Ed Huntress wrote:

Of course, the long-wheelbase deal is the real biggie, but I don't like
seeing them without a special class. They're sure to win if they're done
well.

Don't you have a bounding box rule?

Saw a kid have to carve off a nice spoiler he'd added to the back
because it was outside the box...

wrote in message
oups.com...
Ed Huntress wrote:

Of course, the long-wheelbase deal is the real biggie, but I don't like
seeing them without a special class. They're sure to win if they're done
well.

Don't you have a bounding box rule?

The bounding box isn't the issue. A long-wheelbase car, done properly, will
fit in the bounding box.

In packs and districts where they really get into this, they have a separate
class for long-wheelbase cars. They're MUCH faster. The wheels go right out
to the corners of the bounding box, and to the corners of the car.


Saw a kid have to carve off a nice spoiler he'd added to the back
because it was outside the box...

Too bad. I've thought about trying an airfoil on top, one that lifts rather
than presses down, but I've restrained myself. g

--
Ed Huntress

jim rozen wrote:

In article .com, tillius
says...


Let me know what it is?


The three wheel one. You rig the car so that
one wheel is off the track. This makes a
huge difference in the time, and it's legal
as far as I know.

JIm



What principle makes that work? Is it not having to use up potential
energy by turning it into "flywheel energy" in the non-touching wheel?

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

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

removal of a friction source - actually 2 friction sources - friction
of that wheel against the track - uses some of the energy as heat.
friction of that wheel against the axle - more loss due to heat.

Tillman

"tillius" wrote in message
oups.com...
removal of a friction source - actually 2 friction sources - friction
of that wheel against the track - uses some of the energy as heat.
friction of that wheel against the axle - more loss due to heat.

But you still have the same load, distributed now across the area of three
bearings instead of four. Specific friction (friction per square mm of
bearing area) goes up...but how much? How linear is the load/friction curve
for mild steel against acrylic, or whatever it is?

Hmmm...maybe it's time to bow out and leave it to the kids...

--
Ed Huntress
(remove "3" from email address for email reply)

Beeman.

"Karl Vorwerk" wrote in message
...
Bee Firearms has moly.
Karl

"Wayne Cook" wrote in message
...
On 7 Nov 2005 07:56:27 -0800, "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. That, and moving
the weight from the body to the wheels would decrease the friction
between the wheel axels and the wheels.


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
cause).

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.


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.

Sounds great to me.

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.

Moly is what you want in powdered form. Harder to find than graphite
but much slicker.


Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm

In article , Ed Huntress says...

But you still have the same load, distributed now across the area of three
bearings instead of four. Specific friction (friction per square mm of
bearing area) goes up...but how much? How linear is the load/friction curve
for mild steel against acrylic, or whatever it is?

Consider the limiting endpoints where you have some *huge* number
of wheels, as many as would fit inside the bounding box. You
just know that car's gonna be a dog.

I think the simple answer is easy - you just run a car and time it.
Then take one wheel off and run it again. Just have the weight
rigged so it will run ok with three. If it really makes a difference
then it should be easily visible in the times.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"jim rozen" wrote in message
...

Consider the limiting endpoints where you have some *huge* number
of wheels, as many as would fit inside the bounding box. You
just know that car's gonna be a dog.

I think the simple answer is easy - you just run a car and time it.
Then take one wheel off and run it again. Just have the weight
rigged so it will run ok with three. If it really makes a difference
then it should be easily visible in the times.

Well, that's pretty much what we did, on our 16-foot test track. There was
no difference that we could measure.

But one car doesn't control for a lot of variables. So I don't know. I *did*
test weight placement, wheel camber, toe, etc. No toe; center the weight;
camber didn't have a conclusive result; straight-running on a flat floor was
the best indicator of speed on the track, all else being equal...which
stands to reason, because friction of the wheels against the center strip
seems to be the limiting factor, after wheels and axles are fully prepped.

--
Ed Huntress

tillius wrote:
The three wheel one. You rig the car so that
one wheel is off the track. This makes a
huge difference in the time, and it's legal
as far as I know.

Yep, it's legal. And it's funny too. A lot of parents will accidentally
get the wheels aligned that way then come to the workshop and ask us to
help them fix the car so all the wheels are touching. So we do .

D'oh... and to think we always shunned the nail slots which provide
this capability and sought out a drillpress to drill aligned holes
instead!

I recall a friend roughing something out with a pocket knife while his
dad was out of town, and doing suprisingly well given the crude results
- perhaps this is why. Having the event have an ability to ignore
common parental input can be an advantage...

At the time, I was wondering if leaving the mold flash on the wheels
was the reason - started to wonder about evening it up rather than
removing it.

(Despite storing the official track at our house, my dad wouldn't use
it for test runs - made our own 10 foot test track)

On 7 Nov 2005 07:56:27 -0800, "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. 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.

Tillman

At a bearing store I saw a spraycan of dry film lubricant based on
molebdynum disulfide, and I think it has some teflon in it as well.

I bought a can. When the stuff dries, it stays put on the metal,
doesn't come off on my fingers. It is seriously slippery stuff!

I cleaned up a 40-year-old Palmgren drillpress vise and lubed the
"ways" and screw with this stuff. WOW!!!! I put a knurled knob
on the screw in place of the little crossbar, and now I'm thinking
about putting a little spinner on the knob! I can run the vise
open and closed from end to end just by dragging my sleeve on the
knurled knob, though the vise is snug enough that there is
essentially zero wobble and play in the vise.

(This little vise is used on the bench for small things, rather than
on a drillpress)

On Mon, 07 Nov 2005 15:25:31 GMT, "Greg Krynen"
wrote:

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.

Lead melts at about 640F. Some silicones will take that .... but you
might have better luck with 30/70 body solder. It's almost as heavy
as lead and melts at considerably cooler temperature.

You could cast your weights directly into the wheels using CerroBend.
It's not as heavy as lead, but it's pretty heavy -- and it melts at
157F. It's specific gravity is 9.4 vs lead's 11.35. CerroSafe goes
at between 168 and 190F, probably about the same density. McMaster
Carr carries these products.

On 7 Nov 2005 08:49:09 -0800, wrote:

Pete C. wrote:

You might want to try some of that "fixturing alloy" you can get from
MSC/Enco/McMaster. Since it melts at such a low temp you might be able
to directly cast it into the plastic wheels.

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.

CerroSafe was developed specifically because it is safe for kids to
handle, as in casting toy soldiers.

Don Foreman wrote:

CerroSafe was developed specifically because it is safe for kids to
handle, as in casting toy soldiers.

No, it's not. It's 37.7 % lead and 8.5% cadmium, worse actually than
CerroBend.

The "safe" refers presumably to slight initial shrinkage allowing a
duplication casting to be removed without cracking the part you are
using as a mold, allegedly it then expands back to the original
dimension.

The plain tin/bismuth alloys (example CerroTrue) would be less toxic,
but the lowest melting point there is 281 F, so you can't use the water
bath and as a result have to be carefull not to overheat.

wrote in message
oups.com...
Don Foreman wrote:

CerroSafe was developed specifically because it is safe for kids to
handle, as in casting toy soldiers.

No, it's not. It's 37.7 % lead and 8.5% cadmium, worse actually than
CerroBend.

It might have meant safe temperatures.