Eventually tires wear out, and needreplaced. Typically the rear tires start to square up, losing their roundedprofile, as the center of the tread wears away faster than the shoulders.

 Fronttires generally wear more evenly across their tread, but may begin to developscalloped wear known as cupping. Knobby tires are more obvious as the knobs startto wear, tear or break-off over time.

Inspect your tires for adequate treaddepth. When the tire is worn to the built-in indicators at 1/32nd inch (0.8millimeters) or less tread groove depth, or the tire cord or fabric is exposed,the tire is dangerously worn and must be replaced immediately. Also inspecttires for uneven wear. 

Wear on one side of the tread, or flat spots in thetread may indicate a problem with the tire or bike. Consult your local dealeror mechanic for help. Inspect your rims also. If you have a bent or crackedrim, it must be replaced.

A good practice is to plan ahead and havereplacement tires lined up and ready to install before the old ones are totallyworn out. Tubes should be replaced at the same time as the tires, on tube types.

 Old tubes deteriorate and are prone to cracking, which can lead to suddenfailure, so a new tube should be installed whenever the tire is replaced. Makesure the tube (if it is used) is the right size and is compatible with radialsif need be. Rim strips should also be replaced if they look deteriorated.