I retired from a career in aerospace engineering a couple years ago and decided to put my skills to use making backyard roller coasters for my grandkids! This is the first of three posts documenting my three roller coasters. Way more technical engineering went into this than I can show in an r/diy post, which is why I wrote an e -book! Recently published and available now at backyardrollercoasters.org
I posted a less detailed version of this last week that was removed by the mods. This version is updated to meet the r/DIY submission standards.
Some comments on safety. I'm a big believer in both having fun and being safe. I worked 34 years at Boeing, developing new materials and joining methods, and coming up with steps to qualify airplane, space and military vehicle structures. Building things and then breaking them to see how much load they could take was a big part of my job. I used the same methodologies here, like: 1.5 factor of safety on loads, "heavy cart" 830 pound cart static testing, qualification testing with 1.5 times the allowed weight(repeated bi-monthly), and a lot of other element level testing and analysis. We also have strict safety rules. No one near the track when we're running. I store the carts away from the tracks. Seat belt. I feel pretty safe. Is this more dangerous than sitting on the couch? Yes. But due to the controlled nature of the ride, extensive testing and safety precautions; I feel that it's at least as safe as swimming pools, trampolines, skateboards, or any number of other things we let our kids do just because we're more used to them.
Building the Track Ties
I lot of coasters I've seen online just had flat 2×4's for track ties, but I thought cupped ties would be stronger. I designed this jig to cut them. The two sides are 1 1/2" apart and uses a 2" hole saw. I used the same gauge guide for all the ties because it's very important that all of the ties are the same length so that the cart doesn't either bind on the rails or slide back and forth too much causing undue energy loss. Ties were cut to 14 inches.
Here's the completed Jig.
And here it is in action.
Looking nice and uniform. The track tie ends were dipped in deck stain prior to track assembly to prevent environmental damage from water seeping in and all lumber was treated after track assembly.
Track sections were constructed by simply screwing PVC rails to the ties. I put a tie about every 16 inches. More in high stress areas just to be safe. I used galvanized 3 1/2" #10 deck screws. Four per tie. Holes are pre-drilled at about a 25 degree angle to keep them out of the way of the wheels.
Putting the Track Together
A close up of the cupped ties and connectors. This coaster used schedule 40 ABS, but all future coasters use schedule 80 UV resistant PVC which is stronger. The connector is just a smaller piece of pipe glued inside of the rail and then screwed down.
My first length of track. The gauge set helps keep things tight when screwing things down.
40 feet of beautiful track ready to be framed. Future coasters were constructed with pre-formed pipe(hot sand) which is much stronger and more reliable than just bending the pipe down to fit the frame. I'll explain the process further in future posts about my other coasters.
This CAD drawing tells me where to cut the track pieces and how much to bend the rails.
I started the out-n-back coaster on my deck to save on lumber. You can see my makeshift system for propping it up here while I worked on framing.
This part gets a little technical, but basically, I used an excel spreadsheet to predict the cart's potential and kinetic energy, velocity and vertical G's at different points along the track. This helped me make sure I wasn't going overboard and making the ride too crazy for the intended riders.
Two tracks, joined by cross ties and vertical members to a 2X4 oriented roughly parallel to the tracks, formed a very stiff three-member beam-type construction. Each tie connects to the frame and I kept using the same 3 1/2" #10 screws. The resulting track is very sturdy, with very little vibration or movement when a loaded cart moves across it.
I sort of just winged framing on this coaster, but did a lot of testing to make up for it. The process was basically: Make the frames, fill in between the frames with horizontal members, then put in the vertical stubs up to each tie.
I used a laser level and a tape measure (at dusk so I could see the laser beam on a tape measure), to help set the track heights at points of interest.
A typical track frame. This turned out to be very stiff and strong.
Here's a CAD model of the framing for a later coaster design. My wife wouldn't let me pour concrete in the lawn so these were the solution. I did a lot of experimenting with framing and what got used on this coaster isn't as good as what I developed for the later ones.
The end of the track goes up higher than the track starts, and is wider at the end to make sure the cart can't fly off the end.
The completed track looking so so pretty.
And looking the other direction.
Cart Design and Construction
Now for the cart design. I was used to a $10k+ CAD program from my engineering career so I fiddled around for a while searching for a good substitute. Auto-Desk Fusion 360 works really well and is free for hobbyists and startups! You can view my CAD Models here.
Tubular steel is welded into a rectangular frame sitting atop 4 wheels with side and upstop wheels connected on the sides. The frame is bolted and welded for strength.
This view includes the frame dimensions.
Front View- The 15.25 inch dimension is the track gauge for this coaster. The 14 inch ties plus 1 inch to account for the width of the PVC and a quarter inch extra so the wheels don't bind on the track.
Notice the wide wheels on top to accommodate some side to side motion, small wheels on the side for fast spin-up, and the upstop "wheels" on bottom to make sure the cart never escapes from the track.
I used 70mm x50mm longboarding wheels for the main wheels, standard roller blade wheels for the side wheels and just a small section of a pvc tube for the upstop wheel.
After the wheels were welded to the frame, I clamped it all together over a piece of track to make sure the main wheels were aligned to the track really well before welding the horizontal pieces to complete the frame.
My little 90 AMP welder gets the job done. Time for testing!
My first seat/body was a car seat. Here it is with 84 lbs of weight on it for dynamic heavy cart certification testing.
I strapped my smartphone to the cart with an accelerometer app to verify predicted performance. It hit 2G's in the 2nd dip and nearly one negative G on the hill without a push. On to human trials!
That smile means success!
I didn't like how the tall seat back in the car seat reacted with the riders head, so I made a new body from a durable storage container. I installed a car-grade seat belt through the frame and added padding for safety. There's also a board installed behind the seat cushion and I eventually wrapped the whole thing in thick gorilla tape for strength. Other safety features include testing with 1.5 factor of safety, caution tape around the track, always checking to make sure no one is near it when in operation, and always storing the cart away from the track.
I maaay have been even more excited than the grandkids.
I love how his hair flips up during the negative G section.
The Out-n-Back Backyard Roller Coaster
Type: Negative G
Height: 8' 6"
Max Velocity: 15 mph (est. 20mph pushed)
Recommended rider age: 5-9 years old
Recommended rider weight: 30-70 lbs.
Maximum G’s: 2G’s(Higher if cart is pushed)
Negative G’s: -.7G’s(Lower if cart is pushed)
Track Distance: 54’ (actual ride distance 125’+)
Materials Cost(est.): $480
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