Pinewood Derby Performance Design An introduction to making a high performance Pinewood Derby car

Copyright 2003 by Stan Pope, all rights reserved.

Based on information presented at the author's website at ...

  Pinewood Derby Design http://stanpope.net/pwdesign.html

Learn to Build a Winner http://stanpope.net/lbw_apl.html

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.

Basics

To win a pinewood derby race, your car must get from the starting line to the finish line before the other cars.

Quickest:  Accelerate early  Attain maximum velocity  Hold speed through finish line

Make or Break

Follow your district's rules.

If you can't pass inspection, you can't race. If you can't race, you can't win!

Assure Ground Clearance. Failure: Grinding stop Dramatically flying off the track If you can't get to the finish line, you can't win!

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.

Top 10 in district

Make sure that the wheels are in balance, in round, moving freely, and correctly aligned.

Maximize weight vs. wind drag.

Optimize weight distribution. As the center of gravity moves farther back in the car, there is more energy available to be converted into speed.

Some Guidelines

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)

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"

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)

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)

Fundamental Energy Equation Potential Energy at start minus Lost Energy leaves Kinetic Energy at finish

In other words ...

Start with as much potential energy as you can, and waste as little of that energy as possible.

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

Part of this presentation is oriented around this idea ...

Regardless of the lane in which you race, you should race in: The steepest lane, The shortest lane, The smoothest lane, and The slickest lane

Then, you will be racing in ...

The fastest lane!

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.

Modern Track Starting Line Finish Line

Race in the

Steepest Lane

Which car wins?

Which car is fastest at the bottom?

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.

Car A has farther to fall, so it has more potential energy to turn into speed.

How do I make my lane steeper?

Physics says that “my lane” is the path followed by my car's center of mass.

Ha a b Center of Mass Hb

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.

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.

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.

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.

Race in the

Shortest Lane

A B Which car wins?

Set axles so that car runs straight and level.

Align as required.

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.

Race in the

Smoothest Lane

Which car wins?

How does the car's center of mass move as these wheels roll?

A B C

Which car handles bumps best?

A B

    Wheels round Bore centered and smooth Tread smooth (but not polished!) Wheelbase extended (but not too much!)

Race in the

Slickest Lane

Identify the frictions and losses.

Make frictions as small as possible.

Move friction to where it hurts least.

Are these criteria always true?

Well, no, they aren't. There are some exceptions. However, the exceptions are very few.

When aren't these criteria true?

 Different track styles  CM too far back  Front wheel too far forward

Some Key Steps

Preparing Axles

To hold nail while filing sholders near nail point

Then chuck in drill and clean under nail head.

Preparing Wheels

A drill press can work as a lathe.

Wheel is held in a mandrel and rotated as it is moved past a cutter.

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.

Of course, sandpaper against a block of wood can also be used.

Make sure the block is square!

Body Preparation Drilling Axle Holes

Here is a homemade drill guide to help keep the holes perpendicular to the car body.

Layout car body plan on block.

Locate holes.

Drill first!

Here is a tool that is supposed to produce perpendicular holes even with a hand drill!

Producing complex shapes using Cub Scout age abilities and tools.

Notes: You can still see the original picture. All the cuts are across the grain of the wood.

Now ...

Chip the pieces off to leave a rough surface in the right shape.

Rasp and sand the surface smooth.

Closing

Admonitions

Hot Lead: Severe burns possible

Lead: Lead is Poisonous Lead often has nasty additives Minimize contact.

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.

Rules change from time to time.

Review the rules carefully each year.

Especially check the “boundary conditions.”

Don't believe everything the “experts” tell you!

Smile, say “Thank you,” and go home and check it out!

Help your builders get the most out of their efforts. Share what you have learned.

Good Racing!