Notice: Much editing of this Pinewood Derby page is underway.

(This is in response to many questions about design of cars, etc.)

Look again soon - Grandpa

 

(ChassisIII)

PINEWOOD DERBY PAGE

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(More on the Chassis 2 design - the flexible chassis.)

Grandpa's Advisory for Pinewood Derby Officials

The question sent in:

Grandpa -

I am entering a PineWood Derby.  We use little kits to make a car that coasts down a ramp.  I want to win it.  Can you help?

- Tim  - From San Diego, California

Dear Tim,

Grandpa has been in these races too - -

Good Luck, Tim.  (But it's not really about Luck - Build a Good Car.)

( Several people have written asking about the Pinewood derby - Tim is a fictitious one to represent them all.  Every single one had the same substantive question: "How do I win?"   "How" is often the question asked in scientific endeavors. )    

( Grandpa's answers are usually much shorter than this; this longer answer is for all of you who would like enough details to win one of these races or a similar race.  Grandpa is also using it to have some fun with his own competitors. )

(Note: Things in this color were added after this year's race.)

 

HOW to win a PINEWOOD DERBY

Here are some of the Pinewood Derby vehicle principles you may want to use:

Considerations:

  1. The energy available to you to maximize your speed is limited to the fall in gravity on the track – perhaps five feet.  Note: This year, our track was taller - about 7 feet of drop.  This makes a difference for the physics of the car - see below.

  2. The trick is to minimize the losses of this small amount of energy.

  3. The rules do not allow very much innovation. You must take advantage of very small things.

  4. How is the energy lost?

    1. Frictional losses.

      1. Wheels and axles.

      2. Rubbing against track guide.

      3. Air friction.

    2. Inertial losses.

      1. Rotation of wheels.

      2. Rotation of vehicle through upward bend of track (as track straightens out after the falling part.).

      3. Bouncing of wheels on rough surface.

      4. Rotational losses due to track unevenness.

  5. Is there any variation in the energy available?

Rules:

 

Design Considerations:

Pictures:

 

This is chassis 1. Ignore the sawn lines in the front. Those are an attempt at the chassis 2 design. Here is shown the forward mounting of one of the rear wheels. On that same side of car, the front wheel is raised; it does not touch the track. The other three wheels provide a stable (and quiet) ride. The bronze mass (any heavy metal will do) is shown about an inch and a half ahead of the most rear wheel. It is mounted on a soft foam. (You can wiggle it.) The wheel base is such that the distance from rear wheel edge to front wheel edge is 7 inches. The car weighs 5 ounces.

The car that ran the final race against Grandpa this year and which won second place is also worth some talk.  It had the type 1 chassis with all of the physics.

Here is a picture of that car.  Notice the offset rear wheels - the back-mounted weight - and the right front wheel is mounted high - it does not touch the roadway - and it has that maximized wheelbase almost - could have had another half inch in the front....that left wheel.  The right front doesn't matter; you know why.

 

The chassis 2 design requires more work. The piece below will not be a vehicle, but shows approximately how to cut the wood to get the wheels independent. The cut shape from the side must include some thicker areas to receive the axles.

                  

 

This picture shows how to reshape the chassis with two sticks. Now boil this for a minute or two – then let it dry with the sticks still in place. Then remove the sticks, and the wood will partly hold this new shape. You only need a little of this – to avoid any part of the vehicle touching the roadway during the race.

           

 

Once you have settled on a final shape, this is how the car goes together.

             

 

 

The weight is mounted on a high part of the chassis near the rear wheels. The weight should still be mounted on soft foam. The wheels can be mounted evenly, as shown, but be sure they are 7 inches from front to back. The wood in front and back of the wheels may need a little reshaping to avoid track contact.

If it don't go, chrome it.

Note: Grandpa’s cars have always been ugly.  We used to say in the old days, "If it don’t go, chrome it." These cars have won three years in a row against about 25 entries of adults.  (Now four years in a row - with 48 entries this year.)  If you have the talent and the time, make it great looking too - it adds to the fun for people at the contest.  There is, however, a certain pleasure when some "dweeb" wins with the ugliest car.  Grandpa will never win the "best looking" award.

Some of the cars this year were amazing works of art.  See pictures below.

Upcoming Race:  (Race now over.  Results below.)

The car above will be raced April 20, 2001.  If it wins, I'll post the result here.  (Along with some self-cheering - stuff like, "I told them so," - and "Maybe you guys will know better than to challenge Grandpa next year," - "Ha ha - the ugliest car wins again."  It's all in knowing how to be a good winner.  

The other competitors are mostly quite a lot younger than Grandpa, but they don't see it as a challenge against the "old guy."  They see it as "beating the physicist." 

If the car loses, I'll try to post a picture of the car that beat it - (I hope it's not really great looking.) And I'll discuss the principles again - to see if we can figure out what its advantage was.  I may also include some mild Grandpa cursing.  (Like "Drat it all," - or "Oh fiddlesticks," - or even "Dang Blast it all to Heck," - tough talk like that.  Grandpa always had a little uncontrolled anger.)  It's all in knowing how to be a good loser.

Here it is ready for the race.

Here's the car that won in 2001.

 

Grandpa Wins Fifth Time !

1996 - first place  

There were 15 cars (1 chance in 15 for a random win.)

1997 - no contest held       1998 - no contest held

1999 - first place

There were 25 cars (1 chance in 275 (15 times 25) for 2 random wins.)

2000 - first place

There were 27 cars (1 chance in 7425 (275 times 27) for 3 random wins.)

2001 - first place

There were 48 cars (1 chance in 370,400 (7425 times 48) for all 4 random wins.)

2002 - first place

There were about 40 cars (1 chance in 14,816,000 (370,400 times 40) for all 5 random wins.)

 

Tim - the car you build is important - this is not random.

In our church, where this contest was held, smoking is a definite no no, but an exception is when you smoke the Bishop ! This was 2001 race near final.

See below:

HP wins again, Elders.  Now we look to 2002 too.  

( It happened 5th Year in a row - 2002. )

The car:

This was far and away the ugliest car yet. Most of the extra little pieces you see were patches, as the car was very fragile and broke several times during the building. It was also too flimsy to hold the four ounces or so of weight - it just bent down and looked floppy. Some of the pieces were to stiffen the chassis a little. It ended up relatively sturdy, though it didn't look so. There were also three pieces of thicker wood to hold the wheels, as the main part of the chassis was very thin - no room for wheel holes.

It incorporates all the physics features of the earlier cars - both chassis one and chassis two. It further took advantage of a property of wheels - which is that the part of the wheel that touches the ground is not moving relative to the ground. The top of the wheel is moving twice the speed of the vehicle. Grandpa realized that this meant it didn't matter if the bottom of the wheel touched the track guide, so the wheels were given a little negative camber - slanted with the tops out a little. This way, no part of the wheel could touch the track guide except the bottom, which would never slide against it - so no frictional loss.

All of this was barely capable of winning this race. The competition has become better and better, and the win was mere inches - perhaps even just an inch in the final. Grandpa wanted to retire from the contest at this point, but there are folks who want him to stay in until he's beaten - which most likely will be next year.

Several people entered two cars this year. So did Grandpa. His other car was an improved version of chassis two. It made the finals, but was beaten by at least one other car in a race before that.

The car that beat it was built by a smart little girl who had been studying this site - and who had incorporated some of the physics. It had one feature which was a surprise - and it caused Grandpa to reconsider some of the arguments about a long wheelbase. It turns out that a long wheelbase adds comfort in a real car for the passengers - but actually wastes more energy over bumps. The angle of rotation (over any bump) is less with the longer wheelbase, as argued above, but the lever arm is also longer - and that goes as the square - while the angle goes as linear. More angle, less length is better than less angle, more length. As short a wheelbase as can remain stable is probably the best choice. In most cases, the smoother a car operates, the less energy wasted, but Grandpa is thinking the wheelbase may be an exception because of the rotational inertia consideration.

Here is Morgan King - and the car:

 

As you can see, the wheelbase is less than 3 1/2 inches - very short. Grandpa expected this car to be slow - and it was not slow.

Next year, Grandpa will incorporate this probable improvement. Congratulations on a fine race, Morgan !!   Grandpa was duly impressed. One thing Grandpa has learned over the years - every single person is our teacher. Big people teach little people - and little people teach big people.

 

In the actual finals, we were required to reduce to only one car, and Grandpa selected the ugly one.

Here is a picture of the other car - which did not win - because Morgan beat it !!

The weight on this car was another experiment - which proved not as valuable as Grandpa thought it would be. It was a rubber bag filled with a gel laden with lead shot from three shotgun shells. The theory was that the weight could then move a little at a time rather than "stiffly." The chassis is almost exactly like the 2001 winner, but would have lost in 2002. The uglier car wins again!

Again, Tim - Good Luck - Hope you win.

On a PineWood Derby WebSite, there was a statement: "Build a neat looking car - that will be remembered long after the winner is forgotten."

There is no doubt that beauty (looks) impresses, but most of the remembering will be of the car that wins the race.

By the way - Morgan is building another car - and has been informed that it is against the rules to beat Grandpa. She merely laughs and takes on a competitive attitude.

(Understand - this is an adult contest - the scouts have a separate one - we let the little ones contribute what they can in the adult races - and race our cars. - Grandpa thinks Morgan's dad is out for him.)

 

2003 - second place

Grandpa has been beaten!!

[Grandpa was out of town, and someone else raced his Chassis III car. Grandpa has found a flaw in the Chassis III design. You can read about the flaw by clicking on Chassis III at the top of this page. Grandpa now recommends the harder-to-build Chassis II. In a subsequent race, the 2002, a chassis II, beat this chassis III easily.] Disregard this paragraph - see Chassis III section - it's fine after all.

 

It's 2008 now, and Grandpa has not raced for the years in between. He's considering entering again next year if enough adults are involved. That will be a chassis II with every useful feature used. Or it could be a chassis III with modifications, if those mods can be developed to overcome the flaw.

 

- Grandpa

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