by

Clifford W. Lazar
CLIFF[shift+2]LAZARDEV.COM

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 My first car design came in second place. Then I learned the shape of the racetrack. My second car design finished first. The competing fathers were aerospace engineers and mathematicians. My opponents’ cars were tested in TRW’s wind tunnels.

Operating Environment

The Pinewood Derby Cars travel downhill, initially at about a 45-degree angle, through ambient air, not in a vacuum. The track is composed of two parallel runways, each with a raised center median. The wheels of each car are placed straddling the median of their runway.

The cars are held in place, before the start, by a two-tanged vertical fork that is pivoted forward and away from the front of the cars, until the fork tangs are below the respective medians. The tangs move through two slots in the respective medians.

Air Drag is a Minor Factor

There is a small air resistance that means a 5.0 oz. car will travel faster than a 4.5 oz. car, ceteris paribus. At the same time, since the cars don’t exceed 15 to 20 mph, the relative air drag resistance between two cars is a minor factor, contributing little to speed. Wind tunnel testing, at hundreds of miles per hour, is meaningless.

Major Factors: Rolling resistance and Angular momentum

I have tested, motor oil, graphite, and 3-in-1® oil. Motor oil is too viscous, graphite is the second most viscous. 3-in-1® has the least viscosity. It also won’t last from one heat to another. The pinewood derby car manufacturer recommends against using 3-in-1® because it allegedly deteriorates the plastic in the wheels. We don’t care if the wheels only last 12 hours. Actually, they last for years. If you don’t oil the wheels before each race, they will slow down.

Axles

An often-ignored factor is the drag caused by the axles, which are four cheap nails. I spun the nails in a hand drill bit and used emery cloth to eliminate longitudinal ridges and replaced them with parallel V-shaped ridges (WWWWW). This reduced the surface area of contact between the axle and the wheel to nearly nothing. The wheel rides on 3-in-1® oil. It doesn’t touch the axle.

You want to have the wheels as close to the body as possible to avoid wobble. If the wheel hub rubs against the car body, it will lose energy to friction. Polishing the body as smooth as glass and lubricating the body with 3-in-1® Oil can minimize this.

Angular Momentum

A major factor for the travel time of the car is the angular momentum. If the car doesn’t go straight, the wheels will ride up on the median strip, converting forward speed to chattering increased height. You can hear the losing cars chattering their way down the track. The car manufacturer incorrectly recommends that the extra weight be placed in the center of the car. This is like putting a mid-engine roadster in a drag race. Drag racers are long. They try to maximize their angular momentum. Mid-engine sports cars try to minimize the effort required to turn.

I placed the fish weights evenly in two pairs of holes drilled in the front and back of the car. This created a barbell effect, maximizing the angular momentum. My car wanted to go straight.

Winning the Race Before It Started

Most of the hotshot fathers shaped their cars to look like formula one racers, low slung, aerodynamic. When the fork tangs were pivoted forward and down by the Starter, the low slung cars stood still until the tangs were nearly rotated 90 degrees.

My car was shaped like a landing barge, a funny sight to see:  A landing barge beating the formula one racers.

The landing barge shape meant that when the tang of the starting fork had moved 10 degrees my car was already moving and ahead of the low slung car, next to it. At 45 degrees my car was already .75 inches ahead of the opponent. Some of the heats were only won by .75 inches. The other guy must have done a good job with his wheels and angular momentum. Still, he lost because the beast will beat the beauty ... if the physics is right.

If you use this design or improve upon it, please email me.  Tell me how you did.

Sites: http://www.pinewoodcup.com/ http://pinewoodextreme.com/

Using Superglue on the Axles

1. 1.       Your weight should be very dense and as far back on the car as possible.   The further back it is, at a 45 degree angle, equates to lifting the weight higher and therefore the weight has more potential energy.  More PE = more speed.
2. 2.       I like your V shapes on the axels as it does reduce the resistance
3. 3.       Place your nails in a bag of graphite for a week prior to the race.  Graphite will “fix” itself to the nail and help stay available for future heats.  We can only use graphite on our races.
4. 4.       (If legal) Get a measurement prior to the race of the height of the starting pin and create the front to look like this:
   

On our track you gain a half an inch on every other car.  Just be careful of the fragile front!

1. 5.       We cannot modify the wheels at all except to remove the birr on the inside that touches the track middle.  It is best to shape the inside edges (inner by hub and outer edge) so the smallest possible bit will touch.  If possible, shape away the wood so the outer edge cannot touch.  Prep the wood near the nail to be as smooth as possible and coat with graphite.
2. 6.       A cool design concept is to make the front into the pinchers and the rear weights (molded lead or tungsten) into the tail of a scorpion!

Good luck next year!

Brian Kronberg, MCSE: Messaging
Microsoft Consultant

Oil Reservoirs

Thanks for your insight.   I'm not sure I follow how you replace the ridges on the nails w/ parallel V-shaped ridges (WWWWW).  I can eliminate the existing ridges, but how do you add the V-shaped ridges, and why is this an improvement from a completely smooth axle?  Also, we are not permitted to oil the wheels before each race.  What's your recommendation on a lubricant presuming we can't add anything once the racing begins?  Thanks.

Webb Campbell

4/8/03

Here's my response: