Wheels consist of the following components:
* The hub, containing an axle, the quick-release inside the axle, and two or more bearings holding the rotating hub body with the flanges that hold the spokes. The quick-release is an ``inner axle'' with a nut on one end and a lever on the other, and two conic springs that simplify installation. The small ends of the springs point to the center of the hub. The nut should be tightened such that closing the lever is hard but can be done with one hand (how is that for an imprecise unit of measurement :-) A loose quick release can cause the rear axle to shift in the dropouts when accelerating hard, causing the tire to rub, or in the worst case can work itself even looser until you are in danger of losing the entire wheel.
There are two kinds of hubs, obsolete freewheel ones where the sprocket assembly contains the coasting bearings and ratchets that screws onto the hub itself, and cassette hubs where the bearings and ratchets are built into the hub and the sprockets are fixed. Do not under any circumstances use the former type; they contain a bearing that runs near the center of the hub that can bend the axle. The latter type has bearings on both ends where they can't bend the axle. A bent axle can bend or break the right dropout of the frame, and you'll have to write off the entire frame! It happened to me twice.
* There are usually 36 spokes. Some people prefer 32 for reasons that are not clear to me, or even 28. Tandems can benefit from 48 spokes. Staying with 36 spokes offers the widest selection of hubs and rims. There are double-butted spokes that are narrower in the middle where there is less load, bladed spokes, wavy spokes, and other exotica. Double-butted spokes make sense (spokes always break at the neck or where the threading begins), the rest is fairly useless. Bladed and wavy spokes are also more difficult to install because they twist and make it hard to gauge tension. Spoke thickness is usually measured in ``gauge''; smaller numbers indicate larger diameter.
Spoke length is important, but computing the right length is a black art. Generally, all spokes of the front wheel have the same length. Sprocket-side (right) rear spokes are shorter, outer (left) rear spokes are slightly longer because the right rear hub flange is close to the center of the hub. When buying replacements bring an old spoke. Up to a millimeter too short doesn't hurt, but spokes that are too long can puncture the inner tube.
Wheels can be built radial, 3-cross, and 4-cross. Radial means that spokes run straight to the rim without crossing; 3-cross and 4-cross refer to the number of times a spoke crosses other spokes. 3-cross is standard. There is no reason to use radial wheels except aesthetic reasons; it reduces the strength of the wheel and makes the ride rougher. Obviously, each of these spoke patterns requires a different spoke length. By the way, always use steel spokes and brass nipples.
* There are many types of rims: rectangular profiles (like Mavic MA-2 or Campagnolo Delta), aero profiles (like Campagnolo Omega), and anything in between. High-quality rims are machined to avoid the normal seam where the rim joins; this seam can catch the brake pads and cause uneven wear and braking behavior. Don't buy black-anodized rims; the anodization will be quickly worn away where the brake pads rub against it.
* I swear by Campagnolo rims. Mavic is ok, but look at their high-end double-butted rims - the holes are apparently punched, not drilled, and if you hold them just right you can see small discolorations where the metal was deformed. This weakens the rim, and just now (9/99) two spoke sockets were torn from my rim at such points, leaving huge holes. Campagnolo rims are drilled and finished perfectly. My current favorite is the Moskva 80, but they no longer make it.
* Inside the rim, rim tape protects the inner tube from the spoke ends. There are two types, adhesive textile tape and tough hard plastic. Both work equally well but the hard plastic is difficult to remove without tools while the textile tape may expose the inner tube to spoke holes if chosen too narrow. Reinstalling the hard plastic type can lead to mysterious flats because the tape lenghtens during removal, and when the tire is reinflated the tape may get pushed aside or into the spoke holes until the tube touches sharp metal edges of the rim. When you take it apart to find the reason for the puncture, it shifts back into place, hiding the problem. Use adhesive textile tape if possible.
Aero rims are stiffer but are more difficult to install because spoke nipples have a tendency to get lost. Always use aluminum rims, steel rims brake dangerously poorly in wet weather despite brake pad manufacturers' claims. Make sure the rim does not just have spoke holes drilled into it but some kind of steel sleeve that protects the hole. Absolutely never buy an aluminum rim if there are discolorations on the outside around each spoke hole when held against the light. Discolorations indicate that the holes were punched rather than drilled; rims like that can fail abruptly when the hole cracks and spokes pull out. I have destroyed several Mavic MA2 rims that way when touring with bags.
* Inner tubes come in butyl and latex, in various weights. 100-gram butyl is best. 60-gram butyl is too vulnerable to snakebite punctures, and latex leaks air and can rip off the valve if the tire slips for any reason. Choose a valve stem that is threaded all the way to the tip without any smooth parts because pumps tend to hold poorly to smooth valve stems.
There are two types of valves, Presta (French) or Schrader (like those used on cars). Presta valves are narrower and are always used for racing bicycles; Schrader valves are found on mountainbikes. Presta is easier to pump because one doesn't have to work against a spring. Don't forget to unscrew the nipple before pumping. The maximum pressure allowed depends on the tire; it is printed on the side of the tire. One bar is approximately 14 psi; road tires usually hold 8-9 bar. Don't underinflate because you are more vulnerable to snakebite punctures; don't overinflate because the tire can pull off the rim and cause nasty crashes if the loose tire edge gets caught in the brake.
Don't buy a pump that inflates the tire with both in and out motions. You need to wait for the builtin valves to restore air pressure in the pump at both ends of the stroke, which takes several seconds so it takes forever to inflate the tire. It sounds like a good idea but you'll hate it after three seconds. The best frame pumps ever built are the aluminum Zefal X3 and X4 pumps; they have a cap that twists to lock the internal spring so that all the force put into it directly translates to tire pressure. Brilliant. Unfortunately few stores carry them.
* Tire liners are Kevlar belts that are inserted between the inner tube and the tire to prevent punctures by sharp objects. More on that below.
* Tires for road bikes should always be bald (smooth, untreaded). It may be counter-intuitive but they have higher traction. Fatter tires make for a smoother ride on rough roads or when touring loaded; narrow tires deform less and waste less energy. I prefer 20mm front and 23mm rear tires (but watch out, different manufacturers seem to rate their tires differently).
There are Kevlar-beaded (foldable) and Kevlar-belted (puncture-proof) tires. Puncture-proof tires aren't, because the belt is too narrow. On the contrary, it seems to be hard to laminate smooth Kevlar to the outer rubber layer, I have had tires come apart. My latest trip to France (2001) saw two brand-new Vittoria tires with Kevlar belts come apart after a few days, which first pulled the tire into an S curve and then developed a fuzzy hole. I do not buy Vittoria tires anymore, and some shops have stopped selling them.
Make sure the side wall is strong enough. My favorites are foldable non-belted Avocet road (not Criterium!) tires; they last practically forever and still have a rubber that is soft enough to afford traction on wet roads. Recommended. Too bad that they only make the wide versions anymore. These days I mostly use Continental tires; a little heavy but very robust.
See also an article on "Preventive Punctures"
