The Physics of
Pinewood Derby
Indiana State University
John Swez, Ph D Presenter
1
What effects negate a winning race?
• Wheel/Axle Friction
including Friction
between the wheel and
the rail
• Aerodynamic Friction
• Loss of M g h
• Track Differences
• Excessive Wheel UnBalance
2
Read Your Rules! Rules Differ from District to District
Excerpts from Wabash Valley District Rules regarding Wheels and Axles
“The wheels must be from the official BSA Pinewood Derby Kit or official replacement
wheels.”
“The axles must be from the official BSA Pinewood Derby Kit or official replacement.”
“Axles must be firmly attached to the base of the car.”
“No suspension system may be used.”
“Wheels must not be altered in any way. Mold projections on the wheel treads may be
removed by sanding lightly but the profile of the tire may not be altered. The act of
“shaving “ the wheels will be grounds for disqualifying. “
“The wheels must be attached to the car with the axles (nails) provided in the kit(s). No
cars will be accepted in the District Derby where added fender liners that contact the
wheels, bearings, bushings, washers, hubcaps, solid axles or spacers are used as part of
the wheel assembly.”
“The official wheel width is 0.034 “. All four wheels must be vertically mounted on the
sides of the car.”
3
Winning Pinewood Derby cars from Pinewood Pro
4
Reducing Rolling Wheel Friction
Reduction of Friction on the axle surface and behind the head of the axle (nail)
The axle (nail) surface can be polished using the above jig and progressing in
the direction of coarse to finer abrasives.
Sandpaper coarse to very fine grit (400 to 1200 grit) is a suggestion. Then use
metal polish, jewelers rouge and comet (or similar abrasive cleaner). Add
water to make a thin paste.
5
Metals used in Pinewood Derby axles are
relatively soft. Nail and rod-type axles can
be hardened with heat. When heated to a
red-hot state and then immediately
dropped into a cup of water, the axles will
increase in hardness such that the
lubricant will have little or no effect on the
metal.
Axle
Warning: Adult Supervision Must Be Used!
Sequence
Remove Burrs and Form Taper
Grooving (optional)
Heat with propane torch
and Quick Chill in Tap Water(Hardening)
Polishing
6
Grooving (Optional)
The area of contact between the wheel and the axle creates friction. This area can
be reduced by creating a few grooves in the axle at the location where the wheel
touches the axle. Not only is the area reduced, but the grooves provide a reservoir
for the lubricant.
Grooving should be performed before hardening. Note that the act of “grooving”
seems not to be prohibited in the Wabash Valley District rules.
First place the wheel on the axle and make a pencil mark on the inside of the wheel
hub then remove the wheel.
Place the axle in the chuck of a drill, mount the drill securely and start it.
Using the file make two or three slight grooves on the axle between the axle head
and the pencil mark. Do not make deep grooves.**
** Note! Some people have used a tap to make threads inside the hub of the wheel
to reduce the area of contact. This has a disadvantage of the threads causing the
wheel to move from side to side. Under interpretation of the Wabash Valley District
Rules this may be illegal. “Wheels may not be altered in any way.”
7
Lubrication (use proper gloves for handling)
The Wabash Valley District Pinewood Derby Rules for 2010 on Lubrication:
“Lubrication – Only graphite powder or the new BSA white-lube T102/polytetra
fluoroethylene may be used to lubricate wheels. “
“NO GRAPHITING OF THE CAR PRIOR TO CHECK-IN IS ALLOWED INSIDE THE
BUILDING”
The following is taken from Pinewood Derby Times - - Volume 1 page 24
1. Insert a wheel onto the axle.
2. Hold the axle with the wheel hanging towards the ground.
3. Deposit some lube into the gap between the axle and wheel hub.
4. Tap the wheel gently to help move the lube down into the wheel hub.
5. With one hand firmly grasp the axles and hold the axle horizontal to the
ground.
6. With the other hand, spin the tire firmly, but not so much as to drop the
wheel/axle.
7. Repeat step 6, nine more time.
After performing this procedure the first time, keep the wheel axle pairs as a unit
until they are attached to the car.
8
Other Important Areas to Lubricate
1.Inner and outer
sides of the
wheel hubs
2.Inner and outer
edges of the
wheels
3.Wheel Tread
4.Car Body
5.Clean-Up
Only certain types of lubrication should be used. Check with your
District Rules
9
The placement of lead weights (Make Mgh as large as possible)
M g h (Mass in kilograms, g = 9.8 m/s/s and h is height in meters. Mgh is
potential energy measured in joules (a physics unit of energy).
I recommend using lead weights. Lead is quite dense, easily shaped and
inexpensive. On the downside, lead is toxic if taken internally. Use care after
working with lead: (1) wash your hands after handling lead, (2) Keep lead away from
food and food preparation areas, (3) Collect and use (recycle) Any remaining lead
pieces, and (4) Do not melt lead unless you have a professional do it.
In some cars with the rear axle slot nearer to the rear use this slot. You want the
rear axle allow the center of gravity of the car to be located closer to the rear end.
The optimal balance point is generally 1” to 1 ¼” j front of the rear axles. Center of
gravity can be found by balancing the pinewood derby car on a ruler edge.
10
Other Useful Hints
1. Use a #44 drill bid (available at your local
building supply or hardware store) for
relocating axles.
2. Blow all lubricant off (esp. if using dry
graphite powder) before taking your car
in. Do this step outside.
3. Use Elmer’s white or yellow glue for
wood to wood parts. Use quick drying
epoxy for weights, etc.
4. Pay attention to balance. Test your car
out on a makeshift track. Look for
excessive vibration, movement from side
to side.
11
Data Run
Using the Derby Magic1 Single Lane test track
1The
Derby Magic Company
2785 Walnut Lake Rd.
West Bloomfield, MI 48323
248-432-2754
After a week of work (sanding, filing, tuning up, assembly) the Pinewood Derby Magic test
track is operational.
Here is some first run data. Drop in to S-135 to see it operational.
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
2/26/2010
Date
15:47
15:48
15:49
15:50
15:51
15:53
15:54
15:55
15:57
15:58
2.1599
2.1587
2.1725
2.1618
2.1679
2.1649
2.1752
2.1693
2.1717
2.1717
Time (s)
First successful run
Car 59 un-lubricated Courtesy of Joe Harder
All data in seconds
Average
2.16736 seconds
Std. Deviation
0.00574
seconds
Standard Deviation of 6 milliseconds is not bad! It shows repeatability.
12
13
14
15
16
Two cars were made identical. Car 01 has a raised right front wheel
Car 02 is made normal
18
If you desire a copy of this PowerPoint
Presentation you can go to my web page
at:
newton.indstate.edu/swez
Look for a Link here
Derby Track in Room S-135 of the
Science Building at ISU