- Vertical loop
The generic roller coaster vertical loop is the most basic of roller coaster inversions. Specifically, the loop refers to a continuously upward-sloping section of track that eventually results in a complete 360 degree circle. At the top-most piece of the loop, riders are completely inverted.
The vertical loop is not a recent roller coaster innovation. Its origins can be traced back to the 1850s when Centrifugal Railways were built in France and Great Britain. The rides relied on centripetal forces to hold the car in the loop. One early looping coaster was shut down after an accident. Later attempts to build a looping roller coaster were carried out during the late 19th century with the Flip Flap Railway at Sea Lion Park. The ride was designed with a completely circular loop (rather than the teardrop shape used by many modern looping roller coasters, see below), but caused neck injuries due to the intense G-forces pulled with the tight radius of the loop.
The next attempt at building a looping roller coaster was in 1901 when Edwin Prescott built the Loop-the-Loop at Coney Island. This ride used the modern teardrop-shaped loop and a steel structure, however more people wanted to watch the attraction, rather than ride. No more looping roller coasters were built until 1976 when The Shockwave opened at Six Flags Over Texas,and following shortly afer, the Revolution opened at Six Flags Magic Mountain. In 2000, a modern looping wooden roller coaster was built, the Son of Beast at Kings Island. Although the ride itself was made of wood, the loop was supported with steel structure. Due to maintenance issues however, the loop was removed at the end of the 2006 season. The loop was not the cause of the ride's issues, but was removed as a precautionary measure—thus, it is in fact the only successful installation of a loop on a modern wooden roller coaster.
Loops on non-rollercoasters
Designers of other rides have been tempted sometimes to include similar loops, although safety concerns usually lead them to quickly reconsider.
In 1980 at New Jersey's infamous Action Park, a loop was built into a covered water slide. Riders were warned to descend with their legs crossed and arms across the chest to traverse the loop safely. Too many ignored this instruction, however, and either got stuck atop the loop (requiring the construction of an access hatch to free them) or injured themselves, or both. It was closed after a month, and would open at somewhat random intervals until around 1995. It is not known if the ride was reported as being open during Action Park's last year of operation in 1996.
It has been dismantled since then and will probably not be rebuilt. As of 2006, no other looping water slides have opened at any parks. However, since 2002, the Swiss company Klarer Freizeitanlagen AG has been working on a safe design for a looping water slide and have now built and tested a 1:1 prototype of their design. Since then, multiple installations of the slide, named the AquaLoop, have appeared in many parks.
In actuality, most roller coaster loops are not circular in shape. A commonly used shape for loops is the clothoid loop, which resembles an inverted tear drop and allows for less intense G-forces throughout the element for the rider. The name comes from the incorporation of sections from the mathematical clothoid spiral curve in the shape of the loop.  The use of this shape was pioneered in 1975, on Revolution at Six Flags Magic Mountain, by Werner Stengel of leading coaster engineering firm Ing.-Büro Stengel GmbH.
On the way up, from the bottom to the top of the loop, gravity is in opposition to the direction of the cars and will slow the train. The train is slowest at the top of the loop. Once beyond the top, gravity helps to pull the cars down around the bend. If the loop's curvature is constant, the rider is subjected to the greatest force at the bottom. If the curvature of the track changes suddenly, as from level to a circular loop, the greatest force is imposed almost instantly (see jerk). Gradual changes in curvature, as in the clothoid, reduce the force maximum (permitting more speed) and allow the rider time to cope safely with the changing force.
Obviously this "gentling" runs somewhat contrary to the coaster's raison d'être. Schwarzkopf-designed roller coasters often feature near-circular loops (in case of Thriller even without any reduction of curvature between two almost perfectly circular loops) resulting in intense rides—a trademark for the designer.
It is rare for a roller coaster to stall in a vertical loop, although this has happened before. Turbine (then known as Sirocco) at Walibi Belgium once stranded riders upside-down for several hours. The design of the trains and the rider restraint system (in this case, a simple lap bar) prevented any injuries from occurring, and the riders were removed with the use of a cherry picker.
- ^ Cartmell, Robert (1987). The Incredible Scream Machine: A History of the Roller Coaster. Popular Press. pp. 156. ISBN 0879723424. http://books.google.com/books?id=Qtyoe7tlkVcC&lpg=PP1&pg=PP1#v=onepage&q=&f=false.
- ^ Timbs, John (1843). The Year-book of facts in science and art. London: Simpkin, Marshall, and Co.. http://books.google.com/books?id=zloAAAAAMAAJ&pg=PA15&dq=centrifugal+railway#v=onepage&q=centrifugal%20railway&f=false.
- ^ Roller Coaster History - Early History
- ^ Freizeitpark Forum - Die Looping-Rutsche kehrt zurück!
- ^ EAP-Magazin.de: Special Feature2
- ^ "Roller Coaster Loop Shapes". Archived from the original on 2007-08-27. http://web.archive.org/web/20070827183113/http://fy.chalmers.se/LISEBERG/eng/loop_pe.html. Retrieved 2008-08-13.
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