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Pinewood derby race car physics
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design219
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Mar 16, 2008, 10:42 PM
 
My son is a Tiger Cub (beginner Cub Scout) and we are building a pinewood derby car. The woodworking part is not a problem and we've got the body cut sanded and almost ready to paint. Looking at a couple of derby websites I'm finding what seems like odd advice.

Many people are recommending only having three wheels touching the track to reduce friction... weighting the car toward the back and having one front wheel a little higher. Is there significant friction (speed loss) from a rolling wheel? It would seem to me that four wheels would mean less friction per wheel than three.

Also, I would think four wheels down would make the car stay straighter and thus have less friction from the wheels possibly rubbing on the center guide.

Ok, I don't care all that much about winning, my son and I are just having a grand time on this project. I do want to understand the principles and do the best we can... and my son want his car to go fast!

The basic construction rules: No more than 5 ounces, no ball bearings or movable parts. Graphite powder is acceptable. Kit wheels cannot be modified.
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JoshuaZ
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Mar 16, 2008, 11:05 PM
 
I remember back in the day. My cars never did that well.

The key, if I remember correctly, was to made a wedge. A very small wedge. Drill holes in the back to put weights in.

Some people took the approach of cutting a large hole in the bottom to which they mounted a big lead weight.



Something like that one above would be best.
     
vmarks
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Mar 16, 2008, 11:20 PM
 
Our tracks always started with a steep incline and a series of dowels that rotated down out of the center of the track to act as starting gate.

Dad melted lead weights from the sort that are used to balance real car tires.
(drill or route wood. measure weight of wood and cut down weights until proper weight is achieved to satisfy rules. melt lead in coffee can with a propane torch. pour into the wood cavity.)

The wedge idea is good. However, I always made the bodies. One year, I did a firetruck-like shape. One year, one parent did a very good replica of a Jeep Renegade (he did it as his own car, not his boy's. He just wanted the wood carving challenge.) One year, I did do a thin wedge, and it was quite fast.

Dad always helped us do good paint jobs - nice and high gloss.

Sand the wheels. The plastic molding line on the wheels isn't sure to be round. Sanding the wheel is.

I get the idea of having one wheel too high. I don't like it.

I suppose if you wanted to, you could introduce negative camber (less wheel surface in contact with the track, less friction), but the problem is that this could introduce side to side behavior on the track - the raised center of the track is what keeps the cars going straight. Negative camber would create more width between the wheels and track, meaning the car can spend more time drifting sideways, hitting the center and bouncing back. Not great.

I was in Tiger Cubs the first year they held that program. This would have been 1982. I was awarded Eagle in 1991.
     
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Mar 16, 2008, 11:33 PM
 
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Mar 17, 2008, 12:27 AM
 
I always hated the kids who just made lead wedges. My cars were always attractive and flashy, so I lost every time.
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Mar 17, 2008, 12:37 AM
 
Also, whatever you can do do polish the axles, do it. Use plenty of graphite as well. We won tons of pinewood derbies, and my car was usually just silver from putting graphite all over it.

There are wheels you can get (or make) that are ridged, which go faster (less friction), but they're illegal most of the time. The only time I ever lost was when the kids had ridged wheels, but we didn't make a big deal about it.

Like vmarks said, if you go with the 3 wheel style, make sure it goes straight.
     
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Mar 17, 2008, 08:36 AM
 
My kids have done this now for three years. I also built my own cars as a kid. The most important thing you can do is let your son design and build the car. This isn't a race for the parents. It's a chance to bond with your boy. Teach him what the tools are and how they work. Let him sandthe wood until the car is smooth. And if it doesn't win, there is always next year. And I guarantee your son will pay attention and choose a new design each year, learning as he goes. Good luck!
     
design219  (op)
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Mar 17, 2008, 08:41 AM
 
Thanks Alligator. Yes, my son did the design (it's a pick up truck) and I cut the shape with a band saw, but he has done most of the sanding and I've taught him about sanding sealer and how to sand, so it's getting very smooth. He picked out camo colors (dark green and brown gloss spray paint). I think next year he will probably want more of a race car look, but this is about him learning right now.
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Mar 17, 2008, 01:59 PM
 
Originally Posted by vmarks View Post
I suppose if you wanted to, you could introduce negative camber (less wheel surface in contact with the track, less friction), but the problem is that this could introduce side to side behavior on the track - the raised center of the track is what keeps the cars going straight. Negative camber would create more width between the wheels and track, meaning the car can spend more time drifting sideways, hitting the center and bouncing back. Not great.

I've never done this, so I'm just throwing some general problem solving at it, but isn't what you describe what would happen if you make the contact point the center of the wheel?

Couldn't you file them down so the contact is on the edge of the wheel closest to the hump?

What vmarks described, looking at the car head on:

---------------
\ /----------\ /
iV0000000000 V

What I'm describing:

----------------
\ |-----------| /
i\|0000000000|/

Does this make sense?
     
Charles Bouldin
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Mar 17, 2008, 03:43 PM
 
I agree with the post that the boy should do it and that the point is not winning. That said:

Make sure the care weighs as close to 5 ounces as you dare to go.
Minimize cross sectional area.
Put a GOOD lubricant on the wheel axles.

Circa 1962, I won the Pinewood derby at my local pack. My dad, a petroleum engineer, let me do all the work and then he put the (then) very new silicon lubricant on the wheel axles. It wasn't the greatest design, but it was mine and I had a great time doing it with my dad.
     
olePigeon
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Mar 17, 2008, 04:09 PM
 
Originally Posted by subego View Post
I've never done this, so I'm just throwing some general problem solving at it, but isn't what you describe what would happen if you make the contact point the center of the wheel?
You're not allowed to modify the kit wheels, so you can't shave or sand them.

However, it doesn't say you can't mount the wheels at an angle. Nail the wheels in at a slight angle, maybe 5 degrees off axis, so that the wheels ride on the edge instead of the flat base. This should reduce friction as less of the tire is making contact with the track, and, less of the tire would be making contact with the nail.

/ --- \


I made a locomotive for my pinewood. It didn't do very well. However, I made it a point to tell the other kids that my locomotive will win in a game of chicken against their car... every time.
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subego
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Mar 17, 2008, 04:20 PM
 
Originally Posted by olePigeon View Post
However, it doesn't say you can't mount the wheels at an angle.

I had this exact same idea and was about to post it, when it occurred to me doing that will likely increase the friction between the wheel and the axle enormously.

With the wheel being vertical, the only contact point between the wheel and the axle is the lower half of the axle. If you put it at an angle, the axle is going to contact the wheel all the way around.

[Edit: more accurately, I don't think the axle will be the contact point at all, it will be the axle cap holding the wheel in place. Of course, all of this is more or less true depending upon how radically you angle your wheels.]

[Edit2: I should also point out that I realize the tilted wheel look would be totally dorky on a pickup truck]
( Last edited by subego; Mar 17, 2008 at 04:53 PM. )
     
vmarks
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Mar 17, 2008, 04:49 PM
 
Originally Posted by subego View Post
I've never done this, so I'm just throwing some general problem solving at it, but isn't what you describe what would happen if you make the contact point the center of the wheel?

Couldn't you file them down so the contact is on the edge of the wheel closest to the hump?

What vmarks described, looking at the car head on:

---------------
\ /----------\ /
iV0000000000 V

What I'm describing:

----------------
\ |-----------| /
i\|0000000000|/

Does this make sense?
FALSE.
NOT WHAT I DESCRIBED.

What I described is two separate things:

(1) if you're going to adjust the wheels for a lower surface contact patch, you put the nails in at a downward angle, so that the wheels have negative camber.

Look up negative camber. It's something race cars commonly have.

(2) SANDING THE WHEELS is not to give them an angle or point. It is simply to make them completely round, by removing the plastic excess (called flashing) from an inaccurate molding process. The flashing is the mold line. sand it off. This used to be allowed in the rules.

I cut the wood bodies myself as a child, but my dad melted and poured the lead weights. No one in the Pack had a problem with that.
     
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Mar 17, 2008, 04:57 PM
 
Originally Posted by vmarks View Post
FALSE.
NOT WHAT I DESCRIBED.

Well, my mistake then.

No reason to yell it though.
     
abe
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Mar 21, 2008, 12:46 AM
 
I know little about the subject but thought I'd include this OBVIOUS bit of info as my 2¢




# Home | Customer Reviews | Lectures | Contact | Links | Search
The Physics of the
Pinewood Derby Book
Composition
Content
Book Reviews
Table of Contents
Example of Theoretical Page
Example of Practical Page

Pinewood Derby Car Design
and the Virtual Race Booklet
Description
Table of Contents
Example - Page 19
Example - Page 9

Virtual Race and the
Gravity-Driven Car Design CD
CD Description
Car Construction E-Book
Measure Parameter E-Book
Virtual Racing Program

Packages
Basic Package
Design Package
Speed Package
Complete Package

Individual Speed Items
Super-Z Oil
Super-Z Graphite
Axle Polish
Bore Polish Compound
Pin Vise & Lube Block
Metal Axle File
Pinewood Derby Physics Home
Announcements: Please have a look at our new Customer Reviews Page. Also check out the Lectures and please drop me a line under Contact if you have a physics question about the pinewood derby that would make a good lecture. Remember Lectures 14 through 18 form the basis of a pdf E-Book with photos and short videos showing every step of the Simple But Fast car construction and lubrication. And remember you get the Grab Bag at no extra charge with the Speed, Basic, or Complete Packages.

THE PHYSICS OF THE PINEWOOD DERBY BOOK - The days are over for tips, hints, old wive's tales, and all the other rumors that accompany the popular pinewood derby racing scenario. Rather than superstition and hearsay, we can now share facts with our kids based on physics and natural laws. The large library-quality simulated leather-bound Physics of the Pinewood Derby book captures 15 years of professional research and development on the dynamics of gravity-driven model race cars. The equations of motion have been solved for various types of tracks and cars, even large gravity-driven cars with drivers. And practical designs are presented based on subjects that range from friction studies, wind tunnel testing, and statistical analyses.

PINEWOOD DERBY CAR DESIGN AND THE VIRTUAL RACE is a booklet written to the youngster that explains how to make the measurements associated with his (or her) car design and input them into the virtual race program. There is now available an E-book version of this program called Measure Parameters for the Virtual Race that could be substituted for this paper-back booklet if you have an Adobe pdf file reader. The E-book version comes with the Virtual Race program. THE VIRTUAL RACING AND GRAVITY - DRIVEN CAR DESIGN CD has been expanded with new features that will let even the first year Cub work effectively in a team with their parent or adult partner. Click on the links below to get more detail on the 3 main parts of this CD. One part, a well illustrated Car Construction E-book (also see Lecture 14 & 15) starts right when you open the official Pinewood Derby car kit. This part also contains a car lubrication (see lecture 16,17&18) section with video clips. A second file on the Virtual Race CD is a Measure Parameters E-Book which is a condensed printable version of the pdf booklet Pinewood Derby Car Design and the Virtual Race described in the section above.

And, finally, the third part is the original Virtual Race program. This race simulator lets the user put in whatever design or measurement they desire for any kind of gravity car on several types of track. The program uses the advanced mathematics developed in the Physics of the Pinewood Derby book to predict the precise motion of any gravity car based on its parameters. This Virtual Race program has embedded in it a Virtual Race Help manual which is a third very extensive E-book that covers every aspect of racing at 2 levels, one for the Cub and one for the advanced user. The program has been tested on Windows 98, XP, and 2000 systems, requires 35 MB disc storage space and works best at 1280 x 1024 or greater screen resolution.

FOUR PACKAGES offer various types of hardware in addition to or in combination with the CD and books above. The Design Package gives all three items above along with vernier calipers that allow the youngster to take measurements of key dimensions of their race car for input into the virtual race program. The Speed Package provides a swivel-head pin vise, polishing block and axle file to be used for drilling, axle polishing and lubrication. It also contains the Super Z graphite, a specially blended mixture that has an extremely low coefficient of friction when applied properly. In addition, this package has bore polish and axle polish compounds which can further reduce the wheel/axle friction. The polish cloth and cardboard dowels which are provided are key to proper lubrication techniques.

The Basic Package is new and has been developed especially for the new users who want to start with fundamentals so this package contains everything mentioned above except Super Z oil and the big Physics of the Pinewood Derby book. Finally, the fourth package is the Complete Package which contains all the items (except Super Z oil). Also, we are now advertising a free Grab Bag of 8 or 9 incidental items to help in lubricating and building a car that will come with all packages except the Design Package. Remember the Super Z oil is a speciality item which is not contained in any package and must be ordered separately since some race rules do not allow oils.


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© pinewoodderbyphysics.com 2005-All Rights Reserved
Last Updated: Friday March 21 2008
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Mithras
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Mar 21, 2008, 01:46 AM
 
Teh Goog is scary. This thread is #3?
     
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Mar 21, 2008, 02:10 AM
 
We had nice aluminum tracks in my Pack.

-Owl
     
design219  (op)
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Mar 21, 2008, 07:13 AM
 
Originally Posted by Mithras View Post
Teh Goog is scary. This thread is #3?
I've noticed this with other topics. This forum indexes really fast and high.

Buy the way, the car (truck) is finished and rolls very straight and very fast. We have one front wheel off the ground. My son is very happy with how it looks, although he had never used spray paint before.

The race is Monday evening.
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Mar 21, 2008, 09:29 AM
 
Yes, learning how to spray paint was a blast with my boys. Neither could really grasp the concept of applying the paint in a uniform way, but they certainly tried hard. Getting the car right at 5 ounces is key and will probably be the biggest factor in running fast. Some of it is just luck. Just make sure you take lots of pictures, this is something they will most likely remember for the rest of their lives. I hope your son wins!
     
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Mar 21, 2008, 02:59 PM
 
for my pinewood derby, i made the better of the other cars. from there on out the other kidddos hated me because i always won, even when one of the dads tried to rig the race so that his son would win. my pack was very corrupt to say the least. it seemed as if every other meeting the son of the pack leader would move up a rank, when it took me and most of the others a couple of months per rank level; more so on the upper levels
     
olePigeon
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Mar 21, 2008, 03:17 PM
 
Originally Posted by design219 View Post
We have one front wheel off the ground.
Is that a bad thing? What are the rules on purposefully only having three wheels?
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design219  (op)
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Mar 21, 2008, 03:19 PM
 
Originally Posted by brassplayersrock² View Post
my pack was very corrupt to say the least.
Whoa.

Our pack is brand new and I'm the Cubmaster (somebody had to be). We only have 8 boys and all Tiger Cubs. The parents all seem OK so far. I never considered there being issues like that going. It's still new to me.
( Last edited by design219; Mar 21, 2008 at 04:43 PM. Reason: typo)
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design219  (op)
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Mar 21, 2008, 03:56 PM
 
Originally Posted by olePigeon View Post
Is that a bad thing? What are the rules on purposefully only having three wheels?
That goes back to the original question. Some of the "experts" on the web say it's better because of the lesser friction. I have doubts, but we did it anyway.

There are no rules about having to have four wheels touch the track, and for some kids, making them all level might be difficult.
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Mar 21, 2008, 11:28 PM
 
Did none of you do the raingutter regatta?


Our pack would alternate the Pinewood and Raingutter from year to year. Admittedly, I enjoyed the boats more since I won the first time I entered but it didn't lend as much potential for creativity as the cars.

In the summer during the regional city powwow we would have actual go cart races which kicked the hell out of all of the model races. That was a team building exercise though since it was required at least 4 kids contributed to the building of and in the race itself.

I don't have kids but it seems like scouting is on the decline. I was actually surprised this many people talked about still doing the pinewood derby with their sons.

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abe
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Mar 22, 2008, 12:14 AM
 
Originally Posted by Captain Obvious View Post
Did none of you do the raingutter regatta?


Our pack would alternate the Pinewood and Raingutter from year to year. Admittedly, I enjoyed the boats more since I won the first time I entered but it didn't lend as much potential for creativity as the cars.

In the summer during the regional city powwow we would have actual go cart races which kicked the hell out of all of the model races. That was a team building exercise though since it was required at least 4 kids contributed to the building of and in the race itself.

I don't have kids but it seems like scouting is on the decline. I was actually surprised this many people talked about still doing the pinewood derby with their sons.
I'd never heard of it til this thread.

I knew Soap Box Derby, but when I read Pinewood Derby I thought it was a full sized race and something dreamed up by the Brits. (Pinewood Studios)
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Mar 22, 2008, 10:23 AM
 
I did them all as a kid, and have helped our current pack with all as well. Pinewood derby, raingutter regatta, and the space derby. Of all races, the Pinewood derby works the best. The boys take it personally if their boat doesn't win (since the push them by blowing). Not so with the Pinewood derby, where gravity is responsible.
     
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Mar 22, 2008, 04:20 PM
 
Originally Posted by Captain Obvious View Post
Did none of you do the raingutter regatta?
upldPictures%5CUnit356%5Cpics3493%5Cpiny2470.jpg
I never liked this one. Always made me dizzy, was a mess, hard to do. meh.

Now the one I really liked was the Space Derby…



You would wind up the propeller (powered by a rubber-band going through the ship) and then attach it to a hanger on a fishing line track. (here is a pic of a tack)

Lots of fun. Part of the fun was when peoples rockets would shoot the peg out the end holding the rubber-band to the back of the rocket when they wound it up too tight.

-Owl
     
design219  (op)
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Mar 22, 2008, 05:35 PM
 
Cool! That looks like fun. This shows how new I am at this (our pack is less than three months old). I'm the packs cubmaster and I was not familiar with either the Raingutter Regatta or the Space Derby.
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Mar 22, 2008, 06:06 PM
 
Originally Posted by design219 View Post
Cool! That looks like fun. This shows how new I am at this (our pack is less than three months old). I'm the packs cubmaster and I was not familiar with either the Raingutter Regatta or the Space Derby.
Yeah, search around online. Both have official kits though the BSA.
     
   
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