K’NEX roller coaster

I recently found some old video footage of a K’NEX roller coaster I designed and built around the year 1999. It predated YouTube by a considerable margin and never found its way onto the web, until now. Here it is (please forgive the last-millennium image quality):

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It was constructed from about 10,000 individual pieces, and although it was unremarkable compared with the creations of K’NEX superfans or with the more modern K’NEX roller coaster construction sets available, the aspect which made this design an interesting one is that the building set it used was quite basic: the classic K’NEX Roller Coaster 63030 Builder’s Challenge, which featured nothing more than the standard general-purpose rods and connectors augmented with a couple of simple extra parts for making the rails and the chain.

The original 63030 set featured 2,400 pieces with instructions for building two different roller coaster configurations – ‘spiral version’, and ‘loop version’. I recently tried building both of these on the kitchen table (much to the annoyance of my wife):

The box boasts “8 feet long! 3 feet high! 26 feet of track!”. All very impressive, and made for an exciting toy. However, knowing a few things about roller coaster design and the mathematics behind the track curves, the purist in me found himself somewhat dissatisfied. The loop was circular in profile. There was a discontinuity at the base of the lift hill. The curvature did not change smoothly along the track. It was only capable of handling a single car. Above all, it wasn’t nearly big enough!

This led me to investigate the possibilities of pooling several sets to construct something larger which adhered a bit more closely to real-world roller coaster principles. I don’t recall how long I spent tinkering with it, but I would guess it took me a couple of months including all the fine-tuning needed to make it run reliably.

These are the final stats:

  • Height: 6’11”
  • Ride duration: 47 seconds
  • Track length: 79 feet
  • Inversions: 2 (loop, corskcrew)
  • Cars: 3
  • Cars per hour: 180
  • 1st drop: 34″
  • 2nd drop: 21″
  • 3rd drop: 34″
  • Loop height: 18″

Cars first wait in a pickup area until caught by a hook on the chain lift:

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They are then pulled by the motor-driven chain through a short tunnel (whose true purpose is to hide excess track tubing!) and up a lift hill, taking 20 seconds to reach the summit:

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A motor provides the power to lift cars to the top. The motor was available as an accessory to the original set and appears to work fine with the much longer chain:

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A 180 degree turn off the top of the lift hill leads to the terrifying first drop and into the first inversion, a loop:

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As in most real roller coasters, both the drop profile and the loop exhibit curvature which changes gradually along the length of the track, increasing steadily to a maximum at the loop apex:

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Another turnaround leads to a second, smaller drop:

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Riders are then given a brief instant to prepare for another element inspired by real roller coasters – a banked drop into a corkscrew:

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The pace then lets up considerably with a slow turn into a gentle 720 degree helix:

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This opens out into a section with a long back-straight, which might make a good place for the loading platform of a real-world roller coaster:

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Especially challenging was the design of the car. I found it insufficient to copy the design straight out of the K’NEX instruction book, because it lost energy too fast, and could not twist freely enough about its long axis to negotiate the corkscrew element. You can see the high degree of twisting required:

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I experimented with several designs before settling on one that worked better for my purposes. The result was a longer car, stripped down to a mere chassis, with more rigidly constructed bogies joined by a looser coupling. Crucially, each bogie features an off-axis steering point which permits a continuous axle to join the left and right wheels. This allows the axle itself to rotate in its bushings, lowering rolling resistance compared to the original design which forces the wheels to rotate on stationary axles.

Original:

New:

 

K’NEX is a mathematically elegant building set. Its basic part are colour-coded rods in a variety of lengths, and a range of colour-coded connectors which snap one or more rods to each other. The ingeniuity lies partly in the lengths of the rods – each rod is √2 times the length of the next shorter one (when lengths are measured from centre to centre of the adjoining connectors), which means that diagonal cross-members can be added to a square lattice.

You might then think that all structures would necessarily turn out looking very rectangular, but there’s another crucial property that allows a curving roller coaster to be built – there’s a certain amount of flexibity in each connector, which means rods don’t have to join at exact multiples of 45 degrees. So a straight track can be made to bend gradually. Furthermore, combining rod lengths in differing ways allows for a range of curvatures. For example, here is how I transition from a large to a small radius of curvature in the loop:

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By offsetting the rods supporting one rail relative to those supporting the other, a twist can be put into the track, which is how the corkscrew element is achieved:

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I with to thank Jon Steele for invaluable advice and assistance regarding video file conversion, and Shannon Studstill for the entertainment provided by her adorably savage puppy.

And thanks for reading!

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