Pinewood Derby Physics Lectures

[winderby]

I have to share with you a site that is dedicated to dispelling myth and assumptions regarding the Physics of Gravity Driven Cars. Dr. John Jobe has written a book called "The Physics of the Pinewood Derby - With Engineering Applications". I was blown away with his 15 years of research and practical application of mathematics. This may be a little too technical for the casual derby builder. But anyone with even a hint of curiosity about how thing REALLY work will find his works fascinating.
Dr. Jobe is publishing a series of lectures based on his research and hands-on lab experimentation. They are written in an easy to understand format. You can download them at:

http://www.PinewoodDerbyPhysics.com/lectures.shtml
Check them out.

If the lectures only wet your whistle then the book will keep you up all night. This will be a great resource for both the novice and the pro to expand their thinking and learn a great deal.

“Opportunity is missed by most people because it is dressed in overalls and looks like work.”
—Thomas Alva Edison

[Pinewood Daddy]

Your discussions in the the Derby Worx forum have been very interesting. I've checked out your site also. My wife would KILL ME if I spent $80 on "The Pinewood Derby Bible". Most of us have seen "Down and Derby". I already have too many similarities. Spending $80 on the book would push me over the edge.

I realize you've spent many years researching, experimenting and documenting the finer points of PWD. But if you sold it for $40 maybe I could get away with it. I'm not joking. A lower price would make it more accessible to the average person. It would be up to them to translate it and figure out how to apply it. The higher volume would make up for the lower price.

Could you offer a PDF version??

[doc jobe]

Hi Pinewood Daddy

The book is huge and a pdf file is about 100 meg. Also I don't want to let a pdf file get into the public domain or everone in China may end up with a copy to sell. I'm not saying you would but giving you a pdf version would set a precedent. The simulated leather cover with gold embossed text costs over $40. Its like I tell the CUbs when they are building PWD cars "Even if it doesn't work right, there is no excuse for it not looking good."

Also, you mention something about the book reader needing to be translate and/or interpret the contents? Not so, all the applications are shown in detail, all you have to do is build the cars as shown. Now the first half of the book does get into advanced mathematics, buts that's so the average person can see just what goes into analyzing the fairly involved physics of these gravity driven cars.

Seriously, what are you objectives? To help in WIRL racing? To help a Cub scout build a fast car? Let me know your objectives and I'll see what I can come up with.

Maybe you just need the Virtual Race CD. Its got an awful lot of material and the results of all the advanced stuff in the big book are laid right out for the average person (and youngster) to begin using immediately.

[Pinewood Daddy]

What are your objectives? To help in WIRL racing? To help a Cub scout build a fast car? Let me know your objectives and I'll see what I can come up with.

Maybe you just need the Virtual Race CD. Its got an awful lot of material and the results of all the advanced stuff in the big book are laid right out for the average person (and youngster) to begin using immediately.

I'm mostly interested in building a faster Cub Scout PWD car, but I'd like to build the ultimate gravity powered speed machine for fun. I'm a Tool Designer by trade, so I'm naturally interested in finding a better way to do things.

It looks like the Virtual Race eBooks cover much of what's been discussed here.

We're fairly experienced (3 years), this year winning 1st & 3rd in the Pack, 6th and 14th in the District and 5th and 9th in the Council. I built a car for my daughter that's half a car length faster.

I'd like to build for WIRL some day.

I think one of the biggest mysteries in PWD is proper wheels alignment. I've used the bent axle method and Stan's wax paper shim method with "rail riding". Even with a car that starts with decent alignment, getting near perfect alignment is very time consuming and frustrating. I would like to hear what you have done.

[Down-N-Flames]

[I'd like to build for WIRL some day.

Sounds like you already have, you just need to send it in! Peopel tend to be scared of WIRL and yes there are great builders there, but then again you never know. Phil acouple months ago broke a car and ended up winnig first place that month.

Woody

[doc jobe]

Yes, you are correct about tracking being difficult to control. Re the wheel alignment problem, my book has a construction drawing and a photograph of a wheel installation jig that a machinist, or a good tool maker, can build fairly easily. It is essentially a open top open front box made out of 1/2" aluminum plate. Tungsten carbide drill guides are appropriately positioned precisely perpendicular to the sides. Metal plates sheared to fit the bottom elevate the body to control axle hole height. At one Cub workshop we built 50 cars one afternoon and this drill guide was a key part of our assembly.

But sounds like you are under a budget crunch. One of the main reasons this book was written was that it was designed so the youngster could grow with the content. The calculus and racing statistics parts of the book will be valuable references when he (she) hits high school and/or college. I suppose you have read a few of the reviews on the website but I have many more. The book is bound to last for generations.

Also, if wheel/axle friction is under control, you can get more "perfect" races which are the ones that don't bump the center strip appreciably. Non-uniform or sporadic changes in coefficient of friction, especially on one side or the other at the rear, will swing that lightly loaded front to one side or the other. That's why I never could understand why some think making the wheel bore/axle's surface a graphite crushing machine using hubcaps and/or notched axles could help speed. Please refer to the monomolecular film approach to lubrication under "Speed Package"

[Pinewood Daddy]

I'd like to build for WIRL some day.

Sounds like you already have, you just need to send it in!
Woody

I built a car for our Pack's Open Class with the prerequisite bare rectangle on the bottom. It was fast, but not fast enough. It needs a little tuning. It looks like a stock BSA block, but I cut it down to 1/4" thick and built a balsa wood box with carbide weights inside. I did it to show that aerodynamics aren't very important at the low speeds these cars move at. http://home.att.net/~d.fengler/Pics/Block_Car.jpg

[Down-N-Flames]

For the May race there was a car named Bone Stock, and it was a pain block not even paint! at one point there was talk of doing a race with just straight wood blocks just to see how they ran.

The biggest thing is send a car in and see where you are and then tweak. You rarely see a car run the same two months in a row.

Woody

[Pinewood Daddy]

For the May race there was a car named Bone Stock, and it was a pain block not even paint!

It didn't do very well. I could probably beat that.

The biggest thing is send a car in and see where you are and then tweak. You rarely see a car run the same two months in a row.

Woody

I'll shoot for August. I found a padded plastic box to ship it in.

I've been meaning to stop procrastinating, but I never get around to it.

Thanks for the encouragement.

Dave

[Pinewood Daddy]

Yes, you are correct about tracking being difficult to control. Re the wheel alignment problem, my book has a construction drawing and a photograph of a wheel installation jig that a machinist, or a good tool maker, can build fairly easily. It is essentially a open top open front box made out of 1/2" aluminum plate. Tungsten carbide drill guides are appropriately positioned precisely perpendicular to the sides. Metal plates sheared to fit the bottom elevate the body to control axle hole height. At one Cub workshop we built 50 cars one afternoon and this drill guide was a key part of our assembly.

I've had a similar drill jig made, but no bushings. Holes are drilled into the pine block with the bit in a pin vise.

But sounds like you are under a budget crunch. One of the main reasons this book was written was that it was designed so the youngster could grow with the content. The calculus and racing statistics parts of the book will be valuable references when he (she) hits high school and/or college. I suppose you have read a few of the reviews on the website but I have many more. The book is bound to last for generations.

Budget crunch, maybe. My wife doesn't like to see much money spent on PWD. $80 on a book might raise an eyebrow, or worse!

Also, if wheel/axle friction is under control, you can get more "perfect" races which are the ones that don't bump the center strip appreciably. Non-uniform or sporadic changes in coefficient of friction, especially on one side or the other at the rear, will swing that lightly loaded front to one side or the other. That's why I never could understand why some think making the wheel bore/axle's surface a graphite crushing machine using hubcaps and/or notched axles could help speed. Please refer to the monomolecular film approach to lubrication under "Speed Package"

I used the "rail rider" technique this year with allot of success. But I think we used too much graphite. At the District race in the first round (4 races) my oldest son's car was the second fastest in the building. In the final round (4 races) he ended up 6th, over .030 slower. I was a bit disappointed.

Have you tested NyOil II and Krytox 103/100??

[doc jobe]

Re the statement posted earlier

"aerodynamics aren't very important at the low speeds these cars move at. "

Consider that heights of 4 ft to 6 ft can generate speeds of 12 to 15 mph for PWD cars

Consider the air foil parachutists that hit the ground at about 12 to 15 mph

Tell them that aerodynamics are very important at these low speeds and they are likely to chuckle quite a bit. It keeps them from hitting at 120 mph.

If it weren't for air resistance effects, the weight or mass of a PWD car wouldn't matter at all.

Lectures 1 and 2 shows the physics behind this, and Lectures 8,9, and 10 are devoted to air resistance effects on PWD cars.

Do you have data like race times you got with your balsa wood car?

They would be very interesting to analyze

Re liquid lubricants, 25% silicone oil in n-heptane does better than Nyoil.

And you are correct about too many graphite layers increasing the coefficient of wheel/axle friction. A monomolecular film thin as a soap bubble gives the lowest friction I have yet observed. You must use very high purity virgin graphite

Thanks Guys

Doc Jobe

[Pinewood Daddy]

Re the statement posted earlier

"aerodynamics aren't very important at the low speeds these cars move at. "

Consider that heights of 4 ft to 6 ft can generate speeds of 12 to 15 mph for PWD cars

Consider the air foil parachutists that hit the ground at about 12 to 15 mph

Tell them that aerodynamics are very important at these low speeds and they are likely to chuckle quite a bit. It keeps them from hitting at 120 mph.

The drag coefficient of a parachute must be millions of times greater than a PWD car.

If it weren't for air resistance effects, the weight or mass of a PWD car wouldn't matter at all.

I thought these cars were powered by gravity. I read lecture 1b and sort of understand that the force of the air force pushing back is equal to the frontal area times the velocity squared plus it's drag coefficient. My point of building the block shaped car was to prove (to our Cub Scout Pack) that the shape has very little to do with the speed of the car vs. proper wheel and axle preparation and friction reduction.

Lectures 1 and 2 shows the physics behind this, and Lectures 8,9, and 10 are devoted to air resistance effects on PWD cars.

Your free lectures stop at 8. Any chance of posting lectures 9 & 10??

Do you have data like race times you got with your balsa wood car?

They would be very interesting to analyze

It has only run 4 times at the Pack Open Class race, and a few times at home. I need to make some new wheels and axles, based on some of the info I read on your site, and do good job with alignment.

Re liquid lubricants, 25% silicone oil in n-heptane does better than Nyoil.

Have you tested Krytox 100?? I don't know if Home Depot sells silicone oil or n-heptane. Could you post a recipe and source??

And you are correct about too many graphite layers increasing the coefficient of wheel/axle friction. A monomolecular film thin as a soap bubble gives the lowest friction I have yet observed. You must use very high purity virgin graphite

So Hob-E-Lube shouldn't be as good as some of the new graphites (Tube-O-Lube, Pinewood Extreme's graphites, etc.)?? Thin film lubricants (NyOil & Krytox) should be faster??

I know, I should buy the book and I'll understand. The thought of my wife injuring my private parts overcomes the want to spend $80 on a book to make little wooden cars go faster.

I don't mean to sound like I doubt your findings, I don't. I'm sure science will always win over pure dumb luck.

I wonder how many WIRL guys have bought your book??

[doc jobe]

The silicone oil in n-heptane is available as CRC heavy duty at NAPA auto parts. Spray in a glass, let 75% evaporate, and cap the remainder. I sometimes use it after a graphite film has been applied for best results. A monomolecular graphite film can't be seen, its only a molecule thick which is like a billionth of a meter. Oil films are considerably thicker. Silicone can get bad results if not applied just right. Use hypo needle & 2 tiny drops per axle.

The drag coefficient of a parachute is about 2. The biggest drag coefficient is like an anemometer cup scooping the wind and can be up to 2.3. PWD cars go from a square block = 0.84 to an airfoil = 0.24.

Here is a rough idea of the relative effects of 4 key measures of PWD slowdown relative to a perfect car running a perfect race (no bumping)
The standard car(SC) is 1/2 block from kit (about 3/4" high running standard 1999 Cub wheels/axles with an ordinary fine graphite or white teflon lube. CL = car lengths at finish line on a 16 ft ramp (Piontidosi Al) with a 16 ft straight horizontal section less 3ft stop distance. SC=Std Car. PC = perfect car (all stuff below when racing PC)

Air drag = 0.8 CL(SC) can be reduced to 0.1 with low height and low drag coef

Wheel Moment of Inertia = 1.7 CL(SC) can be reduced to 1.2 by lifting 1 front wheel or to 0.15 CL by running on 3 RS racing wheels.

Center of mass position = 0.7CL (SC) differenc in CM at center of car vs being moved back to 1/2" in fron of rear axle. This is on circ arc ramped track. On a Best Track type with fairly straight inclined plane ramp only get 25% of this or .18CL(SC)

Friction = 2CL(SC) with cheap fine graphite/powdered teflon but can be reduced to 0.5CL with a high purity graphite moly mix like Hodges G-M or Jobe Super-Z. Application technique is as important as the graphite itself

All the above on a perfect race with no center strip bumping. Such poor tracking alignment can cost up to 0.5 to 1.0 CL

[MaxV]

Air drag is a factor in pinewood derby racing, albeit not the most important one. I have measured the effect of air drag in three separate experiments, and in each case the car with the least frontal area has the advantage. In each experiment a single car was used with an adjustable "air brake", so all other factors were essentially eliminated.

If anybody has doubts about the effect of aerodynamics, you can easily do test yourself. You just need a track with an accurate timer.

[Pinewood Daddy]

The silicone oil in n-heptane is available as CRC heavy duty at NAPA auto parts. Spray in a glass, let 75% evaporate, and cap the remainder. I sometimes use it after a graphite film has been applied for best results. A monomolecular graphite film can't be seen, its only a molecule thick which is like a billionth of a meter. Oil films are considerably thicker. Silicone can get bad results if not applied just right. Use hypo needle & 2 tiny drops per axle.

Great info! I assume Silicone in n-heptane has to be applied at the race. How long does it take to evaporate enough n-heptane to affect performance??

How many races will monomolecular graphite last?? What is the best procedure to apply it??

Again, thanks for the help. Now all I need to do is conjure up enough kahonies to spend $110 get the complete package.