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Dirt jumping is one of the names given to the practice of riding bikes over cement type jumps of dirt or soil and becoming airborne. The idea is that after riding over the 'take off' the rider will become momentarily airborne, and aim to land on the 'landing'. Dirt jumping can be done on almost any vehicle with wheels, but it is usually executed on a dirt jump bike. * BMX bike built for dirt jumping tends to have a longer top tube than a street BMX bike, and may well be more reinforced. They will rarely have pegs fitted, and will generally run only a rear v-brake. Also, the tires will be treaded, as opposed to the slicks and semi-slicks used for park riding. Large, padded seats are also popular for a smoother landing if the jump is not done properly and are also easier to hold for in-flight tricks; However some riders do not find seat size an issue. The gear ratio is generally around 44:16, 36:13, 33:12, 30:11 or 25:9 though using small gearings such as 25:9, known as 'micro gearing', has become popular. * Hybrid BMX/Jump bike - a scaled up BMX with 24" wheels. Strong alloy rims and lightweight chromoly frame. Suited to bigger jumps or more challenging competition courses. * Freestyle Motocross (FMX, Moto-X) of various sizes & engine sizes. * Dirt Jump/Freestyle mountain bikes look similar to mountain bikes but usually have a rigid frame and, a lower stand-over height, this keeps the seat out of the way while performing tricks. Some high end DJ frames also have short travel rear suspension, sometimes with bottom-bracket-concentric pivots to allow singlespeed use. The wheels are usually more robust than a cross country mountain bike's and the same for the frame. * Mountain bikes - 24 or 26 inch wheels and either rigid forks or forks with short front end suspension (usually 80–100 mm travel, but can be up to 203 mm depending on the type of the bicycle) Firm suspension is desirable for dirt jumping. A mountain bike built for dirt jumping tends to have a smaller frame than what is used for other disciplines. Running singlespeed with one brake is very common. using single or dual disc brakes has replaced the use of only one rear V-brake. In general, a mountain bike dedicated to dirt jumping will have 24" or 26" wheels, a gear ratio of approximately 60 gear inches (~36:15 on a 24" rear wheel or ~36:16 on a 26" rear wheel) and rigid or 80-100mm travel forks. An 'all-round' bike used for dirt jumping will more likely have 26" wheels, a 25-36 tooth chainring with a wide-ratio cassette and a short- to mid-travel fork. Mountain bike dirt jumpers are usually split on the basis of wheel size because the wheel size dictates the shape of the takeoff to an extent. Dirt Jump bikes are made by a variety of companies. A 20 inch BMX bike for dirt generally has a 48 spoke rear rim and a 36 spoke front to prevent rim collapse in the case of casing a jump. the frame is a little bit longer to aid in more stability and to spread the load of heavy lands. most of these "micro geared" bike run 85 to 100 psi tires. these are usually threaded and made with kevlar bead to prevent pressure flats and tears. ==History== Dirt Jumping evolved alongside BMX racing from its birth. Dirt Jumping is similar to BMX or Mountain bike racing in that the rider jumps off of mounds of dirt. It differs in that the jumps are usually much larger and designed to lift the rider higher into the air. Additionally, the goal is not to complete the course with the fastest time, but rather to perform the best tricks with the best style. Trails are most often hand built, with attention to detail. Dirt jumping has evolved to crash now, there are now competitions that you can compete in for prizes. The dirt jumps have become much larger and better built. They now use diggers and other machines to help build and compact the dirt quicker and more efficiently. The dirt jumps which are sometimes even built with wood can even be up to between 20 and 30 feet long for the professionals. The tricks have evolved a lot partly due to freestyle motocross evolving where a lot of the dirt jumpers get their inspiration from. When it first started it would be just tabletops and whips as the main tricks, now nearly every professional can back flip, front flip and even double back flip. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dirt jumping」の詳細全文を読む スポンサード リンク
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