Attached is the right information and analysis on Pinewood Derby cars. You may post it with attribution.

My first car design came in second. Then I learned the shape of the race track. 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
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. At the same time, since the cars don't exceed 15 mph 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
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. This can be minimized by polishing the body as smooth as glass and lubricating the body with 3-in-1 oil.

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 and aerodynamic. When the fork tangs were lowered 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 three-quarters of an inch ahead of the opponent. Some of the heats were only won by three-quarters of an inch. 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.

Copyright © 1997, by Clifford W.Lazar

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