Tyre glossary

Working definitions for the terms used across this knowledge base. Where a term has a formal standard definition, the authority is noted — see tyre glossary references for the ISO 3877/4223 and TRMG source lists.

Construction & anatomy

• Bead — the steel-wire edge of the tyre that seats against the rim flange and holds the tyre on the wheel under inflation.
• Belt package — the steel or textile belt layers between the carcass and the tread that stabilise the crown of a radial tyre.
• Carcass (casing) — the structural body of the tyre underneath the tread — the part that is inspected, repaired and retreaded.
• Crown — the centre region of the tread that contacts the road in normal running.
• Duals — two tyres mounted side by side on one axle end; common on drive and trailer axles.
• Ply — a layer of rubberised cord fabric. In a radial tyre the carcass plies run bead-to-bead at 90° to the direction of travel.
• Radial (TBR) — a tyre whose carcass plies run radially. TBR = truck & bus radial, the commercial-vehicle radial class.
• Shoulder — the outer edge of the tread where the crown meets the sidewall; runs hottest and shows alignment faults first.
• Sidewall — the flexing region between bead and shoulder; carries the markings and is not repairable for punctures.
• Tube type (TT) / tubeless (TL) — whether the tyre holds air via an inner tube (TT) or seals directly on the rim (TL).
• Undertread — the rubber layer between the belt package and the tread grooves; protects the belts and anchors regrooving depth.
• Wide-base single — a single wide tyre that replaces a pair of duals on one axle end (e.g. Michelin X One).

Markings & ratings

• Aspect ratio — sidewall height as a percentage of section width, e.g. the 70 in 295/70R22.5.
• DOT / TIN date code — the US DOT tyre identification number; its last four digits are the build week and year (WWYY).
• Load index (LI) — the numeric code for a tyre's maximum load at its reference inflation pressure, looked up in standard tables.
• Load rating single vs dual — the load index when the tyre is fitted as a single versus as one of a pair of duals; dual ratings are lower per tyre.
• M+S / 3PMSF — mud-and-snow marking; 3PMSF (three-peak mountain snowflake) is the tested severe-snow symbol.
• Ply rating (PR) — a strength marking that expresses load capacity in old cotton-ply equivalents; it is not a literal ply count.
• Regroovable — a sidewall marking meaning the tyre was built with extra undertread rubber so the tread may be re-cut.
• Rim size — the wheel diameter in inches the tyre mounts on, e.g. 22.5 in 295/70R22.5.
• Section width — the tyre's overall width in millimetres at its widest point, e.g. 295 in 295/70R22.5.
• Speed symbol (SI) — the letter code for the maximum sustained speed at the tyre's rated load (e.g. L = 120 km/h, M = 130 km/h).
• UTQG — US Uniform Tire Quality Grading: comparative treadwear index plus traction (AA–C) and temperature (A–C) grades for passenger tyres.

Pressure & monitoring

• ATIS — automatic tyre inflation system; keeps trailer tyres topped up from the axle air supply while driving.
• Cold inflation pressure — the pressure measured before driving; the only pressure spec that matters — never bleed a hot tyre down to "spec".
• Load vs pressure — the load a tyre may carry is a function of its inflation pressure, not just its size; the tables map required cold pressure to carried load.
• TPMS — tyre pressure monitoring system; in-tyre sensors reporting pressure and inner temperature to the cab or fleet.
• Underinflation — running below the pressure the carried load requires; the leading preventable failure mode — flexing builds heat that destroys casings.
• UN R141 — the UNECE regulation making TPMS mandatory on new EU heavy vehicles; establishes duty classes for truck TPMS.

Wear & damage conditions

• Camber wear — wear concentrated on one shoulder caused by wheel lean-in or lean-out.
• Cupping / scalloping — alternating high-low wear spots around the tread, typically from imbalance, worn suspension or shock absorbers.
• Feathering — tread blocks worn so one edge is higher than the other, felt by dragging a hand across the tread; a toe-misalignment signature.
• Flat-spotting — a localised flat patch from a locked-wheel skid.
• Heel-toe wear — sawtooth wear across tread blocks, front edge high; normal drive-axle behaviour, excessive when alignment is off.
• Irregular wear — any wear pattern other than even tread removal; each pattern is a diagnostic fingerprint of a mechanical or inflation cause.
• Separation — loss of adhesion between casing components (belts, plies, tread); shows as a bulge and is not repairable.
• Toe wear — fast shoulder or full-face scrub from incorrect toe setting; the most common alignment-related tyre killer.
• Zipper rupture — a circumferential sidewall burst on steel-cased tyres caused by running severely underinflated or overloaded; extremely dangerous during reinflation — the reason damaged tyres are deflated before removal.

Repair & retreading

• Buffing — removing the worn tread from a casing to a controlled texture (BTS1–BTS6) before applying new tread material.
• Combination plug + patch — the required puncture repair: a stem fills the injury channel and an internal patch seals the liner. Never plug from outside only.
• Mould cure (hot cure) — retreading by curing uncured tread rubber onto the casing in a mould, forming the pattern during cure.
• Precure (cold cure) — retreading with factory-pre-cured tread strips bonded to the buffed casing in an envelope/cure chamber.
• Regrooving — re-cutting the tread pattern into remaining undertread rubber on tyres marked "regroovable"; subject to depth and position rules.
• Repairable zone — the tread crown area where puncture repairs are permitted (≤6 mm injury on passenger tyres); sidewalls and shoulders are out.
• Retread — a new tread applied to a sound, inspected casing; ECE R108/R109 type-approve retreads for cars and trucks respectively.
• Section repair — a reinforced repair to a larger injury in a truck-tyre crown/shoulder per manufacturer limits; heavier than a puncture repair.
• Shearography / NDT — non-destructive casing-inspection methods (shearography, electrical scanning) used to vet casings before retreading.

Wheels, rims & fitment

• Demountable rim — a rim that bolts onto a spoke wheel or hub as a separate band, common on older multi-piece assemblies.
• Drop-centre rim — the one-piece tubeless rim profile used on modern trucks; the well lets the bead pass during mounting.
• Hub-piloted vs stud-piloted — how the wheel centres on the hub: on the hub bore/pads (hub-piloted) or on the studs (stud-piloted); affects nut type and torque.
• Multi-piece rim — a rim assembled from a base plus side/lock rings; mismatched or unseated rings can explode off under pressure — the OSHA 1910.177 hazard.
• Restraint device (cage) — the safety cage required when inflating a freshly mounted multi-piece assembly.
• Thrust angle — the angle between the vehicle's geometric centreline and the direction the rear axles actually push; misdiagnosed constantly as a steer problem.
• Toe / caster / camber — the three alignment angles: wheel pointing (toe), steering-axis tilt fore-aft (caster) and wheel lean (camber).
• Wheel end — the hub, bearings, seals and lubricant at each axle end; failures trace mostly to improper bearing installation or seal leaks.

Commercial terms

• CPK / cost per kilometre — the unit tyre cost expressed per kilometre run; the benchmark for comparing tyres and contracts.
• PPK (price per kilometre) — tyre-as-a-service contract billing per kilometre instead of selling casings outright.
• OOS (out of service) — the criteria at which a tyre or wheel must be removed from use, per TMC recommended practices.
• UTQG treadwear index — a comparative wear-life grade for passenger tyres (course-derived; real-world results vary).

Sources: terminology per ISO 3877-1/ISO 4223, TRMG retread & repair glossary and the TMC conditions vocabulary — all referenced in tyre glossary references.