Q: I have a tiny nail sticking into the side of the tread on one tire and the air leaks very slowly. The tires are only one year old and one shop I had look at the leak said they can’t repair any puncture in the side, no matter how small. Why can’t a little thing like this be fixed, and if I can’t get it done, do I have to replace both tires on that end? — Marcy in  Springfield, Mass.

A: Tires, while incredibly rugged, have a weakness. That weakness is the sidewall itself, which consists of layers of rubber and cord. The cord is laid in a radial pattern, which means it sits 90-degrees to the side of the tire. If a puncture in the rubber occurs, the object separates the cords inside as it penetrates. This can weaken the tire structure to the point that the sidewall rips and fails catastrophically. Repairs to sidewalls, whether they be patches inside or plugs, will fail over time due to constant flexing of the sidewall.

Puncture repairs can only safely be done in the tread area and licensed shops will not repair sidewalls.

As for the necessity of replacing both tires on an axle, it’s a question of how much tread is worn off. If, for instance, the original tread depth was 10/32nds (quite typical) and the tires are worn to 8/32nds, it’s perfectly acceptable to replace just one tire as long as it’s identical to the original. Greater wear, however, will necessitate replacement of both tires because the differential in diameters will create problems with the ABS and TPMS sensors.