehicle
Lateral clearance
Smallest distance laterally between the tire and the nearest fixed point of the vehicle.
Lateral run-out
A tire assembly that does not run true to its plane; i.e., a damaged wheel moving in a wobbling, side to side manner.
Lateral stability
Limiting side movement.
1. Tread grooves running circumferentially around the tire resist side forces for maximum traction on turns.
2. Stabilizing tread plies limit side to side movement of the tread ribs caused by the expansion and contraction of tread areas as sidewalls flex.
Load base
Used in calculating load distribution; it is the distance between the centre of the rear axle (or centre between tandem axles) and the centre of payload.
Negative load base
If centre of payload is behind the rear axle, it is negative and takes load off the front axle.
Load distribution
(Distribution of load on truck or trailer chassis.) The relationship of the gross load on the front and rear axles to the total gross load.
Load distribution calculation
Payload on the front axle is equal to the total payload multiplied by the load base (in inches) and divided by the wheelbase (in inches).
FORMULA: Payload front axle =
Payload x Load base

To complete the calculation of load per axle, it is necessary to obtain the unladen weight of the vehicle and estimate its breakdown by axle.
Load & inflation table
A chart in the tire manufacturer's data book listing the carrying capacity of a tire at each of several air pressure increments from the lowest range of practical usage to its maximum capacity.
Load range
An index of tire strength which replaces the ply rating system and its predecessor which listed the actual number of plies.
4 plies
4 ply rating (1-2 plies)
Load range B.
12 plies
12 ply rating (1-12 plies)
Load range F.
Load radius (static loaded radius)
Measurement in inches from the wheel axle centreline to the ground when the tire is properly inflated for the load.
Long ton
2,240 Ibs. Also called gross ton.
Low-bed low-boy
A low platform trailer for heavy equipment hauling, usually on 15" diameter tires.
Low pressure tires
Larger cross-section tires for operation at lower pressure. Increased air capacity permits lower pressure.
Low pressure indicator
A unit or combination of units which provides a visible or audible warning signal, whenever the system pressure is below a predetermined value.
Low profile
Refers to the aspect ratio of a tire; section height is smaller than section width.
Lubricate beads
Thorough coating of the beads with a vegetable base lubricant is necessary to prevent bead damage and allow for proper bead seating when mounting a tire. Petroleum base lubricants must never be used.
M + S
Mud and snow tires. Tires having a tread design for maximum traction in mud and snow.
New treads (Nu-treads)
A term used by some tire companies to denote a retreaded tire.
The jerking action which occurs in a vehicle, due to the distortion within some tires as they are forced to traverse pavement ridges at a slight angle.
Non-directional tread
An arrangement of bars, grooves and ribs in a manner that gives equal traction in forward or reverse direction.
A synthetic fibre.
O Ring
An O-shaped rubber ring used in earthmover tubeless tire mountings to seal air between the loose taper seat and the rim base.
(Mileage recording instrument.) A register showing miles driven by a vehicle, usually located in the speedometer.
Original equipment manufacturer. The term OEM tires refers to tires offered originally by the manufacturer of the vehicle at no extra cost, or the tire selected as an option and mounted by that manufacturer.
See rim and wheel terms.
Ounce inches
An expression of the force exerted by a heavy spot (or counterbalance weight) on a tire. Multiply the weight times its distance from the axle centre, i.e.. 3 oz. x 7" = 21 ounce inches.
Out rigger
A device (retractable mechanical legs) used to stabilize equipment such as cranes, ditch diggers, etc., while working.
Outer cap nut
See dual mounting.
Over inflations
Excessive tire pressure in relation to the tire size and load carried.
Over cure
Vulcanizing longer than necessary. Can result in the deterioration of certain physical properties.
Spew-out of tread compound at the mould parting line or at the edge of the matrix skirt which should be trimmed or buffed of f the finished product.
Carrying more weight on a tire than its listed maximum carrying capacity or, carrying excessive loads on a tire in relation to its inflation (under inflation).
Installing a tire larger than needed to carry the load. A common practice on passenger vehicles to increase one size when replacing OEM (it is not necessarily beneficial).
Over steer
A vehicle tends to turn on a smaller radius than that steered by the driver who finds he is turning too acutely. An undesirable condition that can be caused by having larger "slip angle. in rear tires than in front tires. Also see tire deviation angle.
A faintly blue form of oxygen produced by the silent discharge of electricity into the air.
Ozone checking
The cracking of rubber brought about by continued exposure to ozone in the air which, if extensive, is destructive to a tire.
Ozone compound
(Anti-ozone compound ) - Rubber compounded with certain chemicals to regard ozone damage.
Parallelism, axle
Axles are determined to be parallel, thus minimizing tire wear, if a measurement between two or more axles is equal at both ends of the axle.
A simple repair unit such as used for a nail hole.
The actual weight of cargo being carried, including packaging, etc. (GWN - Unladen weight = payload).
Plug repair
The filling of a nail hole by forcing repair material into the damaged area to fill it, often while the tire is mounted and containing air. This not a satisfactory method of repair.
A layer of parallel cords coated in rubber forming the carcass body, stabilizing plies, etc.
Ply rating
The strength index of a tire. It replaced the old system of marking the actual number of carcass plies in a tire on its sidewall, and is an indication of comparable strength. This system is currently being replaced by the term load range.
Ply separation
A breakdown of bonding compounds causing plies to detach from each other. Usually as a result of excessive heat.
Ply turn up
The extension of a carcass ply to its end after wrapping around the bead.
Pneumatic tire
An air-filled tire. The air carries the load.
A synthetic fibre.
A material consisting of large units (molecules) made by joining many smaller building blocks (simple molecules). Usually used to describe synthetic rubber.
Power train
Components used in transmission of power from the engine to the wheels.
Pressure build up
Heat causes air to expand resulting in a normal increase in air pressure. Any increase exceeding 15% above starting cold pressure should be investigated. See bleeding.
A pointed tool, like an ice pick. It is used to determine the extent of injuries during the tire inspection. An awl.
Pounds per square inch. This is the accepted standard for measuring inflation pressure in a tire.
Power take-off, used to transmit power from engine to auxiliary equipment.
Any penetrating of a tire's air chamber by a foreign object, nail, glass, etc. resulting in loss of air. Such loss can be rapid with the collapse of the inner tube, or relatively slow in the case of tubeless tires.
Instrument used to indicate temperature in various areas of the tire.
Quality grading
Department of Transportation requirements for labelling of various tire safety and performance criteria by the manufacturer.
Radial ply
Refers to the ply or plies used in tire in which the cords run at right angles to the bead and parallel to the tire radius.
Radial Run Out
A tire assembly that does not form a true circle; the radii of the circle are not equal. Most usual causes are bent wheels (out of round) or tires not mounted properly (beads not seated).
See loaded radius or free radius.
Radius rods
Metal arms attached to frame and axles for alignment
A tool used to prepare a tire for section repair and for buffing prior to retreading.
A synthetic fibre.
The act of cutting new tread grooves into the crown of the tire after the original tread is worn down. A practice forbidden by many states' laws.
Any material, usually rubber and fabric, vulcanized to the tire to return strength to the cord body at an injury.
Repair gum
See cushion gum
Revolutions per minute (RPM)
The measured revolutions for a tire travelling one mile.
Excessive heating of a cured rubber compound leading to deterioration of its physical properties.
The continuous raised portions of rubber that run circumferentially (straight or in a zigzag pattern) making up the tread on the tire. Also the term applies to various raised surfaces circling the sidewall, i.e., guide rib.
Rim pull
Actual amount of effort in pounds available at point of contact of tire and road surface.
Rim & wheel terms
  • Cast spoke assembly
    part of the vehicle consisting of the brake chum and wheel spider, having 3, 5 or 6 spokes.
  • Demountable flange
    A side ring or side and lock ring combination that retains the tire on the rim. It is removable to permit tire mounting or removal.
  • Disc wheel
    The combination of a rim and a metal disc riveted or welded to its centre.
  • Flange
    The curved metal extremes of a rim that retain the tire on the rim base.
  • Gutter
    The recessed on a truck rim base that holds the lock ring in place.
  • 28 gutter bevel
    Truck rims are bevelled to provide for the gutter and this bevel matches a 28 taper on the inside of the cast spokes, holding the rim in place.
  • Lock ring
    A metal ring which snaps into the rim gutter, holding the side ring in place.
  • Offset
    An important measurement for positioning the tire to insure proper tracking of the vehicle and adequate dual spacing. See below: measurements, rim or measurements, wheel.
  • Rim (truck)
    A metal assembly consisting of a base and either a side ring or a side and lock ring combination, which are removable from one side for tire mounting. The opposite side has a fixed flange to retain the tire. Note: Passenger and tubeless rims do not have removable side and lock rings.
  • Side ring
    A demountable metal flange to hold the tire on the rim base. It can be self contained, locking into the gutter, or may be held in place by a lock ring, depending on the type.
Measurements, rim:
  • Flange height
    Measured from top of flange to bead seat, and is the difference between overall diameter and nominal rim diameter, divided by two.
  • Nominal diameter
    Measured bead seat to bead seat 180 opposite.
  • Nominal width
    Measured between the flanges at the bead seat.
  • Overall diameter
    Top of flange to top of flange 180 opposite.
  • Rim offset
    A measurement in inches from the centre of the rim (between the flanges) to the tip of the 28 gutter bevel.
  • Bolt hole
    Diameter of the bolt hole.
  • Bolt hole circle
    Diameter of a circle scribed through the bolt hole centres.
  • Centre bore
    The diameter of the opening in a disc which allows for protrusion of the axle hub.
  • Wheel offset
    A measurement in inches from the centre of the rim (between the flanges) to the outside face of the disc.
Society of Automotive Engineers.
Safety ledge
A raised area (hump) around the circumference of the bead seat area of passenger wheels. Its function is to prevent the tire beads from becoming unseated during hard cornering or while running with low air pressure. A must with tubeless passenger tires.
Discarded tire casings having service life exhausted through wear or damage.
Semi-drop centre rim. See rim types.
Secondary failure
The failure which occurs as a result of a primary failure.
Section height
The vertical measurement from bead seat to top of crown when mounted, inflated and not under load. (Overall diameter less nominal rim diameter, divided by two.)
Section repair
Reinforcement made to the casing when an injury has extended through the tread or sidewall of a tire. The damaged cord is removed and a new cord is replaced in the form of a repair unit or patch. (A major repair in the sidewall or tread of a tire. This repair unit must have cord material for reinforcement, made specifically for the type tire-bias-ply or radial.)
Section width
The measurement across the tire width at the widest point when mounted and inflated (not under load), excluding any decorative mouldings.
Self-cleaning tread
A tread pattern with tapered grooves and ribs, bars, etc., arranged in a manner that resists packing loose materials, (mud, etc.) in the grooves.
Self Vulcanising
Vulcanization at room temperature or above, activated by chemical agents without the application of heat from an outside source.
The parting or debonding of any adjacent parts of the tire (ply to ply, ply to rubber, etc.), usually due to excessive heat.
Separation solid tires
Rubber pulling away from steel band.
Serial numbers
The individual, consecutive numbering of tires during production. It may be a combination of letters and numbers moulded on the sidewall.
Batch number
May be found in addition to a serial number to identify the tire with other tires manufactured during a given period.
Premature vulcanization of a rubber compound during processing or storage.
Shelf life
See retread terms.
Thin metal plates used as spacers in vehicle alignment.
Side to side movement (or vibration) of a tire assembly.
Shock absorber
Vibration dampening device used with chassis springs to lessen road bounce.
Outer edges of tread.
Shredded wire
Short pieces of fine steel wire mixed into a tread or under tread compound.
That portion of a tire between the tread and bead.
Single bead
Refers to a tire built with only one wire bundle in the bead.
A hairline groove cut into tread ribs, bars or blocks to improve traction on wet surfaces.
To slip, or lose traction, sliding out from the intended direction. Not to be confused with the terms drift or sup angle.
Skid depth
(Anti-skid depth) See tread depth.
Slang reference to vehicle tires
To skive; to cut into. Cut away rubber from an injury in preparation for a section repair.
Sleeve nuts
See dual mountings
Slewed axles
Axles out of parallel.
Slip angle
See tire deviation angle.
Lubricant used to prevent sticking between tire and tube.
Industrial tires made without an air chamber.
  • For demountable rims
    Used in cast spoke mountings, it is a band having the circumference of the spoke wheel and the rims. It fits between two rims in dual mounting to provide clearance between duals.
  • Between wheel spacer (obsolete)
    It is a circular metal plate having a bolt hole circle and centre bore and fitting between the faces of disc wheels to provide additional dual clearance.
Specific gravity
Weight of given volume of substance compared to that of an equal volume of water.
Instrument to indicate velocity in miles per hour.
Special mileage tire
(Re-groovable tire)
A tire manufactured with an extra layer of rubber between the cord body and the original tread design. This extra layer is designed for the purpose of recutting and regrooving, and is specifically labelled as a special mileage commercial tire.
Spread axle
Trailer axle, usually six to nine feet ahead of another axle and located near the centre of the trailer.
Spreader, tire
Any device, manual or hydraulic, used to spread tire beads for inspection, repair, service, etc.
Flexible or elastic member supporting spring weight of vehicle with recovering properties of resuming to original shape when released after being distorted.
Spring brake
Emergency or auxiliary brake system utilizing a spring load as a force for braking. May be automatically actuated by low air pressure or mechanically controlled for use as a parking brake.
Spring clip
Small metal brackets insuring proper alignment of spring leaves.
Spring rate
Rate of deflection versus amount of load applied in other words, how much force is needed to bend a spring a given distance.
Spring seat
Support on which spring is anchored.
Device used to stabilize vehicle during turns sometimes referred to as a sway bar.
Stabilizing plies
Two or more plies of steel, fibreglass, etc., forming a belt around the circumference of a tire, between carcass and tread rubber. It reduces tread distortion of radial and bias belted tires.
Standard rim
A rim that has been calibrated and found to meet the precise measurements specified by Tire and Rim Association, Inc. or, where applicable, by European Tire & Rim Association.
Static balance
See balance.
Steering arms
(Tie rod arms)
Arms attached to front axle steering system to effect turn of wheels.
Steering axis alignment
See alignment.
Steering axle
An axle that directs control of the vehicle. It can be powered or non-powered and more than one steering axle can be present.
Steering knuckle
A steel forging which includes the wheel spindle. In cars it is the spindle support arm which pivots on ball joints. In trucks with non-independent front suspension, it pivots on the kingpin.
Retread (recap)
A means of extending the life off a tire that has worn its original tread, yet still has a sound carcass. The casing is prepared by buffing of f tread rubber and vulcanizing new tread rubber stock in its place. A casing to which tread rubber has been affixed to extend the useable life of the tire after the original tread has been worn out.
Retreading terms
Buff contour
The specked shape of a buffed.
Buff line
The dividing line in the cross section of a tire between the buffed surface of the original tire and the new retread rubber.
Buffered radius
A dimension that ensures the proper contour of the buffed surface according to tire size and type and matrix dimensions.
A machine used to rasp the old tread from the tire.
Grinding or rasping off remaining tread rubber to give the casing proper texture to accept new retread stock and proper dimensions to fit the matrix.
Buffing template
A machined device of a specific shape used to obtain the required buffed contour.
A machine used to apply tread rubber to a casing.
The application of retread or repair rubber.
Uncured retread rubber in crescent shape, available in various widths and depths according to size and type of tire being retreaded. See die size, below.
An adhesive compound of rubber and certain chemicals which effect cure, dissolved in rubber solvents.
A pressure chamber used to vulcanize pre-cured tread stock to the buffed casing.
Cold cap
So called because the tire is placed in a pressure chamber in a temperature range of 195-212 until bonding of pre-cured tread rubber is achieved. Also see hot cap.
Vulcanize uncured rubber through the application of heat, pressure and time to permanently shape and set the rubber to achieve the degree of hardness desired to protect it from the affects of normal operating temperatures and wear.
Curing rim
A special rim that supports the inflated tire during curing process.
Cure tube
A heavy tube within the tire that provides pressure to force the casing against the matrix during the curing process.
Curing gum
A soft, tacky rubber compound used in retreading and repair to facilitate bonding between different rubber compounds and between plies, etc. Also see rubber terms.
Die size
Retread rubber is designated by its crescent shaped dimensions in inches and eighths, and its thickness in 32nds of an inch. (Example: 66-72-16; the crown would measure 6 and 6 eighths, the base 7 and 2 eighths wide; and the thickness 16/32 of an inch, or gauge of the stock rubber.)
A machine used to cut the lugs from tires prior to buffing.
A thin rubber wrapper that surrounds the tread, sidewall and is tucked inside the curing rim during pre-cured cold process. retreading. It protects bonding materials from humidity within the chamber.
Filler strip
free flowing rubber used under the tread when added thickness is needed.
Full cap
New tread rubber is added to the buffed casing, covering the crown and shoulder areas.
Hot cap
The conventional method of retreading in which uncured rubber is added to a buffed casing and cured in the mould at temperatures of approximately 290-300. This temperature allows uncured rubber to flow in the matrix forming the tread design during vulcanization.
That portion of the mould which surrounds the tire transferring heat to the uncured rubber and forming the tread pattern.
A device, that includes a matrix, enclosing the tire and supplying heat and pressure to effect curing.
A condition, usually in the cushion rubber resulting from local material starvation and excessive flow from adjacent areas.
Procured tread rubber
Pre-cured rubber, usually of high density and available in various tread designs is lined with cushion gum before applying to a buffed casing using the cold cap method of retreading. Recently it has been applied in some hot cap moulds employing a smooth matrix or other modifications.
The parting of retread rubber from the buffed casing.
A time limitation for the storage of uncured retread materials (usually 6 months), beyond which certain properties are lost. Storage of materials in a cool, dark, dry environment insures quality.
Stripping stock
A rubber stock used to extend the wing of tread rubber.
Template (buffing template)
A pre-cut pattern, usually metal, used to determine the contour of a buffed tire.
Top cap
New tread rubber is added to the buffed casing, covering only the crown area.




Rim bead taper by rim types:
  • Flat base
    No taper at either bead seat (obsolete).
  • Intermediate
    One tapered bead seat of 5 at the fixed flange.
  • Advanced
    Both bead seats tapered 5.
  • Semi-drop centre
    Both bead seats tapered 5.
  • Drop centre
    • Passenger: Both bead seats tapered 5.
    • Tubeless truck: Both bead seats tapered 15.
Rim types
  • Drop Centre
    A one piece rim having a deep centre depression for passenger, light truck and tubeless truck mountings. The tire is mounted by moving a tire bead into the drop centre, thus allowing the bead area 180 opposite to slide over the rim flange.
  • Flat base
    In common usage, it applies to any truck rim not having a centre depression. A demountable flange (consisting of 1 or 2 piece side/lock ring) permits the removal of the tire and holds it in place after installation.
  • Semi-drop centre
    A rim having a demountable flange, for use on light commercial application. A slight depression in the centre of the rim facilitates the mounting and tire removal.
  • Semi-drop centre
    A term used to describe a two or three piece rim that employs a demountable flange.
Rubber Manufactures Association
Road hazard
Injury to a tire sustained in normal operation, excluding collision or vandalism and that is not reparable by accepted standards.
Roll-off, tire
Bead unseating due to running a tire while flat. Also, the sudden loss of air due to side forces, such as hard cornering, combined with under inflation. The safety ledge on passenger rims (a necessity for tubeless mountings) guards against a roll-off.
Rolling resistance
Sum of the forces at area of contact between a vehicle's tires and road surface acting against the direction of movement.
Roll Steer
A steering effect induced by load transfer from side to side. The axles may move out of their normal parallel relationship due to spring deflection.
Revolutions per mile or revolutions per minute.
Rubber terms
  • Bloom
    A surface film on rubber, caused by the migration to the surface of sulphur, wax or other unreacted ingredients of the compound. It may be protective to the tire, and detrimental only if appearance is a major factor.
  • Brittlepoint
    A low extreme temperature at which a rubber specimen fractures on sudden impact.
  • Gum stock (cushion gum)
    Rubber compounds containing only the necessary ingredients for vulcanizing. Reinforcing fillers are not present in gum stock.
  • Natural rubber
    Elastomer produced from latex, a milky sap, obtained from the rubber tree. An elastic and porous form of rubber.
  • Synthetic rubber
    A man-made elastomer. The raw materials generally being derived from oily by-products. A non-elastic form of rubber.
  • Butyl
    A non-porous form of synthetic rubber.
Run flat
Damage sustained by a tire due to driving with insufficient air pressure or while flat.
A hard rolling method used to both remove trapped air and improve rubber contact for better adhesion during repair and retreading.
Attaching parts including springs for securing axle or axles to chassis frame.
Instrument to record, on a graph, vehicle trip information such as mph, rpm, stop-and-go periods.
Instrument to indicate engine speed in revolutions per minute (RPM).
A term used to describe the sticky quality of a rubber compound.
Powder lubricant to prevent sticking between tube and tire.
Tandem axles
A combination of two axles having a common suspension.
Tandem drive
Two powered axles in combination.
Tare weight (curb weight)
The weight of a truck, exclusive of its contents, but including gas, oil, etc., ready to roll.
Tire and Rim Association.
Tensile strength
A measurement of the greatest longitudinal stress a tire cord can bear without breaking.
Thermal deterioration
Loss of strength of carcass cords due to heat generated in a tire.
Thermal equilibrium (Heat balance)
The point at which the rate of cooling becomes equal to the rate of heat generation in a tire.
Tie rod
Connecting rod between steering arms.
Tie rod ends
Adjustable connections between tie rod and steering arm used to adjust toe. See alignment.
Tire deviation angle (slip angle)
An angle formed by the direction of travel of a vehicle and the steered direction (that which the wheels are pointing). It is produced by distortions within the tire, due to external forces. Not to be confused by actual slipping or skidding.
Tire dressing
Paint, black glossy or dull finish, to improve appearance of tire sidewalls.
The Industry Safety Council.
Tire paint
A black paint, compatible to tire bodies, used to enhance the appearance of a tire after retreading.
Tire size markings
Those designations that appear on the side of a tire to indicate its basis dimensions; width and rim diameter.
Ton mile per hour (TMPH)
A system devised to ascertain the working capacity of earthmover tires; it is a supplement to the normal load and inflation tables.
Toe in-Toe out
See alignment.
Toe in
Adjustment of front wheels so that they are closer together at the front than at the back. Slight toe-in eliminates tendency of wheels to weave from side to side.
An allowable variation in dimension.
Ton miles per hour rating (TMPH rating)
The maximum that a tire can carry without overheating.
TR number
Code number of the tube valves and flaps set by Tire and Rim Association.
The amount of force exerted to tighten wheel nuts, expressed in foot pounds. One pound of force at the end on a one hot lever equals one foot pound. Torque also refers to the amount of turning force supplied by the engine and transmitted to driving axle.
Torsion-bar, Torsion spring
A steel bar attached between the frame and a moving part (control arm or axle) that acts as a spring. Any vertical movement is a twisting force on the bar which resists the movement and cushions the vehicle. It is used by some manufacturers to replace the coil spring.
Track, axle
Distance between centrelines of tire tread measured across axle.
The alignment of the vehicle with respect to axle centres of a vehicle being in the same plane, rear tires following front tires and trailer following the tractor, in the attitude designed. Correct tracking is when the rear wheels of a vehicle follow the same line as the front wheels.
Traction limited grade
Steepest negotiable grade where traction is the limiting factor.
The grip or friction, pull or bite between tire and ground.
Tractor (truck tractor)
Truck portion of combination or train.
Platform or container on wheels pulled by tractor. See full trailer, or semi-trailer.
Tires bouncing up and down, usually from incorrect static balance. Sometimes from running empty with high inflation pressures in trailer tires.
A non-solid driving axle used with independent suspension on some rear engine vehicles. The transmission and differential are in one unit, driving the axle.
Transfer case
Split power gear box transmitting drive to front and rear axles.
Transmission (auxiliary)
Additional gear box increasing the gear ratio combinations when used with main transmission or multi-speed axles.
Transmission (main)
Selective gear box providing various combinations of gear ratios.
The portion of a tire that makes contact with the road. It contains ribs, grooves, and sipes, etc., for traction, and special compounds for resistance to wear.
Tread bar
Raised portions of rubber that make up the tread of the tire, separated by grooves that are arranged laterally, providing traction in the forward or rearward direction.
Tread block
Raised portions of rubber that make up part of the tread pattern They are often present in the shoulder area and are more or less rectangular in shape.
Tread depth
The measurement from the bottom of the tread groove to the top of the tread expressed in 32nds of an inch.
Tread rib
The continuous raised portions of rubber that run circumferentially (straight or in a zigzag pattern making up the tread on the tire.
Tread roller
A roller, either manual or power, used to help apply the tread rubber, remove trapped air and obtain adhesion. See stitching
Tread separation
Pulling away of the tread from the tire casing.
Tread squirm
Distortion produced by rolling stresses within the tire, resulting in tread wear and rolling resistance.
Tread wear indicators
Bars of rubber across tread at base of tread grooves having height of 2/32" at several spots around the tire circumference. Smooth lines appear to warn that safe tread life is exhausted.
Refers to the arrangements of the stabilizing tread plies whereby their cords form tiny triangles in the Michelin X radial truck tire.
Vehicle having three rear axles driving.
The act of cutting rubber off the tread of a tire while rotating on a truing machine to make the tire assembly round. A sometimes harmful practice which takes off tread rubber to allow for an out of round wheel, or for a tire not mounted properly.
See inner tube.
Turning radius
Radius of minimum turning circle. Also an alignment angle. See alignment.
Twin fitment
See dual mountings.
Two speed axle
Axle having two selective gear ratios.
A trade name for a form of rayon produced exclusively for tire cords.
U-shaped bolt for attaching leaf springs to the axle.
ULP (ultra-low-profile)
See aspect ratio.
See balance
A condition which describes less than acceptable vulcanization or curing.
Insufficient air pressure in a tire for the amount of load carried.
The use of a tire having insufficient carrying capacity for the load. Also replacing original-equipment tire size with a smaller size of sufficient capacity, thus reducing unladen weight.
A vehicle is reluctant to leave the straight ahead path while steering. Can be caused by larger slip angle in front tires than rear. Also see tire deviation angle.
Unladen weight
The actual weight of a vehicle or combination with no payload. See tare weight.
Unloaded radius
See free radius.
Unsprung weight
That portion of the weight of the vehicle not supported by the springs; including tires, wheels, axles and brakes.
Valve assembly
A device through which a tire is inflated. It includes a valve stem, valve core and valve cap.
Valve cap
A screw-on cap to seal air in the valve.
Valve core
A check valve within an air valve which permits air pressure checks without undue loss of air pressure. The core should not be considered a valve seal.
Valve extensions
Extra length added to valve stem for greater accessibility particularly on inside duals.
Variable spring
Spring providing variable effective length through cam action to suit load.
Rate of motion.
The act of perforating a textile bias ply tubeless tire above the beads during retreading. Thus air bleeding through the calendering can escape without forming air pockets between plies.
Process rubber by chemical means, usually in combination with heat, to improve its strength, hardness, elasticity and to effect bonding to other rubber compounds. Used in the manufacture, repair and retreading of tires.
Reference to steering where vehicle moves or rambles from a fixed course without positive control.
Weather cracking
A visual sidewall condition which appears as cracking of the rubber. See 'OZONE CHECKING'.
Well depth
Vertical distance from bead seat to bottom of drop centre rim.
Manual or power device employing a drum with cable or rope for pulling objects where great power is required. Power generated by vehicle engine and transmitted through power take-off on transmission.
Wheel base
Distance in inches between front and rear axles.
  • Tandem axle
    Distance between front axle and centre between tandem axles.
  • Trailer
    Distance between fifth wheel pin and centre of rear axle (or centre between tandems).
Wheel alignment
See alignment
Wheel hop
Tendency of tire and wheel to hop during braking or acceleration of vehicle.
A capillary act of air escapement from the tire casing during re-treading.
Wing stock
See re-treading terms or stripping stock
That type of tread rubber that is tapered to a feathered edge on each side in order that it may be applied to the shoulder of the tire. Used only on a full retread.


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