Tires and Tubes
Tire Sizes
Tires are sized in 2 dimensions: diameter and width. A mountain bike tire might be described as 26 x 2.1, suggesting (incorrectly) that its diameter is 26 inches and the width is 2.1 inches.
Diameter
Tire diameter is measured to the tire beads (the steel or kevlar ring covered by rubber where the rim meets the tire). Rim diameter is measured to the top of the rim, where the tire bead would meet the rim.
Unfortunately, the terms we use to describe tire diameters are conflicting and confusing. First, there is the popular name for a diameter size, and then there is the actual diameter measurement in millimeters. For example, most road bikes have 700c tires, and the actual diameter measurement is 622 mm.
Common tire diameter sizes are:
for road bikes and 29er mountain bikes, 700c or 622 mm
for very small road bikes and some triathlon bikes, 650c or 584 mm
(Note: In 700c and 650c tires, the "c" does not stand for centimeter. I don't know anyone who knows what the "c" stands for.)
for old road bikes and old city bikes, 27", which is slightly smaller than 700c
for adult mountain bikes, 26" or 559 mm
(Note that 559 mm is nowhere close to 26"--it's closer to 22". The popular name of 26" refers to the approximate diameter between the outside of a tire.)
Over time there has been a regrettable lack or consistency in tire sizing. Old tires may be labeled 26" and may have actual diameters of 571, 590, or 597 mm.
To make sure you are buying tires that will fit your rim, study the sidewall of the old tire. Try to find a stamp with the diameter expressed in millimeters--622 or 559, etc.
Width
The width size reflects how skinny or fat the tire is. 700 x 20 is skinnier than 700 x 25. But width designations are not accurate. One brand's 25 may be as skinny as another brand's 20.
Whether the width is expressed as a decimal (.75) or a fraction (3/4) is important. A 26 x 1.75 tire is not the same diameter as a 26 x 1 3/4 tire.
The most important dimension is tire diameter. Any 700c tire will fit on a 700c rim. Any 26 x 1.00 to 2.6 tire will fit a modern 26" mountain bike rim. Note, however, that a 26 x 1 3/8 tire will not fit a modern 26" mountain bike rim.
Tire Features
Density Denser tires are more durable and more expensive. Density is usually expressed in threads per inch--TPI.
Softness Soft tires have better traction but do not last long.
Weight Lighter tires are more efficient. Light tires can still be dense and durable.
Steel or kevlar beads Kevlar beaded tires are "foldable." They are usually lighter, and they are more expensive.
Kevlar shield Tires with a thin layer of kevlar embedded under the surface resist punctures and can be light. They are more expensive.
PSI rating The higher a tire's maximum air pressure the better. PSI rating relates to density. Skinnier tires must have a high PSI rating.
Tread patterns There are many tread patterns designed for various uses.
Tube Sizes
Since tubes are very flexible, precise sizing is not as important as with tires. A 26 x 1.25 tube will work in a 26 x 2.1 tire--its walls will just be thinner because it is more inflated than a fatter tube.
The same tube will fit 700c and 27" tires.
The same tube will fit 26 x 1.5 and 26 x 1 3/8 tires.
Tube Valves
Bikes may have Schrader valves or Presta valves.
Schrader valves (aka American valves) are the same type on your car. The valve is opened by pressing on a little rod inside the valve body, and a spring automatically closes the valve when the pump head is removed. The valve can be removed. Infrequently, the valve will leak, and the leak can be fixed by tightening the valve (tighten clockwise). The diameter of a Schrader valve is larger than a Presta valve; therefore, the valve hole in the rim has to be larger.
Presta valves (aka French valves) are found only on bike tubes. The valve has to be screwed open (counterclockwise) before air can be added. Likewise, it has to be screwed back down, or the tube will lose air. The rim valve hole for a Presta valve is smaller than for a Schrader valve. A Presta valve will go in a Schrader rim hole, but not vice versa.
Which valve is better? Presta valves are considered more stylish by bike snobs, but they do not have a practical advantage.
Tube Features
Long Presta valve Long Presta valves are necessary for deep V road rims.
Super light Very light tubes reduce very important rotating weight, but they are less durable.
Thorn resistant Thorn resistant tubes are thicker on the outside to resist punctures. They are very heavy and cost a little more.
Sealant tubes Tubes with a sealant, like Slime, are flat resistant, and are more costly than thorn resistant tubes.
Jumping and downhill tubes Jumping and downhill tubes are thicker all round to resist pinch flats.