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 things 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:

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

Jerry Petrotto - WinDerby
"Now go have some fun and remember - It's all about the kids"

[tatermaker]

I tried to reading that, way over my head.

[docjobe]

Whoops--Still learning how to do this forum thing. What I meant to say was if a lecture seems way over your head, then give me a PWD subject in which you are interested and I'll try to bring it more down to earth.

Dr John Jobe
PinewoodDerbyPhysics

[patmanruls]

okay then, try this one.

Do not consider what is currently available over the counter pertaining to wheels and axles and their current sizes or weights.. just the o/d of the wheels which I think cannot exceed 31mm.....

What is the optimum size for axles and hubs?
What is the optimum amount of space/play between the wheel and hub?

[docjobe]

Sorry I have been gone on vacation for a week

re subjects for a new lecture

What is the optimum size for axles And hubs? As explained in lecture 4, there is no optimum diameter for axles, they should just fit in the bore with a few mils clearance. It is the wheel bore size that counts. As you reduce the wheel bore diameter (and of course the axle's as well so it will fit), the effective bore/ axle friction that is converted to drag on the whole vehicle goes down. The smaller the bore the better. You can't get much smaller than a 0.022" bore with a 0.020" high carbon steel (like a sewing needle steel) else axle won't hold up the car weight without snapping or bending. You can use epoxy resin to reduce the bore dia or a toluene solvent to remold a large standard bore down to 0.022". The best low friction wheel support would be to use sapphire needle bearings like the escapment wheel support on a fine old fashioned pocket watch.

Re hub size, cone it so that its rubbing radius of contact w/ body side is as small as possible. This the optimum size (again as small as possible). Also note that reduction of contact area is not why the cone helps (See Lecture 2) . It is because the hub/body friction force is applied at a smaller distance from the spin axis leading to less retarding torque. (torque = force x radius of action point)

Re optimum play in wheel on axle I have found from experience it is 0.050" or about the thickness of a stack of 5 playing cards. Too large a clearance lets the body wander around and it ends up travelling a longer distance to the finish line. Too small a clearance and the hub may be pressed between the body side and inside of the axle head

[Mood Swing Productions]

how have your cars done in wirl races or sanctioned races? your builder name there is what?

[docjobe]

Although my PWD research lab is in Houston, I helped my Cub Scout nephew in Los Angeles Western Region who built a car based on my design. This car won the National PWD race held in Irving TX on May 24 last year. The car is on display at the Scout Museum in Irving along with a copy of my 530 page book The Physics of the Pinewood Derby. A picture of the car may be found at
www.pinewoodderbyphysics.com/images/BRETT_BULLET.png
Most of my time is spent as a BSA volunteer involved in Scout Fair racing, especially helping the kids learn fundamentals of racing physics. There are 28 trophies in the attic from earlier racing events. I suppose I could add some WIRL stuff as well. How do you join WIRL?

[builderjim]

www.wirlracing.com from the you will find the builder series link that will give you the rules and racing forms.

I didn't know the the National Event last year held a race. I thought that the cars were there for display purposes only.

Jim

[Mood Swing Productions]

www.wirlracing.com

humble group of close knit friends here. <suggestion>

[woodenwonder]

Doc,
Nice looking car but that thing would not pass most CubScout rules I know of. That is a modified. BTW, we welcome any and all newcomers to Wirl.
Caution; once you get started it is awfully addicting.

[Mood Swing Productions]

i googled that event and show NO racing results. i DO however show photos of cars "For Show" purposes.

that car is nice but far from BSA legal.
1. modified wheels - rear and front
2. modified wheel base as well.
perhaps there is a modified or open class it was entered in?

Team Loree had a car there i believe?!? and if i remember stories being told about it,
it magically disappeared/was stolen. clearly understandable if you have ever seen the
quality of cars they produce!!! exceptional in so many different ways!!!!! not justified to
steal/"perminently borrow" but the quality is fabulous!!!

any hoooo if you have a URL web address for the "race" results you took 1st place in
please do share!!

good luck in your wirl stuff adding quests


had to come back and modify my post. seems Mr. Barga already stated what i typed... sorry .... dont wanna get on his bad side, his cars already beat mine down the track by a good 1.248 seconds.

[docjobe]

Yes, there was a National Race. Each council executive had a PWD car chosen from their council by various methods. About a hundred cars were entered and the race was held in the huge Gaylord hotel. The rules for this event were very liberal, namely the sheet out of the PWD kit box. A lot of other car entries had special tungsten weighting and trimmed wheels. Also plated axles were allowed as long as they were from the kit. Our car had the wheel moment of inertia greatly reduced and used a molded lead/tungsten rear end and hollow front. The center of mass was 1/2" in front of the rear axles. The wheel hub bores, normally around 0.098" were reduced to 0.060" by a toluene remolding technique. The kit axles were turned to 0.057" and the shafts were chromium plated and polished with 1/3 micron alundum powder. The wheel/axle surfaces were lubricated with a monomolecular film of Super-Z graphite + moly. The wheel axle drag coefficient was approximately 0.055 as measured on the Friction Test Rig(see Lecture 3).The aerodynamic drag coefficient of the body was reduced to 0.39 by appropriate streamlining and wind tunnel testing. The track was a 4 lane Best Trac aluminum with a 45 foot horizontal run. The Virtual Race (VR) simulator predicted 3.95 sec for a perfect car on this track. In 4 heats our car turned in 4.05 sec avg and won by an average 3 car lengths.

[pwgrasshopper]

I seem to recall reading that some of the cars from this event were used after the race as decorations in the middle of tables during some breakfast or dinner function? A bunch of them disappeared. I can't imagine the heartbreak of kids and their parents. Happy to read that your nephew's car was not one of the missing.

Your book sounds interesting. Have you tried plating axles with other material besides chrome or nickel? How about boron nitrate? Does the book give some advice on how to plate axles?

Toluene remolding technique, that's a new one on me. Again, does the book go into details on how to do this?

[Mood Swing Productions]

what is the total length on the track.
WIRL is a 42' track
PDDR is a 42' track
some personally use a 35' and with simple calculations you can get pretty
close on converting times from 35' to 42' as well as the reverse.

rougly .5 sec per 7 feet

so on the "national race track" what was the total length of the track.

[docjobe]

Thanks grasshopper for your interest. If you look in the book table of contents (pdf) and read the reviews on my website you will see that this 530 p physics and engineering book covers a bunch of detail on a lot more than toluene molding. It has 24 modified wheel photos and shows thermal molding and machining techniques and things like quench hardening axles to improve their stiffness. Perhaps you mean boric acid or boron nitride rather than boron nitrate which is likely explosive. See www.psidragon.com/msds/144,145CBNBlades.pdf for a MDS sheet that shows the so called Boron Nitrate cutting blades are really cubic boron nitride. Also a colleague named Erdimer at National Institute of Technology has shown boric acid forms a coating of boron trioxide on metal surfaces that he claims has a low coefficient of friction. My tests showed it wasn't that goof. The theory I am honing in on is the harder the substrate surface the smoother it is on an atomic basis and the better the surface is able to support a monomolecular graphite film. So boron nitride has the same cubic structure as diamond but it is hard to make and coat stuff with. I think you can still obtain small cylinders of Sapphire (Edmund Scientific used to handle some) and this would really make a hard substrate but how to get it accepted as an axle?

That's terrible about the theft of those cars. I didn't at all agree with how they organized the race and especially the rules (or lack of them) that they used. The BSA is not exactly a model for an efficiently run organization although the mission to help kids is certainly worthy. And not inviting the kids was not good. I made myself scarce before the final luncheon. What I hope is that my book next to the winning car in the museum will help motivate parents and their kids to consider science and engineering as a career goal before the good ol' USA gets passed further by Japan, China, Europe, you name it.

Thanks again

[kayjon]

My sons car finished 5th at the National PWD. Here are a few photos that were taken by our council rep. The pictures are not great but they are all we have. They reported back to us that the car never lost a race all day. We were proud of our finish. From our testing and racing with the WIRL boys we know that we did not apply a couple of proven techniques that we use now. We left a little speed on the table. Maybe we could have finished 4th. ;D.
Congratulations on your first place finish. I had been curious what 1st place looked like. Not to sound like sour grapes but that "Hotrod" would never have passed our Pack, District or Council inspection.
That is why in my opinion BSA must have clear and concise universal rules. Once a racer moves up from the local level, apples must race apples to keep the playing field level.








The above car is not my sons , this is the car that won 1st for best design. This car was supposed to be on display however it is one of the cars that walked.

[Mood Swing Productions]

great job as always!!!!!!!!!!!!!

sorry for the poor sportsmanship and ignorance you all had to deal with.

[pwgrasshopper]

Doc,

You are right, I fat fingered Boron Nitride. Here is a link of what I was looking at:

www.caswellplating.com/kits/bnen.html

I have tried nickel plating a few axles with a 40 dollar nickel kit from Micromark. I have not tried chrome and the price of this BN EN kit was way more than I wanted to pay for what could be a crap shoot.

I guess I was using toluene molding as an example. If the book goes into detailed how to's, what you need for supplies and tools, etc.. then that would hold a good deal of interest for me.

Thanks again

BTW - I did check out the table of contents as you suggested and my impression was that it does go into detail

[docjobe]

You wrote

Not to sound like sour grapes but that "Hotrod" would never have passed our Pack, District or Council inspection.

Well, it does sound like sour grapes. The rules for the National Race were clearly stated, and if you want to win a race it seems to me you should design your car according to the rules of the race in question and not according to some other council or district race rules. RIght? You should know that there are over 300 councils invited to bring cars to this race. And over 300 different sets of local rules. The winning car's adult/cub team spent many hours studying the rules for this race and optimizing the car for the type track used in the race. Lets have a closer look at how your car did compared to the winner:

Car No Lane 1 Lane 2 Lane 3 Lane 4 Total Place STD
235 4.055 4.047 4.029 4.025 16.156 1 0.012
220 4.194 4.184 4.200 4.140 16.718 5 0.024

The average distance that car 1 beat car 5 at the finish line was 3.80 car lengths according to the above race data. But you claim you designed you car against a different set of rules. Then what are your grounds for complaining? Some principles are universal, like car stability on the track. Your car had a standard deviation of 0.024 s which was twice that of the winning car's at 0.012 s. And the aerodynamic drag of your car based on its shape seemed to be somewhat larger than optimum. Moreover, I would doubt your wheel moments of inertia were adjusted to the limits allowed by the rules. And, I suspect your wheel/axle coefficient of friction was not reduced by using the monomolecular film Super-Z graphite type. It is totally fair that the winning car should represent the maximum expenditure of physics (a 530 page book) elbow grease (125 hours) testing (40 hours) and research on race rules and conditions (20 hrs).

Maybe you need to modify your council or whatever rules you use to state " if anybody dares use physics, we demand a 3 car length head start" Hot rod indeed! I can't believe the things that come from some of these councils. There was even one that offered a cash incentive award in the form of gift certificates if they could build a winning car. Now that's really teaching the kids something isn't it?

[WarpSpeedINC]

To reply to the sour grapes portion, I believe that Loree racing was held to their council rules in order to qualify. So it came down to the winning car being from the council with the most wide open rules, not your PHD, or superior building skills. I believe if team Loree's council had wide open rules as yours does, it may have been a different story!

You don't have to be an engineer, or have a PHD in anything to build a fast car!

Warp Speed