Heavy wheels would store energy, which is what we don’t want. They’d have to get their energy by stealing it from the kinetic energy of the car. The wheels should have as low a polar moment as possible.
Wouldn’t a three wheeled car have 33% more friction per axle?
Sam Stone:
Not allowed! Explicitly in most Pinewood Derby rules.
Lance:
Perhaps, but the larger frictional components are between the wheels and the track (tread and rail). The problem with three wheelers is one of alignment and I think the increase in friction of the three remaining wheels against the rail would more than offset the gain.
My guess is that a three wheeler would have a propensity to jump the track and you’re usually only allowed two such faults before your car is disqualified.
Weirdly, enough, many people advocate “three wheelers.” You can’t put it in the middle–that’s illegal–but they raise one of the front wheels just enough to keep it off the track.
All the other tricks are illegal–no moving parts, except wheels. And you can’t sand down the wheels to make them narrower.
Chronos
Was it a constant slope from end to end? If not, Robot Arm is right. Moving the center of gravity backwards will increase the effective potential energy.
A corollary of this, which a lot of people miss, is that the car should be the full 7 inches long.
You’re right, I should have been more clear… The entire track was at the same slope. Any change in slope would, of course, be relevant, not just a change to horizontal. I guess it’s a good thing that I didn’t know about angular momentum and moments of inertia way back when, because I would have lightened the wheels without realizing that it was illegal.
Why the full seven inches?
If I remember correctly, the starting mechanism consisted of three wooden pegs that pivoted up in the middle of each lane. You might be able to get a tiny advantage by having the front “bumper” (not that there’s really a bumper, but the leading edge of the body of the car) as high as possible and then tapering back into the underside of the car. That way, as the rods are folding forward into the track you might be able to roll forward a little bit sooner than a car with a low bumper.
::spin:: Am I in the wrong thread? ::shakes head::
Your center of gravity is moved farther back. Same principle. On the other hand, the spring loaded starting mechanisms are so quick, that any advantage of a high bumper would be of milliseconds, and would be probably more than offset by drag.
Yeah, I kinda realized that could be taken out of context.
In my PD days, the starting gate was hand operated, although I believe the spring had been invented by then.
I’d just like to state for the record that I have never seen a Pinewood Derby that was won by a car built by an actual Cub Scout.
That always kinda torqued my cork. If I let my boys build the car themselves they finished way back in the pack. If I “helped” them we would finish a little better, but when I “helped”, the boys ended up doing precious little hands-on work.
I usually “helped” because they would feel so bad comparing their own handiwork with the other entries. So we joined in the polite fiction that little eight-year-old Johnny turned out the high-precision, silky-smooth, low-friction racing machine all by himself, just like all the other little boys.
One pack even went so far as to have a dual event – one for the boys and one for the parents. That was better than usual, but there were still kids who brought last year’s winner in the parents’ derby as their entry in this year’s cub derby.
quote from pluto:
I’d just like to state for the record that I have never seen a Pinewood Derby that was won by a car built by an actual Cub Scout.
I actually won a PD by using the default car. That is, my car was disqualified because it wouldn’t run the track – I had added weight to the underside of the car and it didn’t clear the center guide-ridge. So I used the ‘default car’.
It was built by one of the older kids and was very crude. The ‘official’ block of wood had not been altered. The wheels were nailed on and lubricated w/ graphite. Weight was added by affixing pennies to the top with Scotch Tape.
And, of course it was decorated: A felt-tipped pen was used to draw circles with little X’s - (x) (x) - in them on the front for ‘head lights’ and the words Saturday Night Special were written (in poor handwriting) on the side.
I raced this car and won the Grand Prize, but wasn’t allowed to keep it since I hadn’t built the car. Oh well.
Hey pluto, I’m hurt…
Except for having a friend of my dad’s turn the wheels on his machinist lathe, I did everything else myself on my car (though admittedly, not without some good advice from my mechanical engineering dad). I even melted and poured the hot lead into the cavity in the car.
jeez, I haven’t thought about this for over 30 years, since my own pinewood derby entry. My dad discovered everyone else was adding weights, so he taped some flat lead weights to the bottom of my car. Unfortunately, it didn’t have enough ground clearance, it ran down the track about 1 foot and just stuck there. Now THAT is embarassment, I had the only car that did not finish the race!
So my warning is, just make sure you have enough ground clearance, if you add weights to the bottom of the car!
How can you build a car big enough to fit a kid yet only weighing a few ounces?
Tampa Flyer:
Pinewood Derby cars are about 6" long blocks of pine wood that the kids carve themselves into racing-car shapes. The are pre-drilled for axles, and run on four plastic wheels. You can add paint and lead weights, but that’s about it. Five or so of these run on a linear track that consists of a set f big boards with strips of wood nailed to it. The car’s wheels straddle the “track” and can’t move away from it. The first board s angled, so gravity can pull the cars down the track. The later sections are flat on the ground. No one rides in these, of course. It’s not like the Soapbox Derby. This is a Cub Scout activity, by the way, not a Boy Scout one. Or at least it was where I grew up.
For the record, I entered one of these. Didn’t win. My father did a LOT of work on mine, but he was an engineer and couldn’t help himself.
Actually, I’m pretty sure that you can add any non-moving parts you want, as long as you stay within the weight and size restrictions. See, for instance, stolichnaya’s comment about his plastic Batmobile flames. This may be different in different councils, though.
I think all the weight in the back, behind the rear axle, works best. With no active steering, the car actually “rattles” down the track by constantly bumping into the raised center section of the track with the inside of the front wheels. The weight in the back puts the minimum weight on the front wheels and allows them to slide sideways when they bump the raised center of the track. My son won the local event with a very “rear heavy” car, mostly by accident because that was the only place he could fit the weight in. Also file off the burr on the wheel side of the axle nail head and put graphite on the inside of the wheels where the rub on the track.
Thanks for the response, folks. I had noticed last year that most of the winners were long and almost flat. That’s what my son wants to do this year. Tres boring, in my opinion, but hey, it’s his car.
I do see a lot of cars every year that were quite obviously made by Mommy or Daddy. Usually it’s a Tiger Cub holding a hand-carved scale model Corvette that tips me off. My son insists on doing it himself; I just stand by to make sure he doesn’t cut his finger off.
Lots of good tips in here. Thanks again, guys!