Modern tires, which lack inner tubes, are designed to form an airtight seal with the wheel.  The wheel is made of steel or magnesium and has rims on both sides which are somewhat broader than the inner diameter of the tire.

The thickness of the tire rubber makes “stretching” the tire over one of the outside wheel rims challenging.  It cannot be finished by hand.  A specialized machine with enough force is required to leverage the tire between the two outer rims of the wheel.  After that, the tire is inflated till it is sufficiently fat to create an airtight seal by coming into touch with the inside edges of the steel rims.

Finally, because of small differences in their construction, it is unlikely that the wheel and tire union will be precisely balanced.  Because tires rotate at very high speeds, they should be balanced within a few grams to minimize vibration that could otherwise affect the car’s occupants.

Balancing is achieved by fastening small lead weights to the wheel’s outside rim.  Determining where and how much weight to put to each wheel’s rim, however, is the difficult part.  The wheel/tire combination is spun by a special machine in auto shops before it is attached to the vehicle.

It establishes the quantity of lead required in grams and, more crucially, the location of the lead addition around the tire rim. Tire balancing increases tire longevity, relieves stress on suspension parts like shock absorbers and springs, and enhances passenger comfort.