Amusement Ride Acceleration

Amusement rides thrill us by accelerating our bodies. Each curve, drop, loop, launch, or brake alters the rider's state of being, triggering inertial resistance -- the feeling that your body is headed in one direction while the ride is pulling you somewhere else. That's the "illusion of danger" that makes amusement rides exciting.

Acceleration Primer

• Acceleration represents the change in the speed and/or direction of a moving object.
  • Linear acceleration is a change in speed without a change in direction.
  • Radial acceleration is a change in direction without a change of speed.
  • Angular acceleration is a simulataneous change of speed and direction.
• A force is a push or pull that can cause an object with mass to change its velocity (i.e., to accelerate), or which can cause a flexible object to deform.

A 3-axis coordinate system is used to represent the direction of acceleration. The amusement ride industry uses "g"s as the unit of measure for acceleration, with 1g representing the acceleration of normal earth gravity.

As an amusement ride accelerates downward
(-Gz), the effects of gravity are partially or totally neutralized and the rider feels a brief moment of weightlessness.  As the ride vehicle reaches the bottom of its path, slows and starts back up (+Gz), the rider is pushed down into the seat, multiplying the effect of gravity.  Lateral accelerations (Gy) push the rider's body sideways.  An increase in forward speed of the vehicle (+Gx ) pushes the body into the seat back; braking or moving in reverse (-Gx) pushes the body forward, away from the seat back.

If both the ride and the rider's body are well-designed and well-maintained, and if the rider's body is well-supported and restrained by the containment system, then the physics end of the safety equation is satisfied. A flaw in a critical component (e.g., the ride track, braking system, rider's back/brain/cardiovascular system), or poor fit between rider and restraint, can be the trigger point for an accident.

Anyone who's spent time at amusement parks or carnivals knows that the quality and quantity of motion produced by different amusement rides varies widely. Some of the newer steel coasters offer a smooth, sweeping, exhilarating swooooosh of a ride, sending your body through seemingly impossible elevations and angles. Older wooden coasters can rattle your teeth and bruise your butt. Computer-controlled rides can simulate the jerky thrills of off-road jeep racing. Human centrifuges create the acceleration of a space launch for millions of armchair astronauts. Spinning carnival rides can literally pin you to the wall or, alternatively, send smaller riders sliding across the seat through a combination of rapid changes in direction and unfitted restraints.

Each thrill ride design offers a different experience to patrons, and each new experience reshuffles the biodynamic equation. Amusement rides are designed for the masses. That means they're generally safe for most people, but it also means their generic design may put some vulnerable individuals at higher risk for accidents or injury on some rides.

If a rider's body is not well-enough supported or well-enough restrained by the ride's containment system, he or she can be propelled out of the ride by the vehicle acceleration. This is a statistically rare occurance, but each year riders are seriously hurt and sometimes killed this way. The most common causes of ejection are:

• Poor fit between ride and rider - This phenomenon almost exclusively involves young children, but obese riders may be at risk on some highly dynamic rides with lap-only restraints. Size Mismatch
• Equipment failure or operator error - The latches on restraints sometimes fail. Ride attendants sometimes forget to latch and double-check each rider's restraints before starting the ride. This doesn't always lead to patron ejection, but it can. Equipment Failure  Operator Error
• Patron error or misconduct - Patrons can put themselves at risk by unlatching their restraint, standing up or turning around in their seat. Sometimes this is done deliberately to increase the thrill factor. Sometimes the motivation is more benign (a rider turns to wave or take a photo, a young child becomes frightened and tries to change position or get off the ride). Either way, this is a very dangerous thing to do. The forces of motion are immutable. If you or your child moves into the wrong position at the wrong moment -- whether innocently or through deliberate horseplay -- the ride will toss you out. Patron Behavior

The effects of acceleration can also result in less extreme accidents, such as patrons hitting against something within the vehicle (e.g., the seat back, rigid restraint, or another rider). This is one of the most commonly reported types of accidental injury on amusement rides, accounting for one quarter of roller coaster injuries. Some of these accidents can be prevented if patrons brace themselves effectively against the ride accelerations.

• Understanding Acceleration, Containment and Restraint
• Fast Cars and Wild Rides: Whiplash, Headbanging, and Sqeezeplays

Speeding, spinning vehicles and rapid changes of direction can apply significant stresses to the human body. Human tolerance to acceleration depends on many factors, including:

• Magnitude, direction, rate of onset, and duration of the acceleration stress
• Differences in individual physiology
• Psychological reaction (i.e., the fear factor)

Human beings are the most complex and unpredictable component in ride design. For more information about physical and psychological reactions to amusement ride acceleration, visit the following pages:

• Physiological Effects of Acceleration
• Intensity of Motion and Emotion
• Fast Cars and Wild Rides: Whiplash, Headbanging, and Sqeezeplays
• Risk Factors Related to Health and Aging
• Riders With Disabilities

Industry standards for U.S. amusement rides are developed by the ASTM F-24 committee. ASTM F2291, Standard Practice for Design of Amusement Rides and Devices, addresses acceleration in several sections:

• ASTM F2291 requires a Patron Restraint and Containment Analysis be completed for rides designed after 2003, or when a major modification is performed on the containment system of an older ride. Five classes of restraint are defined, ranging from no restraint at all (Class 1) to individual restraints with adjustable locking position and redundant or failsafe design (Class 5). Requirements for choice of restraint class are based on the ride acceleration. There is no requirement or standard test procedure to ensure that restraints provided in amusement rides safely fit the full range of riders allowed on board.
• The Acceleration Limits section references a standardized measurement procedure and establishes limits for sustained acceleration. It does not require that jerk or vibration be considered or measured when analyzing a ride's dynamics.