G-Force Lesson - Page 1

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Ever wondered if your ride pulled to many Gs? Don't trust the green/yellow/red scale? Our own BBSpeed26 has your answers, straight from the same engineering standards that the pros use.

One of the most frequent questions I see posted on these forums is something to the effect of: Is it OK if I hit X Gs for Y seconds? Inevitably the thread descends into citations of RCDB or shoot-from-the-hip guesses, and in the end things usually just end up more confusing than when they started. Let’s change that shall we? It turns out that the modern G limits have been freely available to the general public for a good 7 years now, courtesy of the Blue Ribbon Panel Review of the Correlation Between Brain Injury and Roller Coaster Rides (spoiler alert, there’s no correlation).

The Panel was kind enough to cite and include section 7 of Section-7 of ASTM Z9591Z - “Standard Practice/Guide for the Design of Amusement Rides and Devices”, which just happens to be the section of the standard that addresses the acceleration limits of amusement rides, including roller coasters. ASTM was nice enough to provide these limits and the acceptable durations in easy to digest graph form.

Before we get to the graphs, there are a few terms on the graphs that are worth translating out of engineering lingo and into plain (or at least coaster-nerd-friendly) English.


Base Case (Class 4 or 5 Restraint) – Some kind of restraint device that provides support to the lower body in all directions and maintains rider contact with the seat at all times.

Over-the-Shoulder (Class 5 Restraint) – Included in the base case, but with the added specification of being able to support the upper body.

Prone Restraint – The most common application of this is your standard flying coaster. A prone restraint is one where the rider is oriented “face down” at a point or at several points during the ride cycle. A prone restraint is designed to allow the patron to accept higher acceleration in the -Gx (eyes front) as compared to the Base Case and Over-the-Shoulder restraints.


+Gx (Eyes Back) – For practical purposes, these are the Gs felt during a period of forward acceleration, such as a launch. Alternately, these are the Gs felt while on your back during a flying coaster’s loop.

-Gx (Eyes Front) – These are the Gs felt during a period of deceleration on a standard coaster, such as a brake run. Alternately, these are the Gs felt on your chest while in the flying position of a flying coaster.

+/-Gy (Eyes Left or Eyes Right) – Simply put, lateral Gs.

-Gz (Eyes Up) – Airtime, or in the case of a flying coaster, braking deceleration.

+Gz (Eyes Down) - Positive vertical Gs, or in the case of a flying coaster, the Gs felt on a launch track.

Important Notes

Two important things to consider before I present the graphs:

  • First, these are the LIMITS and should not be used as targets. These G graphs represent the absolute maximum allowable G magnitude in duration. They should not be exceeded, but it should also not be your goal to hit these lines at every instance, nor does building a ride with limits totally under those prescribed in the graphs guarantee a realistic ride. These are simply tools that you can (and should!) use to help guide your designs.
  • Second, rate of application is very important. While 6 launch Gs may technically be allowable, it’s going to feel like you got hit by a truck if you go from a stop to 6G launching in a tenth of a second. Use your these graphs in addition to your own judgment, not in place of it.

And Now, The Graphs

Finally, for more information and additional graphs, refer to Appendix A of the Blue Ribbon Report, available here: http://biausa.org/word.files.to.pdf/good.pdfs/BlueRibbonReport.pdf