THE THRILL OF THE RIDE: A BRIEF OVERVIEW OF THE HISTORY AND SCIENCE OF ROLLER COASTERS
Roller coasters are the descendents of Russian ice sleds, which first appeared in the 16th and 17th centuries. These large contraptions were simply made of very long steep pieces of wood covered in ice. Riders would typically climb a 20 meters high hill and would then slide down on these small pieces of wood or ice. The hill was usually designed to have a slope of about 50 degrees.
How the modern day roller coaster evolved from these sleds is still an issue of contention: however, it appears that the idea may have first been adopted by French entrepreneurs in the early 1800’s. Since it was not possible to rely on the presence of ice in the warmer climates of France, a new generation of coasters were designed that moved on wheels and had multiple carts. And over time, the tracks became more and more sophisticated.
Here, the increasing popularity of the coasters inevitably led to new designs, many of which resulted in injuries. For example, vertical loop and loop-the-loop coasters, invented in the U.S. during the late 1800’s, were early innovations which had to be dismantled shortly after construction due to safety concerns. The popularity of coasters also suffered briefly as a result of the fiscal contraints of the great depression in the 1900’s.
However, interest in roller coaster design began to resurface partly due to a breakthrough at the Disneyland theme park in 1959, where steel was used instead of wood. This allowed the construction of much more sophisticated tracks capable of much more dramatic bends and twists. And historians generally agree that the current golden era of roller coasters began in 1972 with the hugely popular Racer in Kings Island, Mason Ohio, designed and built by John Allen. To this day new innovations create new experiences – the challenge is to make these designs as safe as possible while pushing the limits of the speed and acceleration.
But what of the science behind roller coasters?
An important thing to consider is that the carts on a conventional modern day roller coaster are not self-powered. The movement is generated exclusively by gravitational, inertial and centripetal forces. Although the tracks are getting more and more complex and the speed is ever increasing, the basic principles of physics at work are simple and can be easily understood. Still, the actual task of designing a roller coaster itself is by no means simple, which is reflected by the many obstacles that need to be overcome before a coaster becomes operational. Given this contrasting perspective, this paper is going to take a look at these underlying physics principles as well as some engineering methods that are involved.
Energy is essentially applied to the carts only as they are pulled up the first hill. This hill is often called the lift hill. Once the coaster reaches the top, the forces applied to it for the remainder of the ride are mainly gravitational and inertial. Therefore, in essence, the fundamental principle behind the coaster’s operation is the ‘conservation of energy,’ which simply states that energy can neither be created nor destroyed. The total energy which is consisted of ‘potential’ and ‘kinetic’ parts, is therefore constant.
As the coaster moves up the lift hill, the total energy exerted on the carts is stored in the system as potential energy. This happens since as the height increases, there is a greater chance for the gravity to act on the cart to pull it down. However, it is not desirable to have the carts fall vertically to the ground, and so a good way to think about what is happening here is that the tracks are designed to manipulate this fall.
The second physical principle relevant here is Newton’s first law: This states that an object stays in motion (or conversely stays still) if no external forces are applied. The tendency of objects to do this is referred to as ‘inertia’. Based on this principle, as the cart reaches the second hill (after the lift hill), it continues to rise converting kinetic energy to potential energy. However, some of the energy will be lost due to friction which exists between the tracks and the cart wheels as well as that created by carts moving through the air. Therefore, a few extra hills (which are shorter than the lift hill) are put along the path to ‘recharge’ the cart giving it more potential energy to convert back to kinetic energy. The tracks are designed in such a way that at the end of the ride, all potential energy is converted to kinetic energy so there is little need for brakes and the carts essentially stop on their own.
In terms of categories, coasters are generally classified in terms of their track structure and material. Here, there are two major types: Wooden coasters (a.k.a. Woodies), and steel coasters.
The woodies’ tracks and wheel design is very similar to that of a train. The metal wheels run on a metal strip measuring 4-6” wide. Two safety measures are employed to keep these wheels on track. The inner part of the wheel is designed to keep the cart from moving off the track in the lateral direction, and there is a safety bar contraption underneath the track that keeps the carts from flying off. With careful design, it is actually possible to design flips on the woodies, but this is not a common practice given the overall difficulty in constructing stable twists and turns using structures made from wood.
The other type of coasters is identified by the usage of tubular steel tracks, and as mentioned before were introduced in the 1950’s. Here, the wheels are made of polyurethane or nylon. As well, contact with the track is usually achieved with wheel sets added to the sides and the bottom of the carts, in addition to the conventional ones that run on the track. Whilst this gives extra manoeuvrability, such a set up also provides extra safety and freedom in choosing different ways of connecting to the track. For example, it’s possible to have suspended and inverted coasters where the carts hang or sit below the track as they move.
Coasters can also be classified by the mechanism used for lifting them to the top of the first hill. In most cases, this is done by a system of gears, motors and pulleys. In more modern versions however, a catapult launch can be employed. One example of such a system is a magnetic induction mechanism. In this case, there are two magnets – one attached to the track and the other to the cart – and these magnets can then be pulled up the hill, resulting in the lifting of the carts.
It’s also worth noting that even with most designs factored to allow effective loss of all kinetic energy at the end of the run, brakes are still installed on the tracks. Often, a hydraulic clamping mechanism, controlled by a computer, in place particularly at the end of the track.
The enjoyment that is felt while riding a roller coaster is a result of the combination of a variety of forces acting on the body that are different from the usual sensations felt normally. Perhaps the most enjoyable aspect of this ride is the constant change in the acceleration due to change in speed or direction. When the coaster suddenly changes direction; the body tends to keep its current direction (inertia) but is forced to go with the cart. This causes additional forces (besides gravity) to be felt by the rider. Using clever designs, it is possible to combine the effects of gravity with those of changing acceleration to create new sensations and thrills. From the point of view of the rider, his weight is changing and at times (during the free falls) he may feel complete weightlessness. This translates to a sense of lack of control, which can be enjoyable to some and possibly frightening to others. Also the background structures and views can dramatically add to the richness of the experience by creating different optical illusions along the way.
Perhaps the most exciting part of the modern day coaster is the loop-the-loop design. A full rotation is made by the riders, where at the top of the loop the carts are upside down, and centripetal forces push the riders against their seats. From a sensory point of view, moving through the loop results in a wide spectrum of forces on the body with the direction changing rapidly, often making it the most enjoyable part of the ride.
Concerning design, there are basically two parameters in the loop-the-loop structure that are taken into consideration; the speed of the train when it enters the loop and the angles of the turn itself. Since the train enters the loop with maximum kinetic energy at the bottom, it’s important that this energy is then converted to an appropriate amount of potential energy by the time the train reaches the top. In other words, the velocity at the top still needs to high enough so that there is enough centripetal force to keep the rider in his seat, but also not subject that rider to a level of force that is uncomfortable or even dangerous.
Originally, loop-the-loop structures were designed in a circular shape. However, it was found that this required a fairly large cart velocity when entering the loop, resulting in an excessive force experienced by the riders along the sides of the loop. To overcome this problem, the tracks are now designed in shape of a tear drop which requires less initial velocity and also is more efficient at reducing the velocity at the top to maximize the enjoyment.
With the boundaries of innovation being pushed constantly, in recent years, some concerns have been raised about the safety of modern roller coasters. In 2003 the Brain Injury Association of America concluded in a report that although roller coaster rides are safe for the majority of healthy riders, they pose a health risk to some of the riders some of the time. Michael Freeman and his colleagues from the American Congress of Rehabilitation Medicine also released a study on the occurrence of spinal injuries resulting from low level accelerations (as is the case in roller coasters), whereby the study concluded that “…in a susceptible subset of the relatively healthy general population, significant spinal injury can result from low-level acceleration.” However, it can also be argued that number of fatalities and injuries are not statistically significant. The U.S. Consumer Product Safety Information, for instance, submits that about 134 people are injured and 2 are killed in roller coaster accidents every year in the U.S.
In any event, such studies on the safety of roller coasters have served to help in providing better design criterion to reduce the number of injuries.
As with every other piece of technology in history, it is certain that engineers and designers will attempt to push the limits of their creations to make them faster, taller and better. This progress is perhaps only limited by the economical conditions and subsequently, the willingness of people to spend time and money riding these machines. As we achieve a deeper understanding of how the human body reacts to various stimuli, it is perhaps possible to maximize the enjoyment by more clever designs as opposed to just increasing the speed and height. As most people in the first world live their lives with relative comfort, there is perhaps some merit for introducing danger into their daily lives by riding these scream machines.
References
1. Cartmell, R. The Incredible Scream Machine. Amusement Park Books, Inc. 1987.
2 http://www.howstuffworks.com/roller-coaster.htm
3 http://en.wikipedia.org/wiki/Roller_coaster
4 Kuschyk, J, Hamm, K, Schoene, N, et al. Modern roller coaster rides: A potential cardiovascular risk? First systematic analysis of cardiovascular response to extreme G-forces untrained volunteers CIRCULATION 112 (17): U767-U767 3290 Suppl. 2 OCT 25 2005
5 Freeman, MD, Croft, AC, Nicodemus, CN, et al. Significant spinal injury resulting from low-level accelerations: A case series of roller coaster injuries ARCH PHYS MED REHAB 86 (11): 2126-2130 NOV 2005