Design a Safe Backyard Roller Coaster with Paul Gregg – Part 2

Paul Gregg, the retired aerospace engineer turned backyard roller coaster designer, is back with a new book chronicling the research and construction of his homemade thrill rides. When we interviewed him last time, Paul had just published Backyard Roller Coaster Research and Development: Volume I: Negative-G Out and Back Coaster. Paul recently released Volume II: Three Dimensional Backyard Roller Coasters and was nice enough to give us a preview of the book’s contents.

C101: What are some of the challenges with building a 3D coaster rather than a 2D coaster?

Paul: A 3-dimensional coaster will be more difficult than a 2D coaster, as it will have to accommodate not only peaks and valleys, but also left/right curves, and banked track gaining and losing elevation. With a 2D track, with no curves or tilts, four fixed wheels can contact the tracks fairly evenly.  But, when there is 3D shape to the track, only three of the four main wheels would contact the tracks at any one time (three  points define a plane, but there are four points of contact).  So a fixed-wheel 2D coaster would bind on a 3D track, unless the wheel assemblies have large gaps, which would result in an unacceptably rough ride, and a threat to safety.

paul gregg safe backyard roller coaster


C101: Not only are the tracks more difficult to form, but more engineering has to go into the design of the cars as well?

Paul: The 3D cart design is going to be more complicated than the 2D carts because we have to keep all wheels on the track while accommodating varying bank angles around curves combined with track elevations changes.  The forward wheel assembly needs to be able to hold vertical loads and pitch moments, while rotating in the yaw and roll degrees of freedom.  The aft wheels need to transfer overturning roll moments from the rider and cart to the track, while still accommodating track turning.  The cart needs to be close to the track so the center of mass doesn’t produce high overturning roll moments.  This is all a pretty daunting design task.


C101: So if you’re going to make a three dimensional coaster, you’ve got some pretty important design decisions to make at the onset?

Paul: Track width and cart length are important decisions.  Track width is based on intended rider weight and height.  If the rider is larger and taller, a wider track width is needed, to accommodate a longer cart, and to better handle a higher center of mass.  Tracks for lighter, shorter riders can have narrower track width (gauge) so the cart can be shorter, which means the track turn radii can be smaller, and the track fits in a smaller space.



C101: What about the car design, how do you choose whether to go with Ackerman or Bogie Steering on the roller coaster?

Paul: I experimented with 4- and 8-wheel carts, and I also developed designs with bogie and Ackerman style steering mechanisms.  Bogie steering has all front wheels connected to a pivoting crossbar, like a train car wheels.  With Ackermann steering, each wheel (set) pivots independently, like the front wheels of an automobile.

Ackermann steering works better if you narrow the track gauge a calculated amount in turns, as the effective distance between cart wheels is less in a turn than on a straight section of track, especially with smaller turn radii.  If you didn’t narrow the gauge in turns, it could result in binding.  If you design the cart side wheels further apart to account for this, then the sideways gap would be larger than needed in straight sections, and the cart could wobble side to side more, which might be unacceptable.

Bogie steering can have constant track gauge in turns and straight section of track, but the extra mass of the bogies turning back and forth take up energy.  Whereas, in Ackermann style steering, the wheels turning takes less energy and can be quicker (more responsive).   I went back and forth between these two types of steering, and actually made one cart with bogie front and Ackermann aft steering.


C101: If you wanted to know which cart design was more efficient, how would you test that?

Paul: I am defining track/cart drag as the amount of vertical drop divided by the length of the track.  So if a coaster with 4 units of vertical drop stops in 100 units of track length, then it has 4% total average drag.  Undoubtedly a curved track is less efficient than a straight track, but I’m defining average drag to make it easy.  Efficiency is dependent on air resistance, wheel/track friction,  and track fabrication tolerances.

I ran the cart around the 170 foot BYRC-3D-02 track with a  44 pounds payload and noted the position of the cart when it roll a short distance up the lift hill at the end of the run. Using a laser level and tape measure, I measured a total vertical drop of 86 inches from the top of the hill with no velocity to the final stopped position.

86 inches divided by 170 feet divided by 12 inches per foot = 0.0421

so the total average drag is 4.21%, and the efficiency is 95.78%

I had assumed 4.6% drag in the track analysis spreadsheet, so this is pretty close.

C101: Wow, I’d say that’s pretty good efficiency for a backyard roller coaster. Thanks again for taking the time to share your expertise with us.

To purchase either of Paul’s books, please check out his website here:

Do you think you’ll ever build your own safe backyard roller coaster using Paul’s knowledge as a guide?

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2 Responses

  1. Ian gordon says:

    Hi Paul gregg, how are you my name is ian Gordon from jamaica, I am interested in building one of your roller coaster