Sidewall vs tread damage: the decisive distinction
A puncture in the tread area (the part of the tyre that contacts the road) may be repairable if: it is in the central three-quarters of the tread, the diameter of the penetration is less than 6 mm, the tyre has not been driven on while flat (which damages the sidewall and the bead), and the tyre has adequate tread depth remaining. BSAU 159 and the tyre manufacturers' guidelines set the repair criteria.
A sidewall puncture, any damage on the curved wall between the bead and the tread, is not repairable under any standard. The sidewall flexes with every rotation; a repair plug or patch will not hold under the cyclic stress. A sidewall-damaged tyre must be replaced. If the spare is not available, a tow is required.
Impact damage, a pothole strike or a kerb strike, may not immediately deflate the tyre but can crack the carcass cords internally. The tyre may appear serviceable but is structurally compromised and at risk of sudden failure. If the vehicle has struck a sharp kerb or a significant pothole, the tyre should be inspected by a tyre technician before further motorway driving.
