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At the large hotel where I work in the engineering department the big Halloween event is the annual Pumpkin Derby...and we've lost every year to the culinary department. It's embarrasing!

This year, though, we caught them in the act. Since they're in charge of distributing the pumpkins, they kept a half dozen or so for themselves, tricked them out, and we caught them testing the racers to select the best one. We, on the other hand, are limited to a single pumpkin. So I'm soliciting advice for building a hot-rod pumpkin!

The rules are that you may use any kind of wheels, from ball bearing casters up to wheelchair wheels. The catch is that they must be attached only to the pumpkin. You can use axles which go through the body of the pumpkin, but you can't have any kind of a chassis or even a tie wire to align them. That's what killed us in the past; our pumpkin wouldn't travel in a straight line and dissipated its kinetic energy in a turn. The pumpkin is let go from the top of a ramp to roll across the lobby floor, and the prize goes to the pumpkin which travels the longest distance from the base of the ramp. The floor is low-pile carpet.

My preliminary idea this time is to use casters such as you might find in an office chair, three of them. They have 5/16" threaded stems, and we have some threaded anchors intended for concrete. My idea is to use three of them, drilling mounting holes vertically into the base of the pumpkin, and to use a pane of glass to level them into the same plane and then use RTV or similar to glue them in place.

One challenge I'm looking at is how to keep the pumpkin from spinning as it rolls down the ramp. I had thought that by using a triangle of casters and starting it down the ramp heaviest end first we might have the most luck. But I'm open to suggestions.

Help the engineers beat out the cooks!

Edit To Add: Thinking about it, possibly the biggest challenge is keeping the pumpkin from spinning when it leaves the ramp. While accelerating the net force is directed forward, while when it's on the carpet net force is to the rear. We plan on using swiveling casters; perhaps a touch of hot glue on the one in the lead to hold it straight?

Here are the casters we plan to use:

(Casters we plan to use)

We also have these wheels available (only two, but we could use a caster before or behind):

4.10/3.50 wheels

Here's the ramp. Pumpkin needs to fit between first two lines. Long wheelbase is a problem:

Pumpkin Derby ramp

Preliminary components:

Derby racer components

ehbowen
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  • That carpet pile - would a thin disc give less rolling friction compared to a wider wheel? worth testing that. – Solar Mike Oct 29 '22 at 14:18
  • @SolarMike Will keep that in mind for next year, but right now we've limited time for testing. Still have a hotel to run...and the World Series is in town! – ehbowen Oct 29 '22 at 14:35
  • If you don't have time to take suggestions on board, then why post? – Solar Mike Oct 29 '22 at 14:45
  • @SolarMike Fair observation. I don't know where I could obtain a suitable thin disc on short notice, but I can check hardware stores. Thinking that it would work best to have a pair on an axle going through the pumpkin with one of my casters positioned either before or behind the disc wheels. – ehbowen Oct 29 '22 at 14:55
  • Checked around. I can cut a thin disc out of aluminum diamond plate with a circle cutter. Or I could use the tired wheels in my second photo. – ehbowen Oct 29 '22 at 15:08
  • fill the pumpkin with water and freeze it – jsotola Oct 29 '22 at 17:54

2 Answers2

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@SolarMike's point is good. Narrow wheels would have much less drag on carpet. I'd try something like compact discs.

enter image description here

The tail-dragger style rear wheel extends the wheelbase which would give much greater directional stability than a short wheelbase.

Get going with the 3D printer. You've only a couple of days!

With the new constraint of maximum length you may need to start with the tail dragger pivoted under the pumpkin somewhat. The challenge then becomes how to ensure that the rear wheel pulls out. This may be possible by adding a little contact brake (rubbing on the side of the wheel) until it's fully out. This may be easier to implement on a 4-wheel design.

enter image description here

A simpler approach for the three-wheeler would be to cut a wheel-arch and inboard the third wheel. Straight-line steering may suffer.

enter image description here

Option 3.

Simple setup. High mass on the axles keeps the centre of gravity low. Keeping the diameter of the axle weights small minimises the moment of inertia (which would reduce the initial acceleration). The extra mass shouldn't affect the speed leaving the ramp but should give more inertia to fight the carpet resistance.

Transistor
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  • These renderings make it seem like it would roll over down the slope due to being top heavy. I think you need the front wheels, whatever the arrangement, need to be extend a bit farther in front but that would require a frame or something. Or maybe tilt the pumpkin backwards a bit or place it horizontally. Depends on the pumpkin. – DKNguyen Oct 29 '22 at 18:14
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Here's what we eventually came up with:

Pumpkin Derby Racer

I know it looks off-kilter, but first priority was mounting the 'tailwheel'. Once it was in and the glue set we drilled it for the allthread axle, then mounted the disc wheels. A few test runs, then we used some hot glue to align the tailwheel straight.

Keeping it in the refrigerator till Monday. I feel somewhat confident. Thanks for the help!

Edit To Add: Almost race time. The stylists didn't consult with the engineers, so it'll have to go down the ramp backwards!

Pumpkin Derby Racer decorated

ehbowen
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