This simple remote control model of my favorite amusement ride as a kid (and even now) comes with a twist: The spinning platform doubles as a spinning top!
About this creation
Please feel free to look over the images and skip the verbiage.
The ever-popular amusement ride commonly known in the US as the Round Up was always my favorite as a kid and remains so to this day.
Here, I combine a simple remote control model of the Round Up with my current obsession with spinning tops by designing the "wheel" (spinning platform) to double as a finger top.
Frank Hrubetz introduced the original Round Up in 1954. It became an instant sensation, and Hrubetz went on to build 24, 30, and 42 seat versions, the last under the name "Super Round Up" (2011 off-ride video with excellent text here). Alternate names for the Round Up include "The Bamboozler", "Satellite", and in the UK, "Meteorite" and "Meteor". Well over 100 Round Ups remain in operation worldwide.1
In 2003, Dartron Industries introduced "Zero Gravity", a modernized all-hydraulic 33-seat version that folds itself out of and back into a dedicated trailer.2 (Good off-ride videos here and here.)
Since my LEGOŽ Round Up holds only 4 riders (as opposed to the usual 24-42), I like to think of it as the personal Round Up (PRU) I dreamed of as a kid.
∨ The Round Up has drawn casual thrill-seekers of all ages and walks of life for at least 2 generations now.
∧ Some, however, may be aboard in an official capacity.
∨ Riders about to defy gravity find reassurance in operators that exude professionalism.
Mechanically, my little PRU seems to be something of an original Round Up-Zero Gravity hybrid.
The original Round Up and PRU use 2 electric motors -- one to spin the wheel and another to elevate it. Zero Gravity's single motor runs a hydraulic pump powering both spin and elevation.
∨ Linear actuators elevate the wheel in all 3 cases, but the real ones are of course hydraulic.
∨ The PRU's upper motor spins the platform; the other raises and lowers it.
∧ The less expensive M motors that later replaced the original Ls work just as well.
The PRU's mechanical similarities with the original Round Up pretty much end there. The latter's simple spin driveline consists of motor driving a rubber tire applied to the outer rim of the wheel, whereas Zero Gravity's wheel is driven by a hydraulic motor acting on its central axle.
∨ The PRU's wheel is also center-driven, but by shafts and a 2-stage gear train instead.
∨ A single universal joint between the spin motor and the 1st transmission stage carries mechanical power across the elevation arm hinge. Unfortunately, the PRU's spin driveline amplifies the jerkiness inherent in LEGOŽ U-joints -- especially at low speeds and high elevation angles.
∨ The klaxon heard in the video at 0:47 reminds the operator to stop at ~75° elevation to avoid serious binding of the U-joint. Real RUs stop at ~50°.
∨ The small tan gear on the wheel axle beneath the red wheel gear serves only as the wheel's main bearing when the wheel's on the ride. This shot also shows how the wheel "seats" are attached.
∨ Never cared much for the Power Functions IR Speed Control handset (8879), but I strongly recommend using one to operate the PRU, as the 3-state handset (8885) makes it all too easy to overshoot safe elevation angle limits (0° and ~75°) and puts anything approaching a plausible spin or elevation rate far out of reach.
∧ My favorite modification of the Speed Control appears above.
Spin rate scaling
From online videos, I gather that real Round Up and Zero Gravity wheels spin at ~20 RPM near maximum elevation. At the 4.4 m radius of a rider in a middle seat on Zero Gravity, the centipetal acceleration provided by the seat back at 20 RPM is 19 m s-2, or ~2 G.
At the 1:40 minifig scale I'm assuming here, the PRU's rider radius of 0.060 m corresponds to a 2.4 m radius at human scale. The wheel speed resulting in a 2 G centipetal acceleration at 2.4 m is ~27 RPM, which scales back down to a painfully slow 4.3 RPM at 1:40.3
The bad news: With an M or L motor still on the spin driveline, it would take a major redesign of the entire PRU undercarriage and elevation arm to achieve the 14:1 final reduction needed to achieve 4 RPM at the lowest Speed Control setting (±1/7).
A slower XL motor would bring the final reduction down to a more manageable 7:1, but drastic undercarriage changes would still be required.
The good news: Minifigs are virtually incompressible, have no working fluids like air or blood to worry about at high Gs, and are generally a pretty stoic lot, so why settle for a measly 4 RPM?
∨ The 61-322 RPM wheel spin rates available with a Speed Control and the ridiculously high gearing below (1:1 final ratio) correspond to a uniformly fatal 386-2,037 RPM at human scale. (Guess I should've done the math beforehand.)
∨ The quickest fix -- reversing the 12- and 20-tooth gears beneath the elevation arm -- changes the final ratio to a 2.8:1 reduction and cuts the available speeds to 22-116 RPM.
That 22 RPM minimum speed still scales up to a deadly 139 RPM, but I'm sticking with it. It almost looks plausible, and if the minifigs don't like it, they aren't letting on.
∨ The wheel lifts out of the elevation arm to become a smoothly spinning finger top. The process starts at 1:45 in the video near the top of the page.
∨ The simple transformation begins by replacing the tan wheel bearing on the underside of the red wheel gear with the conical red tip assembly shown on the right below.
One then pushes the tip assembly upward until it's firmly seated against the bottom of the 40-tooth wheel gear. The last step is to remove the red 2x2 dome covering the upper end of the wheel axle and attach a suitable stem instead.
Time to give the top a twirl...
Someone who hasn't spun a top in a while will have trouble with this one, as its 136 mm rotor diameter, 95 g mass, and very peripheral mass distribution make for a rather high axial moment of inertia.
I routinely get release speeds of ~400 RPM and spin times of 8-10 sec -- but only because I've spun literally tens of thousands of LEGOŽ tops in the last year. Considering the top's high center of mass, I'm quite happy with those spin times.
Though nearly 4 times faster than the PRU's maximum wheel speed, the minifigs seem to take my top speeds in stride.
342x134x162 mm (LxWxH) including wheel, excluding marquee
Spin final drive ratio:
Available spin rates:
22-116 in 6 steps with Speed Control
Modified LEGOŽ parts:
2, 1 each for spin and elevation
Electrical power supply:
7.2V from 6 NiMH AAAs
Modified LEGOŽ parts:
122x136 mm (HxW) with tip
Manual release speed:
Manual spin time:
Modified LEGOŽ parts:
1 Most of the info about the original Round Up and its direct descendents, the Hrubetz Meteor and Meteorite, came from the excellent Roundup page by Australian amusement ride owner and operator David Burton. Burton posted a mechanical drawing from Hrubetz' 1964 Meteor patent here.
3 The scaling of speeds can be tricky business. The most common scaling (the one used here) insures that the magnitudes of the dominant forces involved (here, centripetal force and weight) retain the same ratios at full and model scales.
If Lf denotes a characteristic dimension of some full-size object, and Lm, the corresponding dimension in a scale model of that object, then the length scale factor is λ ≡ Lf / Lm. Speeds U then scale such that Uf / Um = √(λ). Any system of units can be used as long as it's applied consistently.
Quoting Nerds forprez
Man the photos are spectacular. You chose the exact correct shutter speed. Just enough blurring of those that are spinning their guts out with the background in great focus. Well done!!
Still no sign of my original reply, so I'll try again: Thanks! The fact that the wheel spins way too fast made it a lot easier to capture the blurring with reasonable exposure settings. My habit of shooting MOCs with small apertures (f/22 or so) isn't always a good one, but it kept the background sharp here.
Is THAT how fast I was going on that ride the other day? I think I should be dead right now. But in all seriousness, this is so neat! What a great way to fit a popular ride into playable Lego; as always, fantastic. Making it a detachable top is a clever combination of your skills, too.