Designing a Safe Backyard Roller Coaster with Paul Gregg
Every now and then we come across homemade roller coasters designed by amateurs and built in their own backyards. While we appreciate the effort and creativity that goes into building one, most look poorly made and are of questionable safety. That is until we discovered Paul Gregg, a retired aerospace engineer who holds 29 US and foreign patents, two special invention awards, and was Boeing Aerospace Engineer of the Year in 1988. Paul uses his engineering skills to design and test his backyard roller coasters until he can prove they are reasonably safe to ride and operate, which I’m sure his grandchildren love! We recently got to ask Paul a few questions about how he uses math, science, and his engineering experience to design and build backyard roller coasters and why he decided to write a book about it.
C101: For those who don’t know, who are you and what do you do?
Paul: I retired from Boeing in February of 2014, after a very interesting career, spent primarily in the research and development of new materials (carbon fiber composites, chopped fiber composites, metal matrix composites, titanium), joining processes (adhesive bonding, diffusion bonding, welding), forming processes (superplastic forming, compression molding, thermoplastic forming) and structural architectures (sandwich, trusscore, isogrid, corrugation, hat-stiffened) associated with light aerospace vehicles. I had worked on many space, military, and commercial aerospace programs, developing and using a systematic approach involving the integrated disciplines of design, analysis, fabrication and testing. I have six grandchildren, and three backyard roller coasters.
C101: How did you get involved in backyard roller coasters?
Paul: Like many people, I’ve had a lifelong interest in roller coasters. As a young boy, I was equally fascinated and terrified by them. Well, maybe not equally. I found some backyard roller coasters online, a few pretty well done. But I thought I could make a few improvements in design, analysis, and fabrication methods. I also felt I could add to the safety aspects, and employ the more rigorous engineering certification testing methodology similar to what I had learned in the aerospace industry. And, the grandkids thought it would be a good idea.
C101: How do you ensure your backyard roller coasters are safe? What steps do you take that other backyard engineers probably skip? Can you give us a quick summary?
Paul: I try to ensure safety in several ways. I always store the cart away from the track. I have caution tape up around the tracks and make sure no one is inside the tape when we are operating a coaster. I have automobile-grade safety belts fastened through the steel cart frame. The carts are designed with provisions for prohibiting hands and feet from getting near the track. The lift hill has anti-rollback features. I thought about a motorized lift hill, but decided it was safer to have an adult lift the cart and rider up the lift hill, which ensures adult supervision at all times. I also have done static “heavy cart” testing, where I constructed a 10-foot straight section of track, and put 900 pounds on the cart and rolled it back and forth. Every track and cart are also “certified” every couple months, by running a “heavy cart” around the track loaded with 1.5 times the allowed rider weight, using sand bags.
C101: How did you decide on a factor of safety (FOS) of 1.5?
Paul: A major consideration when designing a backyard coaster is how strong it will be. With my materials and design, how much weight could I put on the cart before it would break? If a bridge was designed to hold exactly 180 lbs before it broke, and you weighed 180 lbs (a factor of safety of 1), how safe would you feel walking across? On the other hand, we could build our little one man bridge out of 1 foot thick solid steel that you predict would hold 10 tons (a factor of safety of 111). You’d feel quite safe walking across it, but you definitely spent more money than you needed to.
When a coaster cart experiences centripetal accelerations, the resulting forces can be higher than you think. For instance, a 100 pound rider experiencing 3 G’s produces an equivalent static force of 300 pounds. Multiply by 1.5 to assure safety, and that’s an ultimate force of 450 pounds the cart and track have to carry!
Design limit load is the maximum expected load the structure will experience in it’s lifetime. Design limit load times the factor of safety (FoS) equals what we call ultimate load. Static ultimate load is what you design the structure to withstand. For these coasters I made sure we had a factor of safety of at least 1.5 on loads. This means that if the most weight we ever expect to send around the track is 100 lbs, then we have dynamically tested it with 150 lbs. If it passes that loaded dynamic cart test (is run around the track several times with 150 pounds of sandbags on it) we can be confident every time we send a passenger down the track weighing no more than our 100 lb design limit.
The purpose of a factor of safety is to account for variability and uncertainties. Nothing is perfect, and we must account for possible tolerances and manufacturing flaws, as well as for the possibility that our design may be subjected to loads higher than predicted or intended.
Possible sources of variability are: fastener quality, fastener installation parameters (over-torque, under-torque, hole size, hole depth, countersink depth, etc.), fastener angle tolerance, material variability (PVC, steel, wood especially), assembly tolerances of track and cart, track ageing effects, and track environmental conditions, like temperature and rain.
C101: What material did you use for the coaster rails?
Paul: Most other PVC BYRCs seem to have been constructed from white plumbing PVC pipe. I knew from my engineering career that most polymer materials are susceptible to ultraviolet (UV) sunlight degradation, resulting in embrittlement. Companies using plastics, fiberglass composite, and carbon-fiber composite materials are aware of this, and employ a variety of solutions, putting in additives and/or painting soon after fabrication, before exposure to sunlight. Embrittlement is a condition where the plastic strain capability, or fracture toughness, of a material is degraded. The material may still be as stiff and strong as ever, or more so with ageing, but is susceptible to impact shocks, like a ceramic would be. I opted for the grey electrical conduit, which has “sunlight resistance” printed on it, though I was unable to find any definitive test data showing quantified fracture toughness degradation due to UV sunlight exposure. So, as I had to at work on many occasions, when data doesn’t exist, you have to do your own tests, which I document in my book. I also decided to paint all the PVC rails soon after forming, for added UV sunlight protection. I have some painted and unpainted PVC pipes in my back yard, being exposed to sunlight, which I will eventually impact test with a drop-weight device I made in my garage.
C101: Real coaster designers are highly secretive about their steel pipe bending processes. What’s the secret to bending the PVC pipes into smooth curves?
Paul: Most of the PVC BYRC tracks I saw online seemed to be constructed with straight pieces of PVC, bent as well as possible during track assembly, maybe with some torch heat applied. This has several drawbacks. First, when you use a lot of force, you create what we call pre-loading, where the materials and joints have load on them all the time, even when the cart isn’t running. When the cart does run over such a track, the dynamic cart loads can add to the existing pre-loads, and cause cracks or yielding of materials, possibly leading to a catastrophic failure.
I tried several methods of heat forming PVC pipes, like torches and heat guns, but the heating is never uniform, so one part always forms more than others, and the resulting curvature is terribly uneven. The pipe tends to buckle locally on the compression side of the bend, resulting in a non-circular cross section, an ugly track, and a rough ride.
I finally came up with a simple, cheap and highly accurate solution. I heat sand in my (wife’s) oven (with tentative permission), pour the hot sand into the pipe, move the evenly-heated limp pipe to a cooling rack of the radius I want, and let it cool down before removing the sand. The sand keeps the pipe round, and the curvature is remarkably uniform.
Hot forming of the PVC pipes is what we call an “enabling technology” in engineering. A breakthrough in manufacturing methods, a new material, or a new joining method can lead to new and/or improved designs. Hot forming allows for tighter turns, more accurate engineering and more interesting track than was previously possible.
C101: Ever thought about doing an inversion?
Paul: I could do a loop, but I’ve calculated that I would need about 18 feet of drop to do it safely, and I don’t want to go that high.
C101: What inspired you to write a book and share your design methods with other BYRC engineers?
Paul: It’s really just an eccentric hobby, and a fun thing for the grandkids. I enjoy using my engineering skills. I’m learning Autodesk Fusion 360, which is fun. I would hope I’m injecting some safety into the growing mix of backyard roller coaster enthusiasts, along with generating some enthusiasm for math, physics and engineering in young people.
C101: Have you ridden very many real roller coasters? What is your favorite?
Paul: I don’t ride much any more, but I used to. I’ve never ridden one I didn’t like.
Thanks again to Paul for taking the time and sharing his knowledge with us. I really enjoyed reading his new book because he included some personal
information and stories and I felt like I was getting to know him and his approach to design as well as how to design a safe backyard coaster. You can watch Paul’s backyard coasters on his YouTube Channel and if you want to learn how to build your own roller coaster you can purchase his new book, Backyard Roller Coaster Research and Development: Volume I, here: http://backyardrollercoasters.org/
Update: We did a second interview with Paul, click here to read more about track, wheel design, and steering.