Dirt Jump or Freestyle bikes are similar in appearance to mountain bikes but have a lower stand-over height, which keeps the seat out of the way whiletricks are being performed. The wheels are usually more robust than those of cross country mountain bikes, as is the frame. Frames are built for a balance of strength and light weight. A mountain bike built for dirt jumping tends to have a smaller frame than for the other disciplines. It is common to run single speed and to use only a rear break. Nowdays, single or dual disk brakes have replaced the use of only one rear V-break.