A drop tower or big drop is a type of amusement ride, based around a central structure or tower. Drop towers vary in height, passenger capacity, lift type, and brake type. Many are custom made, although there are some mass-produced designs. Riders experience free-fall initially, followed by rapid heavy deceleration. With most drop towers, a gondola carrying riders is lifted to the top of a large vertical structure, then released to free-fall down the tower. Brakes activate to slow the gondola as it approaches the bottom of the ride. Some designs expand on this concept with features such as rotating gondalas, or several bounces before coming to rest. Most drop towers require child riders to exceed a minimum height; limits vary widely depending upon the nature of the tower, with a 30 feet (9.1m) tower for smaller children from 37 inches (95cm) tall, and a 115 feet (35m) tower having a 51-inch (130cm) restriction.