On-Line Tire Retailers & Manufacturers
Tires are the single point at which your bicycle makes solid contact with your environment, and, as such, have special importance. Tires are among the most complex components of your bike, and the experts are just beginning to understand why tires behave the way they do. And, as always, there is total disagreement among riders as to the best performing tires.
The Dynamic Duo
Of course, the wheel dictates the diameter (and to some extent the width) of the tire, and together, the two components have a dominant effect on the comfort, safety and speed of your ride. Among the factors dictating the performance characteristics of your tires are: materials, diameter, width, tread, thickness, bead, reinforcement and air pressure.
Materials
Tires are constructed of fabric coated with rubber-like materials. Sometimes this material is actually butyl rubber, sometimes it is a mixed compound, sometimes different areas of the tire are made with different compounds. The object is to improve weight/speed performance, traction/handling, puncture-resistance or durability of the tire. Obviously, some different features (such as lighter weight for speed vs. heavier weight for puncture protection) are a tradeoff in terms of performance characteristics. It is not always clear which materials are best for which purposes, but it is true that, in general, new materials have improved tire performance across the board in recent years.
Diameter
The air-filled rubber tube/tire and the spoke-supported rim are both flexible in different ways, and provide a dual cushion against road shock on your ride. The greater the diameter, the more flex of the tire and rim and the more cushion. Smaller diameter wheels/tires will have a rougher ride.
Sizing Tires to Rims  There are two key dimensions in tire sizing: width and diameter. Tires come in many different widths and diameters, and these dimensions may be measured in either inch based or millimeter based increments. For example, there are 26" tires and 700mm tires. However, these measurements are usually inaccurate - different tires labelled 26" may be greater or less than 26" and all will not fit on the same 26" rim; the 700 is actually a measurement of 622mm from bead to bead.
     The ERTRO system has been developed by ISO to try to eliminate this confusion. This system uses two numbers; the first is the width of the tire or rim in millimeters. The actual tire width will vary a bit depending on the width of the rim; the rim width is the inner width measured between the flanges.
      The second ERTRO number is the critical one - it is the diameter of the bead seat of the rim in mm (BSD). Generally, if this number matches, the tire will fit onto the rim; if it doesn't match, the tire won't fit.
      For example, a 700 x 20 C road tire would be a 20-622; a 700 x 38 hybrid tire would be a 38-622. The width difference between these sizes would make them less than ideal replacements for one another, but since the diameters are the same, any rim that could fit one of them would work after a fashion with the other.
     Most tires have both the old system of identification and the new ERTRO system embossed on the side. Whenever possible, to avoid confusion and product returns, try to order tires based on the ERTRO system. Here is more on tire sizing.
Width
Contrary to what seems common sense, wide tires do not necessarily have a greater rolling resistance than thin tires, and practical tests bear this out. The latest science shows that speed is related to the thickness and flexibility of the tire's sidewalls - the thinner and more flexible, the less energy is wasted in forcing them to bend and more is left to increase speed. However, many wider tires are also made with thicker sidewalls, so your chances of getting thick sidewalls increase when you buy wider tires unless you know what you're looking for.
      All this aside, racers universally prefer thin tires (usually about 1") for smooth pavement - perhaps because of their lighter weight. Tourists and commuters, on the other hand, need wider tires; their greater footprint provides better traction and stability on less than perfect pavement, gravel or dirt. Tourists tend to prefer tires at least 1.5" wide, but some use tires as wide as 3". Wider tires call for lower pressure, generally in the 70 psi range; thinner tires call for higher pressure, as much as 120 psi.
Tread
Tread is important on surfaces where the surface is not firm, such as dirt, gravel and snow. On these surfaces, thicker tread provides greater traction, since the surface material conforms to the shape of the tread, filling the spaces. This is why mountain bikes always come with thick, knobby tires. On smooth, firm surfaces, a slicker surface gives greater stability, since there is more tire surface in contact with the road at all times (no unfilled space between rubber treads). Automobile tires are treaded to divert water and avoid hydroplaning at high speeds; bicycles are not subject to hydroplaning, so tread is not needed for this purpose.
Thickness
The thicker the tire at the bottom, the better the puncture resistance and longer the life of the tire, but this will also add weight and reduce speed. Often, tires are made thicker to add tread.
Bead
The bead is the thicker, cordlike strip around the very edge of the tire. This bead slips into a slot in the rim and, under air pressure, holds the tire in place. This kind of tire is called a "clincher," and almost all bicycle tires are of this type. The bead may be steel under the rubber-like coating, in which case it is stiff and fits on the rim a bit more easily; it may also be made of Kevlar, which is flexible and allows the tire to be folded for more convenient storage in a pack.
Marathon PlusReinforcement
Reinforcement can add strength and puncture protection and extend the tire's life. It can also increase resistance and reduce speed if it adds significant extra weight. Often reinforcement is with light weight Kevlar belts built into the body of the tire, and some new tires like the Schwalbe Marathon Plus or Marathon Supreme have belts of other materials like hybrid rubbers or ceramic. These are the new breed of "flatless" tires, so named not because punctures are impossible, but because they are very rare, even under extremely harsh conditions.
Pressure
A bicycle's performance characteristics can be radically changed by varying the pressure in the tires. Running tires at the maximum recommended pressure usually gives greater speed and a harder ride. Lowering the pressure to the middle of the recommended range can provide a softer but generally slower ride. Greatly reducing the pressure (or allowing it to fall by neglecting to fill the tires) can result in mushy steering, sliding and generally unsafe handling. Underinflated tires are also subject to pinch flats. Experienced cyclists know their bikes and their tires and adjust the pressure to give the kind of ride they want at any given time or for any given surface or load conditions. Eventually you will have a feel for tire pressure on your own bike and be able to do the same. Until then, it is wise to keep the pressure within the recommended range at all times.
What Kind of Riding Will You Do?
The key to sorting through all these options is in knowing what kind of riding you'll be doing and on what kinds of surfaces. Tires can be evaluated in terms of four characteristics: speed, grip, protection (from punctures) and durability (tire life). Read the manufacturers' specs and descriptions for the tire, then choose the features of your tire to produce the performance characteristics best suited to your riding needs.
     Here are a few of the many sites which can give you greater insight on tire choice.

On-Line Tire Retailers
Air Bomb
Bike Tires Direct
Bicycle Tires
Every Bicycle Tire
Tire Information Sites
Sheldon Brown's Bike Pages
Sheldon Brown's Bike Pages:      Sizing
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On-Line Biking Forums
On-Line Biking Blogs
On-Line Touring Site Links
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On-Line Commuting Site Links
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