# The Test of YourPinewood Derby Car Design Learning

My professor said that I should learn as much (or more) from his tests as I did from his lectures. I wish the same for you.

So, here is the test:

## Question 1

Assumptions:

1. Car weight limit is 5 ounces. Size limits: 7 inches long, 2 3/4 inches wide, 3 inches high.
2. Kit wheels and axles must be used with no substantial modification. If the wheels are worked, a flat tread profile for the full width of the original wheel is required.
3. The location of the rear axles is required to be 7/8". The front axles may be moved.
4. The racing surface of "our track" is 28 feet long with cars straddling a nearly 3/8 inch thick center rail.
5. "Our track" starting line is approximately 48 inches above the "flat," sloping at approximately 30 degrees and making a smooth, large radius transition to the flat.
6. On "our track," the increase in elapsed time (ET) due to one wheel's angular inertia exceeds ET increase due to moving the center of mass (CM) 1 inch forward.
7. On "our track," stability requires that the CM be at least 1.3 inches forward of the rear axle. (Otherwise the car "comes loose" on the track and rattles against the rail, loosing all of its advantage and more.)
8. On "our track," stability requires a wheel base of at least 4 inches.
9. You have the ability to align and stage your car so that it tracks straight down the track, i.e. equal to these cars.

With these assumptions, you can see that a 3 wheel car (one front wheel raised) with CM 1.3 to 2.3 inches forward of the rear axle will win over a comparably prepared 4 wheel car with the CM at 1.3 inches forward of the rear axle.

Based on what you have learned:

Design a car which beats the 3 wheel car (with its CM 1.3 inches forward of the rear axle) under the above conditions.

Specify these design parameters for your design:

Car length:
inches.

Wheel base:
inches.

Distance from rear axles to Center of Mass:
inches.

Height of center of mass above the plane of the weight bearing axles:
inches.

Other key design factors:

Name:

Email:

Separate answers are acceptable / encouraged.

## Question 2

Assumptions:

1. Car weight limit is 5 ounces. Size limits: 7 inches long, 2 3/4 inches wide, 3 inches high.
2. Kit wheels and axles must be used with no substantial modification. If the wheels are worked, a flat tread profile for the full width of the original wheel is required.
3. The location of the rear axles is required to be 7/8" from the rear of the car. The front axles may be moved.
4. The racing surface of "our track" is 28 feet long with cars straddling a nearly 3/8 inch thick center rail.
5. "Our track" starting line is approximately 48 inches above the "flat," sloping at approximately 30 degrees and making a smooth, large radius transition to the flat.
6. On "our track," the increase in elapsed time (ET) due to one wheel's angular inertia exceeds ET increase due to moving the center of mass (CM) 1 inch forward.
7. On "our track," stability requires that the CM be at least 1.3 inches forward of the rear axle. (Otherwise the car "comes loose" on the track and rattles against the rail, loosing all of its advantage and more.)
8. On "our track," stability requires a wheel base of at least 5 inches.
9. You have the ability to align and stage your car so that it tracks straight down the track, i.e. equal to these cars.

With these assumptions, you can see that a 3 wheel car (one front wheel raised) with CM 1.3 to 2.3 inches forward of the rear axle will win over a comparably prepared 4 wheel car with the CM at 1.3 inches forward of the rear axle.

Based on what you have learned:

Design a car which beats the 3 wheel car (with its CM 1.3 inches forward of the rear axle) under the above conditions.
Specify these design parameters for your design:

Car length:
inches.

Wheel base:
inches.

Distance from rear axles to Center of Mass:
inches.

Height of center of mass above the plane of the weight bearing axles:
inches.

Other key design factors:

Name:

Email:

Copy and paste the page created by pressing the "Write Answer File" button into your email program and send it to me. (This used to email from within the server, but AOL broke the software but I didn't learn about it for a long time, the site move lost that ability totally, I received too many bad email addys AND the reader didn't get a copy of the answer sheet!)

I will reply with an evaluation of your answers. If I think that your answer shows understanding of the concepts at work, then I'll include a key that you can use in the corresponding "Here is my answer" page.

Be sure to use the provided Subject line for prompt response.

It occurs to me that some might think that I am "just fishing". Not so. Here is my answer to question 1, and Here is my answer to question 2. You will need a key, though. (Yes, if the key were long enough, I could pass the answer in the "key", but it isn't that long!)

Initial publication: 4/26/99.