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.
* Dirt Jump/Freestyle mountain bikes look similar to mountain bikes but have a rigid frame and, a lower stand-over height, this keeps the seat out of the way while performing tricks. The wheels are usually more robust than a cross country mountain bikes and the same for the frame. Frames are built for a balance of strength and lightweight.
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. Examples of commonly purchased bikes for the general purpose of dirt jumping include the Kona Shonky or Cowan, (Kona also have a line of trail oriented bikes that are also suited for dirt jumping, the Shred and Stuff) Specialized P.1, P.2, P.3, Norco 125, 250, Havoc or Ryde, Transition's Trail-or-Park, Bank or Double, Giant Brass or STP, Devinci District, Haro Thread and Porter, Blk Mrkt Mob, Riot, Contraband, Killswitch, Soldier or Malice, NS Majesty, Suburban or Capital, Dobermann Pinscher or Molosse, Trek Ticket Signature, Eastern Nightrain, Thunderbird, and Mad Dog, the Banshee Amp or Rampant, and khs DJ and sj series, khs, a company based out of California, lesser known but makes a great selection of bikes. 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.