Blackcircles.com Tyre Glossary P-S
Plies (or ply): The reinforcing members of a tyre which are composed of layers of cord fabric and rubber that provide the strength to contain the air pressure needed to support a load and resist deflection.
PSI (pounds per square inch): Imperial measurement unit for air pressure.
Radial Tyre: The design of all modern car tyres. Development of the use of parallel carcass belts for sidewalls and crossed belts for the crown of the tyre.
Reinforced Tyres: Tyre where the manufacturer has added extra strength by adding more plies into the side wall to handle more weight e.g. for heavier vehicles (i.e. SAAB 9-3s or Ford Galaxy’s Renault Laguna).
Remoulding: A recycling process where a tyre is buffed back to its casing, repaired and rebuilt using a fresh tread compound.
Retreading: This is the process of replacing the tread layer on a tyre. Usually reserved for commercial, agricultural and industrial tyres.
Rim: The metal edge of the wheel on which the bead is seated supporting the tyre.
Rim Size: The diameter of the rim (usually measured in inches and also known as the wheel size).
Rim Width: The Distance between the two opposite inside edges of the rim flanges.
Rolling Resistance: The energy required to keep a tyre moving at a constant speed. The lower the rolling resistance the less energy being used to keep the tyre moving.
Run-Flat: Tyres which allow a driver to continue a journey in the event of low or no pressure. Please note that Run-flats are designed to run for short distances and should be replaced as soon as possible.
Section Width: Measurement of the tyre from side to side. This measurement excludes raised letters or any other additional sidewall items.
Service description: As well as Tyre Size Designation a tyre may be identified by a Service Description consisting of a load index (or two Load Indices in the case of single/dual fitments) and a Speed Symbol.
Shoulder: This is where the sidewall and the tread meet on the tyre.
Snow Tyres: Tyres specifically designed to perform better in the snow.
Space Saving Tyre (also known as a “Mini” tyre): Smaller than the road tyres, this type of Tyre is designed to save space and reduce weight in the vehicle. A temporary measure to get you home these tyres should be changed as soon as possible, these tyres also have a lower maximum speed.
Specified load: Found in the load index under the conditions specified by the tyre manufacturer.