Pinewood Derby
Performance Design
An introduction to making a
high performance
Pinewood Derby car
Copyright 2003 by Stan Pope, all rights
reserved.
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Based on information presented
at the author's website at ...
 Pinewood Derby Design
http://members.aol.com/standcmr/pwdesign.
html
 Learn to Build a Winner
http://members.aol.com/standcmr/lbw_apl.ht
ml
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This presentation may be
copied and presented freely
provided that it is unchanged,
the copyright notice is
displayed, and no fees are
charged for copying or
displaying of the contents.
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Basics
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To win a pinewood derby race,
your car must get from the starting
line to the finish line before the
other cars.
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Quickest:
Accelerate early
•Attain maximum velocity
•Hold speed through finish line
•
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Make or Break
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Follow your district's rules.
If you can't pass inspection,
you can't race.
If you can't race, you can't win!
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Assure Ground Clearance.
Failure:
Grinding stop
Dramatically flying off the track
If you can't get to the finish line,
you can't win!
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Assure that the car will "run" on the
track.
A narrow or pointed nose may not
stage correctly on the starting
mechanism and may not trip the finish
line sensor correctly.
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Top 10 in district
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Make sure that the wheels are
in balance,
in round,
moving freely, and
correctly aligned.
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Maximize weight vs. wind drag.
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Optimize weight distribution.
As the center of gravity moves
farther back in the car, there is
more energy available to be
converted into speed.
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Some Guidelines
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Car as long as rules allow
Rear wheels as far back as rules allow
Front wheels forward as far as rules
allow, without affecting rear wheel
location (longest allowed wheelbase)
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Car's center of mass as far back as car
stability allows, and car's center of
mass as low as possible, but "as far
back" is more important (maximize
potential energy)
Wheel alignment "dead-on"
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Weight as close to maximum allowed
as possible
Car's cross section as small as possible
Wheels "in round", balanced, and all
sliding contact surfaces polished (hub
and inside wheel edge)
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Axle contact surfaces polished
Hub contact area as close to wheel axis
as possible (minimize breaking torque
due to wheel-body and wheel-axle
friction)
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Fundamental Energy Equation:
Potential Energy at start Lost Energy =
Kinetic Energy at finish
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In other words ...
Start with as much potential
energy as you can, and
waste as little of that energy
as possible.
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Losses from:
Friction between wheel and axle
•Friction between hub and car
•Friction between hub and nail end
•Air friction
•Wheel vibration
•Body oscillation (wheels out of round)
•Wheel rolling friction
•Wheel sliding friction on track or rail
•Angular acceleration of wheels
•
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Part of this presentation is oriented
around this idea ...
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Regardless of the lane in which
you race, you should race in:
The
The
The
The
steepest lane,
shortest lane,
smoothest lane, and
slickest lane
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Then, you will be racing in ...
The fastest lane!
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But if you don't get to pick the lane,
how do you make sure to get the
fastest lane?
By what you do to your car!
That is why this presentation exists.
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Modern Track
Starting Line
Finish Line
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Race in the
Steepest Lane
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Which car wins?
Which car is
fastest at the bottom?
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Why?
Gravity!
Gravity and the mass of the car
create a force along the line of the
track.
If the track is steeper then more
of the gravity force goes to pull
the car forward.
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Car A has farther to fall,
so it has more potential energy to
turn into speed.
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How do I make my lane steeper?
Physics says that “my lane” is the
path followed by my car's center
of mass.
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Center
of Mass
a
b
Ha
Hb
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It isn't about weight in the back
pushing the car down the track...
Or about weight in the front
pulling the car down the track.
It is about how far the car drops
as it goes down the track.
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The amount of drop determines
the amount of “potential energy”
that can be turned into speed.
With modern tracks, drop
increases as center of mass is
moved farther back in the car.
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Make your car as long as rules
allow.
Locate the center of mass
(balance point) as far back as
stability allows. “Stability” includes
“sticking to the track” and not
sliding sideways.
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This applies to modern tracks
which start on a slope and flatten
out toward the finish line.
This does not apply to older style
tracks which have constant slope
or which have both increasing
slope near the starting line.
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Race in the
Shortest Lane
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Which car wins?
A
B
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Set axles so that car runs straight
and level.
Align as required.
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You may analyze the track and find
that raising the CM reduces the
distance that it must travel. Right!
But this usually fails because the CM
is so far from the wheels. You must
be on a very smooth track, and your
wheels must create very little
oscillation.
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Race in the
Smoothest Lane
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Which car wins?
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How does the car's center of mass
move as these wheels roll?
A
B
C
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Which car handles bumps best?
A
B
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Wheels round
Bore centered and smooth
Tread smooth
(but not polished!)
Wheelbase extended
(but not too much!)
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Race in the
Slickest Lane
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Identify the frictions and losses.
Make frictions as small as possible.
Move friction to where it hurts least.
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Are these criteria always true?
Well, no, they aren't. There are
some exceptions. However, the
exceptions are very few.
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When aren't these criteria true?
Different track styles
•CM too far back
•Front wheel too far forward
•
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Some Key Steps
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Preparing Axles
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To hold nail while filing
sholders near nail point
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Then chuck in drill and
clean under nail head.
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Preparing Wheels
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A drill press can work as a lathe.
Wheel is held in a mandrel
and rotated as it is moved
past a cutter.
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Here, the cutter is held in a piece
of wood.
The cutter assembly is held, by
hand, against a guide bar.
The cutter is moved gradually closer
to a stop block on the guide bar.
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Of course, sandpaper against a
block of wood can also be used.
Make sure the block is square!
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Body Preparation
Drilling Axle Holes
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Here is a homemade drill guide
to help keep the holes
perpendicular to the car body.
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Layout car body plan on block.
Locate holes.
Drill first!
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Here is a tool that is supposed
to produce perpendicular holes
even with a hand drill!
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Producing complex shapes using
Cub Scout age abilities and tools.
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Notes:
You can still see the original picture.
All the cuts are across the grain of
the wood.
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Now ...
Chip the pieces off to leave a rough
surface in the right shape.
Rasp and sand the surface smooth.
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Closing
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Admonitions
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Hot Lead:
Severe burns possible
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Lead:
Lead is Poisonous
Lead often has nasty additives
Minimize contact.
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Melting lead is discouraged!
Supply your builders with lead slugs.
Hold lead slugs in pliers.
Pound lead slugs into desired
shape with hammer.
Glue in place with 5-minute epoxy.
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Rules change from time to time.
Review the rules carefully each year.
Especially check the
“boundary conditions.”
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Don't believe everything the
“experts” tell you!
Smile, say “Thank you,” and
go home and check it out!
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Help your builders get the most
out of their efforts.
Share what you have learned.
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Good Racing!
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