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
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:
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
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.)
Here are some of the Pinewood Derby vehicle principles you may want to use:
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.
The trick is to minimize the losses of this small amount of energy.
The rules do not allow very much innovation. You must take advantage of very small things.
How is the energy lost?
Wheels and axles.
Rubbing against track guide.
Rotation of wheels.
Rotation of vehicle through upward bend of track (as track straightens out after the falling part.).
Bouncing of wheels on rough surface.
Rotational losses due to track unevenness.
Is there any variation in the energy available?
Must use wheels and axles provided with no alterations. You may remove burrs and smooth but no reshaping is allowed.
Vehicle may have a maximum weight of 5 ounces.
Vehicle may have a maximum length of 7 inches.
Vehicle may not ride on springs.
Individual contest authorities sometimes make up some of their own rules, which do not come with the car. One frustrating one has been the disallowing of lubricating between races.
No "active" devices are allowed – no starting devices – or anything that adds energy. The contest would be ruined without this rule. (Jet engines?)
You want the center of gravity of your vehicle to fall as far as possible. This means you want the center as far back as possible without losing stability, because the vehicle starts slanted with the back higher than the front. I have found that this center can be about 1 ½ inches forward of the rear wheels and still be stable. Since the main chassis weighs something, put the extra weight about an inch to an inch and a quarter ahead of the axles of the rear wheels. This is a compromise. For the lowest inertial losses, the best spot for the center of gravity is at the center, but from running cars, it has become obvious that the extra fall in gravity is worth more than the inertial savings. ( As noted above - the track in our contest this year was taller - with about 7 feet of fall. This means all the cars went faster. The inertial considerations - and the stability considerations are greater. Also the gravitational gains by placing the weight in the back are smaller as a percent of the total. If Grandpa had known the track would be taller, he would have moved his weight almost to the middle of the car, perhaps an inch further forward than it was - still a bit behind center. This would have given extra stability, saving inertial losses, at the expense of a bit of lost fall in gravity. )
You want the longest wheelbase possible. This is to minimize inertial losses from rotational vibration. This means you want the front of the front wheels to be exactly 7 inches from the rear of the rear wheels. This will require making your own axle holding holes or slots, as the provided slots are too close together.
Four wheels touching the roadway without any suspension system is a formula for a rough ride. Provide for one of the front wheels to be mounted high, so that it does not touch the roadway. The other three wheels are more than sufficient for the five-ounce vehicle, and will provide a more stable ride. With four wheels, most of the time, one wheel will not touch anyway – and which wheel is not touching will be constantly changing – requiring friction to re-speed wheels over and over. Also the general vibration from this costs energy.
Two chassis designs are included here. The one providing a shape that allows the wheels to operate independently could be regarded in some contests as having "springs," though only the wood block provided in the kit is used. This is the kind of innovation that should be encouraged, as no expense is incurred and no extra parts. This is the way the kids could learn some physics.
If the second chassis design is used, the raising of one wheel is no longer needed.
If the first chassis design is used, the rear wheels should be mounted such that one is forward an inch or more in front of the other. The more forward wheel should be on the same side of the car as the raised wheel in front. This will provide best three-wheel stability, but more important, it will save inertial losses over bumps. Consider how one likes to traverse a speed bump in a real vehicle. We go at an angle, so that only one wheel at a time hits the bump. The center of gravity of the vehicle is raised half as far twice. Each of these rises uses only a quarter of the energy – so that the total is half the energy lost. (This comes from the E = ½ mv^2 formula for kinetic energy. The center of gravity vertically is moving half as far in the same amount of time and therefore traveling an average of half as fast during this move – thus the v^2 is only a quarter as much. Two quarters is only a half.) To be sure, these are small energy savings, but there is very little energy to begin with, and these contests are tests of inches.
In case of the lubrication rule: Put a v-shaped groove in your chassis next to each axle – fill with graphite to vibrate to the axle during the contest.
Remember that noise is lost energy. The quieter your vehicle rides, the better. If the center of gravity is too far back, the vehicle will vibrate from side to side, with the front wheels hitting the track guide. If all four wheels touch the track, the ride will be generally noisy. The winning vehicle will usually be very quiet. Many vehicles have accidentally won these contests because the wheels were a little "crooked." Only three wheels were in contact with the track. Many others have carefully adjusted their four wheels to form a perfect plane, and thus lost the race. Again, if the second chassis is chosen, ignore this and get them even. It may still be a small advantage to keep the three-wheel design. It would take testing to know for sure. A third chassis design may take advantage of all of this later.
To minimize rotational losses some more, see that the chassis is as light a possible – before adding your weight all at one place. If you have separated mass, that is, mass at the front and mass at the back, every time a wheel gets a bump, the masses must move up and down – the vehicle is being rotated, and there are losses of energy.
Choose a weight shape that is short as possible in the forward direction – a cylinder across the vehicle is probably best, though I have argued that a sphere may be the best overall. It would be difficult to measure the relative losses of forward vs. sidewise rotations. Use a sphere or a cylinder. Do not use the long flat weights they sell you at the hobby shop (shudder.) Make your weight such that the total weight of the vehicle is 5 ounces. The wheels and axles are far more than sufficient for this weight, and the air resistance to mass ratio is lower the more the vehicle weighs. Try rotating a baseball bat two ways. Once lengthwise, and then once about its long axis (holding both ends and spinning it.) Rotate it a quarter turn back and forth several times. It’s easy to see which takes the more energy, yet the entire bat is rotating that quarter turn in both cases. Since the track is curved in one dimension, you want the rotation of this large mass to take minimal energy in that dimension.
With either chassis, but most important with chassis one, having no suspension, it is important to mount the weight so that it is insulated from vibration. A soft foam mounting would be good. Perhaps mount it on a smaller piece, so that the weight is free to move in all directions a little as the vehicle negotiates bumps and roughness. This will reduce inertial losses, because the heaviest part of the vehicle does not have to move the whole distance of each cycle of a vibration. (Note: This year there was a car that got to the semi-finals that Grandpa would not have picked to be fast. It had a feature that saved high-frequency inertial losses. Grandpa is going to use this feature next year - in addition to the other physics. This car had a hot dog wiener laid on top of the car. It may have been done for humor, or maybe this builder knew some interesting physics involved here. The wiener is "flexible" all through - It can "vibrate" without the whole mass moving in unison. Especially at high frequencies, this minimizes lost inertial energy. This car didn't have most of the other features that Grandpa would consider advantages, so it's probable that this hotdog feature is significant. The wiener did not appear to be attached in any way. The reason it stays put is its very advantage. It's like the good "sticker" hopscotch lagger - a piece of chain - it "sticks" because of its flexibility - its ability to move in parts instead of as a whole. Next year, Grandpa won't use a hot dog wiener - but he will use a flexible weight of some kind. Perhaps a heavy colloid (gel) with lead dust mixed in. It's still an advantage to keep the shape and location, as much as possible. This means we need high density (the lead) to keep the weight small enough for such locating. The wiener was located exactly wrong - lengthwise instead of across the vehicle - yet it was still fast. This was one of about four "scary" vehicles this year. All the other contenders had the chassis 1 design, described below. A cocktail wiener as dense as lead laid across the vehicle near the back might be a great weight.)
Friction: Alignment of wheels should be such that they don't wander and rub the track guide. Lube well. Don't have a high-profile shape. Air friction is almost negligible for the speeds involved unless the shape is extreme.
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.
HP wins again, Elders. Now we look to 2002 too.
( It happened 5th Year in a row - 2002. )
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.
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