(MONEY Magazine) – Attention, aging high school jocks with dreams of glory, ex-joggers with blighted knees, teenage racers with Olympic ambitions and pedal-pushers out for a sedentary spin. The bicycle for you is out there. In fact, for anyone at all who is convinced that two wheels can be better than four, now may be the best time to buy. Worried about physical damage to their joints from high-impact fitness activities like aerobics and running, Americans are expected to spend $1.9 billion on 12.3 million bicycles this year. That's more than in any year since 1974. Manufacturers are responding by offering two-wheelers with designs and components that were virtually unheard of even a few years ago: all-terrain bikes, lightweight frames and user-friendly derailleurs (the mechanism that is used to shift gears). With a minimum of knowledge, $300 to $600 or so will get you a durable, sprightly, high-quality machine. Value levels off quickly as prices increase beyond that point. In the higher price ranges above $1,000 and ascending easily to $2,500, you are paying more for beauty, prestige, hand labor and exotic materials than you are for pure function. Nearly all 10-speed bicycles are an assemblage of parts from different manufacturers marketed under one brand name; the frame may be American, the wheels French, the seat and handlebars Italian, and the brakes, pedals and derailleurs Japanese. Experienced cyclists routinely replace stock parts on their frames, or even choose parts separately and build their own. Following their example, you should begin your quest by scrutinizing parts: -- Wheels (a.k.a. rims) should be aluminum rather than the heavier steel found in cheaper models. Each wheel should weigh in at 500 grams (about one pound) or less. Aside from their light weight, which makes for a more comfortable ride, aluminum wheels also tend to have superior braking power in wet weather. Top brands to look for include Rigida, Ambrosia and Mavic. -- Frames -- the top, seat and down tubes (see the box below) -- should be made of butted chrome molybdenum steel rather than weaker carbon steel. A butted tube's inside walls are thicker at the ends than in the middle. This makes the bike stronger where the tubes meet. There they should be held , together by lugs, sleeves that connect the tubes sturdily, rather than being welded directly to each other. Frames made with tubing by Columbus, Ishiwata, Reynolds, Tange, True Temper and Vitus are all excellent choices. While 95% of all bike frames are made of steel, aluminum is coming on fast. Because aluminum frames are more rigid than those of steel, particularly if they are constructed of oversize tubing, they are well suited to heavier riders of 200 pounds or more. Aluminum's light weight also has made it a popular material for all-terrain bikes. -- Derailleurs determine the ease with which you can change gears. Three companies, Campagnolo, Shimano and Sun Tour, make the derailleurs for most U.S. bikes priced at more than $300. One new feature you might want to look for: indexed shifting, which was popularized by Shimano in 1984, though all three manufacturers now have a version. Recommended for riders who are cowed by multispeed bikes, indexed shifting allows each successive gear to slip into place cleanly and quickly, accompanied by a reassuringly audible click. Where should you buy your bike? Two-thirds of them are sold in mass-market outlets like Sears, K Mart or Toys R Us for an average of $100. The 7,000 independent bicycle shops in the U.S. carry higher-quality machines, and boast the most knowledgeable salespeople and the widest choice of models. You need to be in good hands to get the proper fit. A poor one can painfully overstretch your ligaments and Achilles tendons. Moreover, says Doug Roosa, technical editor of Bicycle Guide magazine, ''a well-fitting $400 bike will always give you better performance than an ill-fitting $1,500 bike.'' An experienced salesperson should have no trouble matching you to the right size bike. To check it out yourself, straddle the top tube: there should be an inch clearance between it and your crotch. Generally, a five-foot-10-inch man with a 32-inch inseam would take a 22- or 23-inch frame. Women might consider bikes specifically designed for them, such as the ones made by Terry Precision Bicycles. Now to the real task at hand: choosing the exact bike for you. To do so, you need to know the assets and liabilities of the four basic types of bike -- racing, touring, general sport and all-terrain use. (Whatever you choose, don't leave the shop without spending an additional $50 or so on a hardshell helmet. It should sport a sticker indicating approval by the American National Standards Institute or the Snell Memorial Foundation.) ) RACERS. These are the glamorous Formula Ones of the two-wheeled set. Their mission is speed, precise handling and absolute reliability. Although they claim the smallest share of the market at only 10%, they provide the design and engineering basis for touring and sport bikes. (All-terrains are in a class by themselves.) Racing bikes are not only for racers; they are appropriate for fitness buffs and avid cyclists who want to move up to a lighter, speedier model. Don't consider a racer unless you are in fine physical shape or are determined to get there. While a racer's dropped handlebars enable the rider to get into an aerodynamically advantageous crouch, this can be tough on your back, forearms, neck and shoulders. Narrow tires an inch or less in diameter cut surface resistance but can make the ride bumpier. And the frame geometry -- the angle at which the frame's tubes are joined -- allows the bike to respond quickly to steering changes but requires concentration for instant responses. Although still referred to as a ''10 speed,'' the standard racing bike now has 12 speeds, or gears. A rider chooses gears on a bike by shifting levers, located on the bike's down tube, which operate the derailleurs, the bike's equivalent of a car's transmission. The smaller front derailleur moves the chain between the two large chain rings near the pedals. The rear derailleur moves the chain among six smaller sprockets called freewheels, located on the rear hub. This makes for 12 possible gearings. Shifting gears changes the force necessary to turn the pedals and thus serves the purpose of allowing a rider to keep pedaling at a steady rate uphill, downhill or on flat terrain. Low gears require the least amount of effort but move the bike the smallest distance for each pedal stroke. High gears are the most difficult to push but take the rider the farthest on each revolution. Each gear is measured in inches. On a racing bike the low gear is relatively high at around 50 inches. The high gear is usually 110. This is a smaller range of gears than on other bikes, whose low gears are lower. On a racer, gears are placed close together so a rider can shift them quickly in the heat of competition. Racers are also the lightest bikes. Some weigh as little as 14 pounds, though most come in at between 18 and 21 pounds. Lightness costs. One rule of thumb: all other things being equal, every pound a bike weighs less than 21 or 22 pounds adds $100 to its cost. ''It's easier for most people to shed three pounds than it is to come up with another $300,'' notes Mark Allen, five-time world triathlete champion. Excellent entry-level racers can be found for $500 to $600. They include Cannondale SR500 ($600), Centurion Ironman Expert ($549) and Raleigh Tri-Lite Technium ($450). Top-of-the-line, ultralight racers, on the other hand, contain the most expensive (and theoretically the most dependable) components and have partially or even totally handmade frames, which last longer. Italian racing bikes have dominated this upper end of the market since the late '70s. Cognoscenti salivate at names like Cinelli ($2,300) or De Rosa Professional ($2,500), built with Columbus tubing and equipped with Campagnolo components. But American maufacturers produce fine high-quality lightweight racers for around $1,000 less, such as the Schwinn Paramount ($1,650) and the Trek 2000 ($1,429). TOURING. A touring bike is built to carry you and your packed saddlebags over long distances in relative comfort. It accounts for about 20% of the market. To do its job, a touring bike has a lower range of gears -- 25 to 30 inches at the low end -- than does a racing bike. This helps bikers pedaling full loads to climb hills easily. High gear is normally 100 or under. To achieve the low gears, touring bikes often come with a triple chain ring on the crankset plus five or six chain positions on the rear hub of the bicycle, yielding 15 or 18 speeds. Touring tires are wider than on racing bikes -- 1 1/4 inches -- for greater comfort, while a lengthier wheelbase and more weight (25 to 30 pounds) contribute to a smoother ride. And the drop of the handlebars is not as severe, allowing the rider to sit in a more upright, comfortable position. Two reliable and moderately priced tourers are the Schwinn Voyageur ($425) and the Fuji Saratoga ($489).

SPORT. In gearing, weight and comfort, a sport bike -- sometimes called a sport-touring or sport-triathlon -- is a popular compromise between the touring and racing categories. Sport bikes, which account for 35% of the market, also boast the widest range of models. For these reasons they are often recommended for the casual biker. They have relatively low gearing, starting at about 40 inches. They are easier to ride than a racer but don't have the extra-low gears a tourist needs. They also have racerlike dropped handlebars. Less expensive but eminently serviceable sport models, like the Panasonic Sport DX ($259), weigh in at a hefty 25 pounds, while more < performance-oriented machines in this category, like the Lotus Elite 600 ($650), weigh a trimmer 23 pounds. Other reliable models: Nishiki's Olympic 12 ($360), the Trek 400T Elance ($449) and the Miele Lupa ($524). ALL-TERRAIN BIKES: If sitting upright is important for your comfort, consider this category. All-terrain bikes, also known as mountain bikes or city bikes, are the fastest-growing type and currently claim 35% of the market, largely because they are versatile, durable and fun to ride. They can be raced in off-road competitions, loaded with bags for long tours or used simply to get around town. Their extra-wide knobby tires and higher bottom bracket -- the juncture of tubes at the pedals -- make for easy riding over rough territory, giving extra protection against bumpy trails as well as potholed streets. The upright position, plus brakes and gear shifters on the traditionally shaped handlebars, are comforting reminders of childhood bicycling for many purchasers. ''It's the yuppie bike,'' says Edward Schwinn, president of Schwinn bicycles. Happily, as ATBs have turned into hit sellers in recent years, attracting more makers, prices have plunged. You can buy roughly the same mid- range quality ATB for $300 now that would have cost $700 only four years ago. For the most part the more reasonably priced choices, such as the Specialized Streetstomper ($325) or the Univega Rover Sport ($330), are general-purpose ATBs meant to be ridden casually on the road as much as off. Higher-priced models, like Diamond Back Arrival ($860), Fisher Mountainbike Montare ($898) and Klein Elite Trail ($1,375), are more suited for competition with lightweight tubing and ultraresponsive handling. While competition ATBs can weigh in at as little as 21 pounds, most are nearer to 30 pounds and, like touring bikes, are distinguished by a wide range of gears for climbing hilly terrain.

BOX: BIKING BASICS

Of the 140 or so parts on a typical bicycle, most buyers need concern themselves with only six. If these pass muster, you can be reasonably sure that everything else will too. The first three key parts belong to the frame, the bike's superstructure, and therefore should combine strength and lightness.

The seat tube (1) carries two essential pieces of information: the size designation of the bike and the material the frame is made of (a decal affixed to this tube should tell you whether it is made of molybdenum steel or aluminum, the two preferred metals).

The down tube (2) is where the gearshift levers are located on better bikes.

The top tube (3), along with the seat tube, determines your fit.

The rims (4) should be of aluminum alloy for light weight and superior braking.

The front derailleur (5) and rear derailleur (6), which make the gears shift, should also be made of light, rust-resistant aluminum.