tillius wrote:
Why would you want to weight the wheels of a Pinewood Derby car? Adding
weight at their periphery will increase the rotational inertial of the
wheels, and they will accelerate more SLOWLY than unweighted wheels. Just
the opposite of what you want. But I may be missing something...


Because the track we run on has a very long runout at the bottom of the
slope. I was thinking that the rotational inertia would cause the
wheel's RMP's to decay slower on the straight away.

You might just be onto something there. I'm thinking that maybe if the
car accelerates slower but rolls further because of that rotational
inertia in the wheels, then the overall energy losses due to air
resistance drag effects should be less than if the car reached higher
velocity during its run.



That, and moving
the weight from the body to the wheels would decrease the friction
between the wheel axels and the wheels.

Now that seems like it might work too.

I may be wrong, but since we've got another year to prepare, I was
thinking we could set up a small test track in the basement and try
different configurations. It would make a great science project for the
kids as well.

I sure hope you'll post the results here, I'm fascinated.

BTW, while I'm on the subject, is there a better lube than graphite
powder? We tried teflon and graphite this year and the graphite
definitely outperformed the teflon by a HUGE margin.

Tillman

I'm remember back 30+ years or so when I too was the "engineering
support" for our two son's Cub Scout Pinewood Derby efforts. Being the
only engineer father in a troop of kids with mostly doctor and lawyer
dads*, it was like shooting ducks in a barrel and the kids' cars always
brought home the bacon.

We weighted the cars with lead to within a quarter of a gram of the
legal limit, I made the kids a "calibration weight" out of screws and
nuts in a small glass jar adjusted on a lab scale at work.

I still remember the kids getting a useful lesson in "positional
authority" when the guy running the Pinewood Derby one year put one of
my kid's cars on the POS kitchen scale he was using, with no standard
weight on hand, and judged it was heavy by about an eighth of an ounce.

It wasn't worth making a row about in front of a bunch of youngsters,
but I did speak to the guy privately later in the evening and asked him
if he'd like to bet me $100 for charity that an accurate scale would
prove my kid's car was under the weight limit. He mumbled something
about having to go by the scale he was given and slunk away.

I'm assuming they are a little more sophisticated about weighing the
cars these days. Am I right?

Jeff

* The only reason we could afford to live where the doctors and lawyers
did back then was because I could fix our cars, house and everything
else the family owned (save for medical problems) while the other guys
had to shell out for all of those things. I'm still enjoying living that
way. G

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."