Tread (Not THREAD)
The tread is the part of the tyre that comes in contact with the road surface. The portion that is in contact with the road at any given instant in time is the contact patch. The tread is a thick rubber, or rubber/composite compound formulated to provide an appropriate level of traction that does not wear away too quickly. Tread grooves run circumferentially around the tyre, their purpose is to channel away water. Lugs are the portion of the tread design that contacts the road surface. Spaces between the lugs allow the lugs to flex and evacuate water. Tread patterns are designed to minimise noise levels.
Tread lugs provide the contact surface necessary to provide traction.
Tread spacing allows the lug to flex as it enters and exits the contact area with the road surface. These spaces also provide for rainwater, mud, and snow to be channeled away from the contact area.
The rain groove of the tread pattern is specifically designed to channel water away from the contact area. Rain grooves are generally circumferential whilst many high-performance passenger tyres feature rain grooves that are angled toward the sides of the tyre.
Tread lugs usually have small slits called sipes that improve the flexibility of the lug to improve the contact patch. This reduces shear stress and heat build up in the lug.
Wear indicators are raised features located at the bottom of the tread grooves that indicate the tyre has reached its wear limit. When the tread lugs wear to the point that the indicators are level with the lugs, the tyres have reached the end of their life. The wear indicators are raised approximately 1.6 millimetres in the tread grooves.
The bead is the part of the tyre that fits to the rim of the wheel. The bead is reinforced with steel wire and high strength, low flex rubber. The bead seats against the rim to ensure that the tyre does not leak, or move as the wheel rotates.
The sidewall of the tyre bridges between the tread and the bead. The sidewall is reinforced with fabric or steel cords to increase tensile strength and flexibility. The sidewall holds the air pressure and transfers the torque applied by the drive axle to the tread to create traction.
The shoulder is corner of the tyre where the tread meets the sidewall.
Plies are layers of cords in the rubber allowing the tyre to hold its shape. The cords prevent the rubber from deforming.
Tyres are fitted to wheels that usually have integrated rims on their outer edges to hold the tyre.
The beads of the tyre are held on the rim, or the “outer edge” of a wheel. The wheel’s rim must be of the proper design to hold the bead of the tyre.
The valve stem is a tube made of metal or rubber that incorporates a valve, through which the tyre is inflated. Valve stems protrude through the wheel for easy access and are mounted directly to the rim, in the case of tubeless tyres.
Radial tyre technology is now the standard design for essentially all automotive tyres, but other methods have been used.
Bias tyre (or cross ply) construction features ply cords diagonally from side to side, at angles in between 30 to 40 degrees, in a crisscross pattern.
Radial tyres have cords extending from the beads across the tread at approximately right angles to the tread, and parallel to each other, and stabiliser belts directly beneath the tread. The belts may be cord or steel. Advantages include improved tread life, steering control, fuel economy, fewer blowouts and lower rolling resistance.