Mountain bikes are designed to withstand a much higher degree of abuse than nearly any other bike on the road. From suspension systems and disc brakes to heavy-duty tires and components, they are built for rigorous use. However, none of these components would do much good were it not for the mountain bike frames to which they are attached.
A bike frame is the main support structure for every other aspect of the bike, including the rider. However, unlike a typical road bike frame, mountain bike frames are designed for strength rather than speed. They generally use much larger diameter tubing and are made from aluminum or steel. This does tend to make them heavier than a road bike frame, but they are much stronger and much more capable of dealing with the stresses of off-road use.
Mountain bike frames generally come in one of four different configurations; fully rigid, hard tail, soft tail, and full suspension. A fully rigid frame means that there is no suspension system at all and this means that any shock absorption is taken on by the tires and the rider. All mountain bikes were made in this fashion prior to the invention of the shock absorbing front fork. Many mountain bike riders still prefer this style of frame, and many manufacturers still have models made in this fashion.
Hard tail mountain bike frames became popular after the shock absorbing front shock was introduced. These frames are identical to a fully rigid frame in most respects, with the exception of the front fork which acts much like the front forks of a motorcycle. They became a popular frame design because the forks significantly reduce the amount of jolt and shock that is transferred from the wheels to the rider. By eliminating vibration and reducing the likelihood of a hard jolt, they reduce the strain on the rider’s hands, wrists and arms, making the ride much more comfortable.
Soft tail mountain bike frames are those that feature some form of a suspension system for the rear of the bicycle. Dual suspension frames are those that have shock absorbers on both the front and the back. It is important to note that while these two frame designs are technically separate, most soft tail frames also come with a shock absorbing front fork, making them identical to a dual suspension frame. In fact few, if any bikes are sold with only a rear suspension system.
Dual suspension mountain bike frames are arguably the most popular style for the majority of mountain bike enthusiasts. These frames are designed to be ridden very aggressively, and thanks to the front and rear suspensions can take a degree of abuse that other bikes simply cannot. They are especially popular with downhill racers because of their ability to absorb the heavy shocks inherent in this style of riding. However, due to the advanced nature of these frames, they tend to be extremely expensive when compared to the average fully rigid or hard tail design.