Rollback (roller coaster)

A rollback occurs on a launched roller coaster when the train is not launched fast enough to reach the top of the tower. It will roll backwards down the tower, and will be stopped by brakes on the launch track. Any roller coaster on which it is possible for a rollback to occur will have these brakes. Intamin, a manufacturer of roller-coasters, refers to the "rollback" as a "short shot".

Most coasters contain at least one anti-rollback device to prevent a train from rolling backwards while ascending the main lift. This is typically with chain-driven lifts, not hydraulic launchers such as Kingda Ka or Top Thrill.

Rollbacks are most common (though still quite rare) on the world's largest launched roller coasters, Kingda Ka at Six Flags Great Adventure, Top Thrill Dragster at Cedar Point, along with somewhat smaller rides such as Stealth at Thorpe Park and launched roller coasters of the same type, such as Xcelerator at Knott's Berry Farm and Storm Runner at Hersheypark.

There are several factors that can cause a rollback, all of which are related to the train's speed:

While the general public may not realize that rollbacks usually are completely safe and that coasters are designed with them in mind, many coaster enthusiasts look forward to being in one. Being in a rollback essentially gives riders a ride and a half, as the train will be launched again after the rollback. On some rides, when a rollback occurs with people on board, the train will first be brought back to the station to give the guests the option of getting off; other roller coasters with dual-train dispatch systems disallow backwards returns to the station and the train will simply be launched a second time without the option to disembark. Giving guests the option to get off the ride (to possibly return at a later time) is less of an incentive in parks that charge an overall admission fee and don't charge for individual rides.

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Wikipedia