I need to make some custom shaped lead weights that must be balanced
and fit precisely inside a small plastic wheel, the kind you might find
on a pinebox derby car.
I've tried making a casting of the inside of the wheel to use as a
pattern so I could make molds of the part and cast them out of lead,
but I've tried using plaster of paris, wax, clay and lead to cast the
Plaster of parris disintegrates when I try to remove it from the wheel,
as does the clay when allowed to dry.
The wax and wet clay deform horribly when I remove them from the wheel.
The best success I has was lead, but the hot lead did melt the plastic
wheel enough to deform it.
When I made the final lead castings, they were still deformed enough to
deform the wheel when inserted. This and the fact that the weights were
unbalanced lead to a speed destroying oscillation.
If I can cast a better patters, I should be able to case weights that
are a lot more accurate then use the lathe to finish them, but I've
still got to figure out the balancing part, and, how dangerous would
the lead chips be?
I also thought about just turning a lead cylinder of the correct size
and boring out the center, then slicing it up into the individual
Any suggestions on getting this done right?
Okay try this...
Use putty style silicone mold material (it mixes by squishing it together).
Put the mixed material into the wheel and let set (about 10 mins). Remove
model once set by simply pulling it out.
Now use this mold to create another mold using more of the putty. Simply put
it on a flat surface, mold the new lump over it and let set. (Oh side note,
powder the model or it might stick permanently to the mold body.)
You should now have a mold of the correct dimensions of the wheels inner
spaces ready to receive molten lead (or lead free white metal/pewter) that
can withstand the low heat and be reused a great many times.
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. That, and moving
the weight from the body to the wheels would decrease the friction
between the wheel axels and the wheels.
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.
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.
As a former Cub Scout pack Master I must advise you that modifying the
wheels other than truing the roundness is not legal. We added unlimited top
fuel and funny car classes for the dads that couldn't resist meddling to
much in their Cub's design. :-)
Yeah... but you'll still need to machine it in place to balance.
And don't let the cub scout handle the stuff unsupervised - same goes
for the lead really.
4 wheel weights of lead may easily exceed the 5 oz limit... what about
using some more ordinary engineering material such as free-machining
Well, let's be realistic. Handling lead isn't going to make 'em start
Drill holes in the underside of the body with a spade bit, and cast the
lead directly into them. Drill to reduce to legal weight.
I ws thinking of only weighting the two rear wheels, since real
location of weight has the greater potential energy.
The samples we did in testing this year, we were able to get up to 5 oz
by just weighting the rear wheels.
Well, it's not really Cub Scout pinebox, it's AWANA Grand Prix. Same
cars, etc. And you can't modify the wheels here either, but you can
place the wieght anywhere you want on the car. My test car passed the
legal check this year.
Oh yeah, other than casting the weights, putting the weight into the
car when we weighted their original designs, and the bandsawing, my
kids do all the work themselves, I like to offer suggestions, but then
they have to do it and test it and make the final decisions. They learn
a lot more that way.
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
Now that seems like it might work too.
I sure hope you'll post the results here, I'm fascinated.
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?
* 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
I think it's a bad idea to weight the wheels but go ahead and try
it. Experimenting with these things is good for the kids.
I'm still disappointed that the local club did away with the
unlimited adult class. It was a lot of fun and allowed the kids to
learn. Of course part of the reason they outlawed it was because of
me. I always had some outrageous design which looked sure to win. The
funny thing is that only one of my cars ever won the race and it was a
powered car thrown together in about 30 minutes just before the race.
I never had a lot of time to work on many of them since I was also in
charge of setting up the track and much of the other work that went
into the race. The powered car was simply sawed out quickly on a chop
saw to get rid of excess wood. Gutted a old VCR for the motor and
friction tire to run on one of the car wheels. Clipped three 9volt
batteries together. Wire a on and off switch and a micro switch on the
front for the starting gate to keep the motor off till the gate
dropped. Some sheet metal and screws to mount the motor and a bunch of
hot melt glue to hold everything else. That car won the race. Funny
though they never did have a unlimited class race after that. :-)
But it sure was fun to race the 5 lb ball bearing wheeled car even
if it never did win. I took a piece of 1 1/4" pipe. Forged one end to
look kind of like a DC-3 nose. Put steel axles through the middle.
Filled full of lead (and I mean full with the car being the max length
allowed). Made wheels for masonite pressed over ball bearings. That
car made everybody stand up and notice. But it never won a single race
even after I melted all the lead out to reduce it's weight. It did
great on the straight away but took way to long getting up to speed on
the slope so it always lost.
The first year I lost to a guy who took his old cub scout propeller
driven rocket and made a base for it. He used standard derby wheels
and filled the base full of lead. But the prop gave him enough of a
boost to beat me.
After they outlawed the unlimited class I concentrated on design.
Helping my daughter come up with designs that brought home at least
one trophy a year (some years we brought home 2 or 3 what with the
adult class and more than one car entered into it). In fact some cars
just kept bringing home trophies for years since we'd enter the old
cars into the adult class (we had to pay to do it so it was for a good
The last few years the local Awana's club has gone to the boat races
instead of the cars. In this case we kept bringing home race trophies
since I taught my daughter how to blow them properly plus I always
streamlined the boats. In fact my standard design for the boats is a
tri-maran with much lower water friction that most boats. But we never
got a design trophy for any of them.
Looks like I won't have to worry about that this year though. Since
my daughter got older and decided to not do Awana's this year. I may
end up being roped into the race again though since the water trough
tracks are stored at my place (I built them from my own money).
Actually the finish gate for the car track was built by me as well
though somehow it ended up being stored somewhere else the last year
or two of the race.
Sounds great to me.
Moly is what you want in powdered form. Harder to find than graphite
but much slicker.
Yep, every car is weighed on the same digital scale which is calibrated
from use to use with a 'standard' 5 oz weight to make sure it's
calibrated the same no matter which day the kids register their cars.
Cars also have to be registered no later than 1 week before the race
and they are confiscated until race day. The guy in charge doesn't have
any kids so their's no partiality involved.
And if one of the kids car is over weight or under weight, we have a
few drill presses setup and either one of the volunteers can adjust the
weight or one of the kids parents can.
And I'll be happy to post the results once we start testing.
Heh, my design this year had a high spoiler in the front then swept
straight back to drop down just before the wheels.
The spoiler rested against the starting pin, but as soon is it cleared,
my car was rolling before the pin cleard the fronts of the other cars.
I'm hoping not too many noticed it, my son wants to incorporate that
into his car next year.
When my son cubbed, we drilled out his pinewood body, and filled it with #7
shot, with a quick-release "Moog Headers" decal over the hole, so we could
reduce the weight if the officials' scales weren't exactly the same
calibration as ours.
I engineered, but the kid actually did the cutting.