1 - Broken bead:
The part of the tire that sits in the rim and gives the tire it's shape is called the bead. In most cases the bead is made of stranded wire. Once in a while, usually in an older tire,
one of the strands can break and poke through the rubber surrounding it. If the flat occurred on the side of then tube, check the bead in that area. Also make sure that the rubber of the tire fully surrounds
the bead on both sides of the tire.
2 - High Pressure leak:
Basically, a high pressure leak is a hole in the tube that is so small that air does not escape through it until there is a lot of pressure in the tire. A telltale sign of this kind of
leak is a tire that loses a lot of air in a short amount of time such as overnight. These kinds of holes are hard, if not impossible to find. If you checked your tube under water and still did not find the hole,
your only recourse is to put in a new tube.
3 - Loose spoke:
If the hole in your tube is on the inside surface and you've already checked for rough surfaces on the rim and spokes poking through, check for a loose spoke. Sometimes a loose spoke can 'float'
around in the rim and can puncture the tube. If you find a loose spoke, tighten it in a counterclockwise direction (while looking at the side of the nipple) with your fingers just enough so it doesn't move
around in the rim. If you tighten it too much you could mess up the alignment of the rim. You should also consider bringing your wheel in to a bike shop to have that spoke tightened properly to avoid this
problem in the future.
4 - Low air pressure:
If you ride your bike with low air pressure in your tires, each time you start and stop, the tire slides on the rim a minute amount. This can cause friction between the tube and the tire
or rim gradually wearing a hole in the tube. Telltale signs of this problem are abrasion marks around the hole in the tube. If you ride often, you should check the air pressure in your tires at least once every
week or two. If you only ride occasionally you should check it each time you ride.
5 - Debris in the tire:
If you see the above mentioned abrasion marks and low air pressure is not your problem, you might have bits of debris floating around inside your tire. This debris can be small particles
of sand, dirt or in older tires, pieces of rubber that have broken off from the tire. Hold the tire upright and shake it gently a few times. If you find debris building up on the bottom inside of the tire, this
could be your problem. If this is the case, shake the tire roughly to remove the debris and using a slightly damp rag, wipe out the inside of the tire. Make sure the tire dries before putting it back on the wheel.
6 - The base of the air valve:
Twist the air valve around in circles watching for any cracks or listening for that hiss of leaking air at it's base. If you find a crack it could mean one of two things. One,
you're not riding with enough air pressure in your tire which can cause the valve to rock back and forth slightly each time you start and stop. Over time the metal of the rim around the valve hole can wear
through the base of the valve. The solution to this problem is easy, check your air pressure often. Two, if you have a valve that tightens to the rim with a nut or splined fastener, each time you start and
stop, since the valve is anchored to the rim, the tube is stressed around the base of the valve. The solution to this problem is to toss the nut or fastener away or to get a second one and lock the two together
leaving a slight gap between them and the rim.