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The frame is the skeleton of the bike. In the diagram above the parts labeled in blue comprise the frame. The frame comes is a unit, and gives the bike much of its "feel" and riding characteristics. When someone says, "I ride a brand x bike," they are referring to the frame's manufacturer.

Mountain bike frames are divided into three broad categories based on the frames geometry (the length of the tubes, and the angle at which the tubes meet): cross-country bike, full-suspension bike, and down-hill bike. Deciding what type of frame fits your riding style is the first decision to be made before buying a bike.

Cross-country bikes, like the one shown in the diagram, are the most common, and probably the most versatile. If you're a first time buyer, then buying a cross-country bike is your best bet. The geometry of these bikes allow them to perform well under most conditions. They are the lightest type of mountain bike, which makes climbing easier, and gives the bike a more controlled feel. Most new bikes come equipped with a front suspension fork, which is a highly recommended option. Specific characteristics of the ride is discussed further in the section "frame geometry."
Full-suspension bikes, like cross-country bikes, are designed to work well in most riding conditions. However, full-suspension bikes are equipped with a shock absorber integrated in the frame for the rear wheel. Full-suspension bikes are generally heavier than cross-country bikes due to the extra parts, which makes climbing hills a bit harder. Additionally, some frame-designs flex during climbing which wastes energy, further increasing the difficulty of climbing. On the up side, full-suspension bikes offers a more compliant ride as both tires are more apt to remain in contact with the ground. Descending and improved handling on rough trails are both pluses for full-suspension bikes.
Downhill bikes are, as their name implies, designed to go downhill fast. Generally the bikes are full-suspended, and the frame geometry is laid out in such a way that the bike handles better at higher speeds while sacrificing low speed maneuverability. Since these bikes are not intended to spend much time climbing, not as much emphasis is placed on weight savings. If you're someone who likes taking ski lifts to the top of mountains and then zooming down the hill, this is the type of bike you're looking for.

Frame material is the next decision to be made when looking for a new bike. There are many materials used in bicycle construction. However, the most common are steel, aluminum, titanium, and carbon fiber. Each material has its strengths and weaknesses. Many consider titanium to be the "ultimate" frame material due to its compliant ride and light weight. However, everyone has their own opinion as to which material is best for them. Since this article is intended for newer bikers, only steel and aluminum are discussed in great detail, as they are the more affordable frame materials - and still great performers.

Steel frames have been around since the first bikes were produced. Historically steel is the most used of all frame materials, and considered by many to be the "best" frame material. Frames are generally made of either hi-ten or chromolly steel. Hi-ten is used for lower priced bikes and will not hold up well under extreme off-road conditions. Chromolly is a much stronger steel used on bikes starting from a few hundred dollars and going through a few thousand dollars. If you plan on riding mostly on streets and paved bike paths, hi-ten could be an option for you. However, if you plan on serious off-road riding, don't skimp, go with the chromolly.
Chromolly is strong, light, and makes a versatile frame material. For a time, when aluminum frames first came into fashion, chromolly frames were looked down on as inferior. However, today chromolly steel has overcome it's "second rate" perception, and is ridden by many as their frame material of choice. Chromolly is more malleable than aluminum, although significantly less so than carbon fiber, allowing for more varied tube shapes. Steel frames can be bent to allow additional frame clearance where needed. Due to its natural flex, steel also offers a softer ride than aluminum, although not as soft as titanium. Steel takes the edge off of bumps, while it remains rigid enough to not flex under heavy pedaling conditions.
Aluminum frames are stiff and light. Riders who want to feel the ground beneath them, including the bumps, will prefer aluminum to steel. The same increased stiffness allows for quicker sprinting due to the reduced flex. Aluminum is lighter than steel; however, it is also weaker and requires more material to reach an equivalent strength. Aluminum frames use a lighter material, but more of it, resulting in a frame that is roughly the same weight as steel.

When deciding which frame material to buy, it really becomes a choice of personal preference. There is not a "best" choice. Try riding steel and aluminum framed bikes to determine which you prefer. Some prefer the stiff ride and feel of an aluminum frame, while others prefer the more compliant ride of a steel frame (aluminum frame riders say that steel does not feel as "alive"). It's personal preference. Also, if you have the money, you may also want to try some of the more exotic materials and see how they compare.

Frame geometry determines how a bike rides and handles under varied conditions. As with most, if not all, aspects of biking, frame geometry is largely personal preference.

Each piece of a bike's frame gives the bike a subtly different handling characteristic - some less subtle than others. Each piece connects to others making it so that by changing the dimensions of a single piece, others are effected. Therefore the frame must be viewed as a whole comprised of the interactions of its parts.

The head tube is the shortest tube on the bike (excluding the bottom bracket), yet gives the bike much of it's handling characteristic. The steepness of the head tube angle - the measured angle between the ground (horizontal) and a straight line drawn directly through the tube from its top to its bottom - is a large determinant as to whether the bike will "turn on a dime," or be an admirable high-speed bike. Imagine a very steep angle, lets say 90 degrees, which would place the center of the tire more or less directly beneath the handlebars. At this angle the bike could make very sharp turns at slow speeds, it would do wonders on a slalom course. However, if the bike was moving with any speed when you turned the wheel there's a good chance that the bike would turn but you'd continue straight ahead on your own. Conversely, a small angle, often referred to as a "slacker" angle, would handle great at higher speeds, but not be very responsive at lower speeds.
When deciding on a frame be sure that the head tube angle matches your riding style. If you plan on flying down hills, go with a slacker angle. If you wish to do more technical (riding through, over, or around obstacles) opt for a steeper angle. The difference between steep (74 degrees) and slack (71 degrees) is fairly small and not easily seen. Riding the bike is the best means to judge its handling.
The top tube length plays a large part in how your bike fits you. A shorter tube provides for a more upright riding position. The body is more raised as the distance between the seat and handlebar is reduced. Persons riding in traffic where it's advisable to keep your head up, as well as persons with shorter torsos should lean toward short top tubes. Longer top tubes give a more "laid out" feel as the body is stretched further along the top of the bike. This position gives a lower center of gravity which can make off-road riding more enjoyable.
Although the top tube length plays a large part in determining riding position, other factors also come into play and should be considered. A shorter stem length can reduce the length between the seat and handlebar also giving a more upright ride. And a longer stem length can lengthen the effective distance. However, the placement of the rider's center of gravity will vary depending on the method used to increase the length. For example, a long stem and short top tube moves the rider's weight forward on the bike. Often while riding, especially down steep hills, is desirable to keep your weight as far back as possible. The shorter top tube will prevent this riding position.
The seat tube, much like the top tube, determines the position of a rider's center of gravity. A slack seat tube angle effectively increases the top tube length by placing the rider further back on the frame. A steep angle moves the rider forward. This positioning also effects the relationship between the seated rider and the pedals, although that can be fine tuned by moving the seat forward or back. Determining if you prefer a steeper or slacker seat tube angle is more difficult and probably less important than the previously mentioned measurements.
The chain stay length determines a bikes ability to easily climb. Shorter chain stays equate to easier climbs. However if they are too short they tend to catch mud between the tire and seat tube. Most modern bikes have seat stays that are as short as possible while while maintaining sufficient mud clearance.
The bottom bracket height is the distance from the ground to the tube through which the pedal cranks pass, and is the last major measurement on a bike frame. The height is a function of the down tube and seat tube. Lower heights provide for a more stable ride as the rider's center of gravity is lowered, especially when standing on the pedals. However, a bottom bracket that is too low tends to limit the bike's ability to pass over obstacles such as rocks, logs, and ledges. For extremely technical riding higher bottom brackets are more suited.