Caterpillar tracks are large (modular) tracks used on tanks, construction equipment and certain other off-road vehicles. Unlike the Kégresse tracks which use a flexible belt, most caterpillar tracks are made of a number of rigid units that are joined to each other. The tracks help the vehicle to distribute its weight more evenly over a larger surface area than wheels can. Tracks do this because as the tracked vehicle moves forward the segments are laid out flat on the ground at the front and are picked up again at the back. The segments inbetween the front and the back end carry load too as they are supported by rollers. This keeps it from sinking in areas where wheeled vehicles of the same weight would sink. From equilibrium, the ground pressure of a car must be equivalent to the pressure of the air in the tires. For most cars this is approximately 30 psi (207 kPa), whereas the seventy-ton M1 Abrams tank has a ground pressure of just over 15 psi (103 kPa).