People often ask: “How do I know when my tires are worn out?” Good question!
It is easier to see a mountain bike’s knobby off-road tires showing wear. The knobs get smaller and rounder. Traction is not as good as it used to be. You may be getting a lot of flat tires.
It is more difficult to judge wear on a smooth, slick road tire that has no tread whatsoever, even when new. However, a newer road tire will have a distinctly rounded profile, looking much like the top half of a circle. When the tire begins to wear, it loses its round profile, and begins to resemble the top half of a hexagon, with a flat midsection. As with mountain bike tires, if you begin to get more flats on your road tires, or if traction becomes an issue, it may be time to replace them.
With any tire, if the sidewall has cracks or is quite brittle to the touch, it is preferable to replace the tire than to risk a blowout on the road or trail.
If the inner tube does not hold air, it is not usually necessary to replace the tire (unless it is gashed or slashed, risking a bulge in the inner tube through the tire). You simply need to patch or replace the tube. The size of tube and tire is usually embossed onto the tire’s sidewall. Designations such as 26 X 1.95 or 700c X 23 refer to the diameter and width of the tire (and the inner tube). You must know the size in order to purchase a replacement tube. You must also know the valve type. For bicycle tubes, there are two different valve types used, and they are not interchangeable. The fatter valve (Schrader) looks just like the valve on your car tires. The skinnier valve (Presta) has a metal valve stem that unscrews at the end.