The main way we judge what kind of shape tires are in is to look at the tread wear. This is usually the best indication of whether or not a tire is in good shape to perform reliably and safely. There is another problem tires can have that does not necessarily show in the tread – it is called tire dry rot. This problem occurs as a result of vehicle inactivity, low tire pressure, or extreme heat.
In cases of tire dry rot, the tread on old tires may look fine, but on closer inspection, tires may show fine cracks running in a web-type pattern along the sidewalls to the tread, indicating deterioration from tire dry rot. The rubber and materials used in tire fabrication naturally degrade over a period of approximately five years. This is affected by factors such as temperature, humidity, use and storage of the vehicle, and tire pressure levels. Try dry rot is revealed as hard and brittle surfaces on the tires, occurring as the oils in the rubber begin to evaporate and chemical bonds break down.
Tire dry rot is not an issue on a vehicle that is driven regularly. That is because tire pressure is more closely monitored and the tread actually wears out before rot from aging can occur. Vehicles that are especially susceptible to tire dry rot are those that are driven only occasionally, such as collector cars or recreation vehicles.
If noticed in the early stages, tire dry rot can sometimes be repaired, but as a rule, tires with dry rot are not safe and should be replaced as soon as possible. For short distances, you may have a limited time before you must replace the tires. When the cracks reach the cords of the tires, the heat of long distance driving will cause rubber expansion and the tires may actually break apart while driving.
If you have a vehicle you must store for long periods of time, keep it in a climate-controlled garage and keep the tires properly inflated to prevent damage from tire dry rot.