Shifting center of mass?

[mozartk626]

I just had a crazy idea yesterday, and was wondering if something like this has ever been done. My son’s first ever car won 1st place his Lions den. I made the car body very light, concentrated most of the weight very aggressively toward the far back of the car using a 3 oz low profile tungsten round with the slot in the bottom so the weight gets distributed over the top of the back axel, and I added a very small counterweight to the front of the car so the center of mass (balance point) was exactly 0.9 inches in front of the rear axel.

To my surprise, our car got clobbered in the pack finals by cars that looked very bulky with a lot of wood, which would have to mean their weight was nowhere near as far back as mine. WHY????

So I went and did some reading, and discovered that the precise optimal balance point (center of mass) is not universal, but is dependent upon the track being used. The point is actually a happy medium between the optimal poison of mass as far back as possible while the car is descending vs the optimal position in the center of the car once it is on the flat part of the track. Too far back and the car is too back heavy and slows down on the flat track. Too far forward and the car doesn’t convert enough kinetic energy to accelerate at the bottom of the slope.

So then I had this idea. What if I could have the best of both worlds? What if I could design a car that would start with the weight as far to the rear as it could possibly go, and then move forward a split second before reaching the bottom of the slope so that it advanced down the flat with a center of mass smack in the middle of the car?

My very foggy concept of this is that your weight would be a tungsten ball positioned at the top rear of a wedge shaped car. As the car hits the bottom of the slop and levels out horizontally, the ball would be caused by gravity alone to roll out of its position at the back, down a short ramp located inside the car body, and rest in a new position toward the middle of the car so that the center of mass has shifted from the back of the car to the center of the car during the race.

Has anyone tried anything like this?

[gpraceman]

I had heard of someone trying to do that somehow with a mercury capsule, only to have the capsule break open and leak out the mercury at the finish line. A hazmat team had to be called out to do the cleanup. Maybe that is a Pinewood Derby legend, but there could be truth to that.

It depends on your rules. If it states no moving parts, then that would preclude that idea.

EDIT: That was an actual event. https://www.derbytalk.com/viewtopic.php?p=2714 Too bad the news story is no longer available.

[exoray]

I have heard multiple stories of BBs/ball bearings and other unsecured weights breaking loose and making a mess, that is why some have the no moving weight rule...

EDIT: That was an actual event. https://www.derbytalk.com/viewtopic.php?p=2714 Too bad the news story is no longer available.

Here is the article...

https://web.archive.org/web/20040217081 ... ory/159199

[gpraceman]

I have heard multiple stories of BBs/ball bearings and other unsecured weights breaking loose and making a mess, that is why some have the no moving weight rule...

EDIT: That was an actual event. https://www.derbytalk.com/viewtopic.php?p=2714 Too bad the news story is no longer available.

Here is the article...

https://web.archive.org/web/20040217081 ... ory/159199

Thanks for digging up that article. Rather an over reaction if you ask me. I remember as a kid we played with mercury in science class and I didn't suffer any ill effects. <nervous tick ensues> lol

[Loud2ns]

Was the car that beat you from an experienced racer? I could probably beat quite a few with a bulky car because I know how to prep wheels and axles well. I would personally recommend focusing on that before exotic ideas. However, that would be a fun/interesting rabbit trail! At the high level of racing, we try to minimize vibrations in the car. I think a movable weight would inherently have vibrations.

Sent from my SM-G991U using Tapatalk

[whodathunkit]

Have you seen this one?
https://www.reddit.com/r/BSA/comments/1 ... rules_say/

[Loud2ns]

That's pretty creative!!

Sent from my SM-G991U using Tapatalk

[FatSebastian]

Has anyone tried anything like this?

"Doc Jobe", at the beginning of Chapter 12 of the book Physics of the Pinewood Derby, describes such a car and analyzes its performance via computer simulation. Instead of a tungsten ball, he considered a 3/8" lead (Pb) cylinder that rolls in a J-shaped channel in a wedge-like car (taller in the rear), so that the cylinder moved vertically inside about one inch during transition. He determined that such an internal weight shift ought to result in a meager one-half inch advantage at the finish line, but the added aerodynamic drag penalty from having doubled the cross-sectional area of the car outweighs any advantage of the moving weight. Even using a tungsten (W) rod greater than 1/2" diameter would not be enough to overcome the air drag of the contraption, so he determined that the "falling weight 'trick' ... is thus impractical." (p.384).

[whodathunkit]

I remember as a kid we played with mercury in science class and I didn't suffer any ill effects. <nervous tick ensues> lol

I remember as a hospital maintenance technician.. when ever we had the old mercury filled blood pressure units that spilled.
Hazmat would just put on rubber gloves and use bread to soke up the mercury for picking it up off the floors.