The majority of bicycle wheels you are likely to encounter are comprised of four parts:
- The tire.
- The inner tube.
- The wheel itself, which includes the hub, spokes and rim. For the purposes of flat repair, we will be talking primarily about the rim.
- The rim strip.
Let's look at each in more detail:
The tire is the firm rubber casing mounted on the rim of the bike. By itself, the tire is incapable of holding air. It is open in the center, and has two rigid and fairly sharp edges that allow it to be mounted onto the rim.
These edges are called "beads". On the majority of wheels, the beads are made of coils of wire encased in protective rubber.
As you ride, the tire rolls over road debris, and may pick up pieces of glass, nails and other sharp objects. Some of these items will penetrate the wall of the tire. Most of these punctures are tiny, and do little or no damage to the tire. The tire does need to be replaced if it develops a gash or tear, has dry rot or cracks along the side, or if there is evidence that the bead is separating from the main body of the tire, or protruding from the rubber casing.
The reason small tire punctures become problematic, is that most often they penetrate through to the inner tube, and cause it to lose air.
The inner tube is a soft, pliable rubber donut with an air valve on its underbelly that allows air in and out.
Note that a filled inner tube can appear larger than the tire. This is perfectly normal. When encased inside the tire, the tube will not exceed the tire's dimensions. When the tube develops a hole for any reason, it will lose air, and need to be patched or replaced.
There are many different types of bicycle rims. In addition to the important function of holding the tire onto the wheel, the rim also serves as the anchor for spoke heads, or nipples. On some rims, spoke nipples rest directly on the inner surface of the rim that comes in contact with the inner tube.
On other types of rims, an extra metal layer separates the spoke nipples from the inner-tube. This metal layer has holes drilled in it for access to the spoke nipples, as in the above image. In either case, the protruding spoke heads, or the edges of the holes drilled in the rim are sharp, and can cause damage to the inner-tube.
To prevent this from happening, the inside of the rim is lined with a rim strip. This can be a snug-fitting rubber strip, or pre-gummed cotton tape that is applied to the rim surface. To do its job, the rim strip has to go all the way around the rim , and cover all the spoke heads or holes (except the valve hole).
If it becomes, torn, worn, or folded over, it is no longer doing the job of protecting you from flats.
Please note that on many brand new bikes, cheap plastic rim strips are often used, which can themselves cause punctures and flats (click here for more).