G Force

During a pillow fight, a whack from an opponent would strike with about 14.7 G’s (14.7 times the force of gravity), more than double the maximum G force people ever experience on roller coasters.

Despite the fact that acceleration or G forces have been part of the roller coaster and thrill ride experience since their inception, in recent years, as coasters grew in height and speed, criticism aimed at the amusement industry began to question whether or not the industry had gone too far and if humans should be exposed to these G forces.

However, there are plenty of engineers, ride manufacturers, scientists, and astronauts who contend that the concern is misplaced and the result of a host of misconceptions surrounding the issue of G forces and roller coasters.

The Controversy
Speed is not the issue. It’s the change of direction that actually limits the body’s tolerance to force and causes stress to the cardiac system. According to Harold Hudson, chairman of the subtask group that developed new standards for measuring and limiting G force (ASTM F-24), a mis-understanding about the relationship between G force and speed was the major cause of the criticism from the media and various politicians. This onslaught suggested that rider injuries and even death were the direct result of the industry’s reckless need to produce higher and faster rides that bring people to the park.

These critics, led by U.S. Rep. Edward Markey (D-Mass.), say that the increased speed and height create greater G forces, which may cause riders to experience brain damage. Congressman Markey is attempting to pass legislation that would federally regulate the industry. Throughout the summer Markey and members of the media high-lighted several cases in which individuals purportedly rode roller coasters and incurred brain damage, attempting to demonstrate that there was a direct correlation in those cases and that today’s higher, faster roller coasters were dangerous to ride.

“To say that today’s coasters are dangerous because they’re taller and faster is really a misconception—they aren’t more dangerous and in fact, the forces are probably less than they were 20 years ago,” Hudson says. Technology, he points out, is much more advanced than it was even 25 years ago when the first modern-day looping roller coasters began to appear in the parks, allowing designers and manufacturers to build even safer rides. “The truth of the matter is that with today’s technology being what it is, the industry is able to build taller and faster roller coasters, and forces are not any higher than with the older coasters.

“In fact, they are usually equal to or lower than the older roller coasters because we know a lot about designing rides. We have a lot more tools at our fingertips, such as computer-aided design, computer-aided fabrication, sophisticated test equipment, plus the experience of the past.”

Since 1978, the American Society for Testing and Materials (ASTM), an independent professional organization, has worked with members of the amusement industry in developing ride safety standards—ASTM F-24 is the committee that deals with amusement rides and devices. Last year an F-24 subcommittee— made up of 15 G force experts who are involved in the amusement industry and various aspects of engineering—passed a standardized means of measuring G forces. Jim Seay, president of Premier Rides and a participant in the committee, says G forces are misunderstood. “The negative comments that have been made to date don’t take
into consideration many important points of G forces,” Seay says.

One major aspect of G forces that’s often misunderstood is that G force is not just one measurement. Acceleration in three axes is considered in design: vertical (up and down), lateral (side to side), and fore and aft (ahead and behind). “You have to be very specific about what axis you’re talking about because requirements for lateral are very different from vertical requirements for G forces. It’s important that you’re focusing on the magnitude of the G force and also duration as well as the onset—how quickly the G force is applied to you and how quickly it’s taken away,” says Seay. These factors all have a hand in the way the human body will react to the force.

The issue of G force is by no means solely a consideration of the amusement industry. Pilots of high-performance jet aircraft experience 7 to 9 G’s routinely and wear G-suits to prevent the blood from pooling in the lower extremities and causing blackouts. Astronauts face 3 to 4 G’s for extended periods of time. But Seay is offended by the comparison of the forces that astronauts endure with those of roller coasters. “It does a disservice to astronauts who go through extensive training to become shuttle pilots. I personally know several astronauts, and the G forces on the space shuttle are dramatically different and far beyond anything imagined on rides,” Seay says.

“People seem to ignore the duration element, but when you watch the space shuttle take off, if you’re there in person, you’re watching it for several minutes . . . you’re still seeing the bright exhaust for up to 10 minutes after launch, and the astronauts are still experiencing G forces, whereas we’re talking tenths of seconds on a coaster. It’s a classic example of a hyperbole, in my mind.”

The Solution
The amusement industry in the United States creates and operates rides in accordance with commonly accepted engineering practices and standards. These are the same standards that are used for designing aircraft, automobiles, bridges, skyscrapers, etc., as well as 14 separate standards developed by the ASTM F-24 committee for amusement rides. However, the existing ASTM Design and Manufacturing standard, first passed in the early ’90s, was considered by some critics to be overly broad and lacking necessary details, including G force limits. Hudson believes the passage and publication of the new and more detailed Design and Manufacturing piece of F-24 should dispel much of the criticism that the industry is unregulated when it comes to G force.

The process for developing the new standard was arduous and intense because the committee knew they had to address every aspect of the issue. “The philosophy that ASTM took was that it was very important to first establish a very technical and very detailed process for measuring G forces prior to even having a discussion as to what levels of G force are appropriate,” Seay says.

Standardizing the system for measuring G force meant that people were all speaking the same language. “The result was the most detailed standard for testing G forces that exists in the world today. And that took several years to develop and many biodynamic experts around the world to work on it.”

The standard for testing was completed and released to the general public approximately a year ago, and it uses an electronic testing apparatus that looks at each of the three axes, Seay says.

The new provision also establishes limits for G force in each of the three axes including duration and intensity of the force. The standard works by limiting each of the three axes to a specified amount of time based on the intensity of the force. A maximum of 6 G’s vertically, for example, is only allowable for one second.

Hudson, who also liaised with the Brain Injury Association of America on the development of a study on G force and coasters, says the new standard is based on all the available materials on the subject. “We looked at work done on G force limits for amusement rides in Europe, Russia, Australia, Japan, England, and other countries. We took their information and enlisted several
biomechanical experts and developed what we thought the G force limits should be based on all the information we had,” Hudson says. “The bottom line is that amusement rides are way below most of the results they obtained from their research.”

Of 170 rides that the subcommittee measured last year with the new means of measurement, only one violated the new standard on G force levels and has since been retrofitted, Hudson says.

In spite of all their work to offer the industry a bible, these standards are not new, Hudson says. “Engineers, being the professionals they are, understood [prior to this standard] what the limits were and designed rides accordingly for many, many years. It’s not as if an engineer would design a ride with 20 G’s just because there was no limit. The rides were being designed prudently even though there were no published limits.”

But beyond all the research that the committee collected and analyzed to develop this standard, Seay says one of the most convincing pieces of evidence that roller coasters are safe is a study published in October 2002 in the Journal of Neurotrauma by University of Pennsylvania researchers Dr. Douglas H. Smith and Dr. David F. Meaney.

The sheer number of stories that came out about the dangers of roller coasters and G force propelled these two doctors to do a study on their own, unprompted by any outside group, Seay says. “The discussion level did get to the point where finally scientists said ‘Let’s take a look at this issue,’ and once the issue was examined from a technical standpoint, the results clearly indicated that rides are extremely safe. It was done on their own, and I can only assume it was done because the level of rhetoric got to the point that it motivated technical scientists to take an approach to an issue that was creating headlines.”

The study attempts to produce “any sound evidence or analysis directly linking roller coasters with brain injury,” by using a mathematical model to determine the effects of three popular coasters in the U.S.—Rock ’n’ Roller Coaster at Disney MGM Studios in Florida; Speed: The Ride at the Nascar Café in Las Vegas; and Face Off at King’s Island in Ohio. Smith and Meaney, a neuroscientist and a bioengineer, concluded that roller coasters do not cause brain injury. “While waiting for this issue to resolve, we highly recommend that all roller coaster riders use a proven method to reduce the risk of brain injury: make sure your seatbelts are buckled at all times when driving to the amusement park.”

The Future
Technology accounts for one of the major reasons that gravitational forces can be kept to a minimum while building more thrilling roller coasters, Hudson says. Today’s ride development technology is sophisticated and calculates G forces in all directions long before the track is even laid. “With computers that are available today and using what is called computer-aided design, calculations can be made and the roller coasters or amusement ride design can be generated much more accurately, much faster than it could prior. Secondly, in the fabrication process, CADCAM (computer-aided design, computer-aided machining) allows you to fabricate or build a ride much more accurately and to build a coaster track in a manner that was impossible prior to the computer, both in design and fabrication,” Hudson explains.

Curves and loops were a particular challenge in the past. On a roller coaster 20 years ago, if a track was going straight and transitioned into a left turn, the straight track simply intersected with a circle and the circle created the turn. However, today’s ride engineers use a “spiral function” to create a curve. “That means that gradually and constantly the radius is changing as you make that turn. Forces can be kept very low and with today’s manufacturing techniques it can actually be built.

“The rides that are built today are vastly different than the rides that were built 20 to 30 years ago—more exciting, taller, faster, and no higher forces. Technology has allowed that.” This technology truly benefits the ride manufacturers, Seay says, because they are able to develop exactly what they create onscreen. “The important point is that as designers today, we’re very fortunate that the high-tech approach to analysis, which in large part is based on the used of aerospace technology software, allows us to predict incredibly accurately the level of G force exposure that guests will see on rides.”

Premier Rides has always used the spiral function, for example, but some of the rides they are conjuring today are taking the concept to the next level. The best example of the continuously changing curvature is Speed: The Ride at the Sahara Hotel and Nascar Café in Las Vegas, a ride that’s just a few years old. The ride shoots guests out of the Nascar Café and makes a sharp left-hand turn that seems inconceivably tight, but is actually described as one of the smoothest rides ever built. “As a guest you say to yourself ‘How can I possibly make that transition’ and when you ride it’s incredibly comfortable,” Seay says. “And that’s because you’re using the spiral function.”

Seay knows that publication of ASTM’s G force standard will likely have little impact on much of the industry’s manufacturers who have already been limiting their G force long before there was a standard. But newcomers, he says, will have a blueprint to work from, which may make the transition a little easier.

Whatever effect it has, he hopes it brings the amusement industry’s critics down to earth.