OT: Pine Derby Car building help

[Dario Octaviano]

I am assisting my nephew to make his first (and mine) pine derby car...anyone have tips to make descent one?

My main question is where is the ideal center of gravity located on the length of the car? I know it should be as low as possible but not sure if the CG should be nearer the rear or the front. I read that it should be 1/3 from the rear but not sure if that is good advise.

They can only use the nail type axle and the approved nails are much smaller than the wheel holes which is causing some wobble...anyone have a good fix for this? I am planning on building it up a bit with thin CA http://www.penturners.org/forum/images/icon_smile_big.gif but not sure if that is a good idea or not.

Planning on using graphite powder lube...is there better choice?

Thanks!

[Joe Pelonio]

It's been a while but I helped my son with many of those. We went for the center of gravity low and over the rear wheels to the middle. We used fishing lead (pencil style) inserted into holes drilled in the bottom rear, plugged with lightweight spackle. With a Postal scale try to get as close as possible to maximum weight without going over. We used the poweder graphite lube too, and the nails have to be tapped in very carefully so that there is little play, yet have to be straight enough that during testing the car goes as straight as possible. What we did was predrill the holes then glue them in and adjust the tracking after the glue set up. If still allowed, we also sanded off the tread on the edges of the tires.

[Lee Schierer]

First of all, let your nephew do as much of the work as you can. With my son, I set up a drum sander on the drill press and let him shape the car. He did the design, he did the majority of the contouring and all of the painting.

The whole thing works on Potential energy; so in theory the higher up the track you get the majority of the weight and the heavier the car the faster the car will go. You also have to condiser frontal area and wind resistance or drag. The lower and smoother the profile the faster it will go. You want to insure that you can have the car at the maximum weight allowed. Make a recess in the bottom of the car and fill it with lead fishing weights or birdshot mixed with modeling clay. At the weigh in if you are too heavy you can remeove a bit. If you are too light you can add some weight.

Most rules won't let you substitute the wheels or axles and won't let you modify the wheels except to remove the molding flash. Do this very carefully. You need round smooth wheels. If you look closely under the head of the nail you will also see some flash from when the head was formed. Carefully file this out and polish the nails. If you have a buffing wheel, buff the axles to get them smooth. Powered graphite lube should be applied after each run if you can. Drilling pilot holes for the nails is the way to get them in straight. All the wheels need to touch for the car to go straight. A three wheeler might be faster, but it may also not go straight.

There are oodles of web sites with designs and pointers. After you read a few you will get the idea of what is fast. Some packs also give awards for most creative and best paint.

REMEMBER THIS IS HIS CAR, IF YOU WANT TO DO ALL THE WORK GO BUY YOUR OWN KIT. Some packs even have a race for the highly competitive fathers or in your case unlces. You want him to be able to say his uncle helped, not that his uncle built it.

My son made four cars and finished no higher than second in his pack and about the same at the district. He still has all his cars and he is 30+ now.

[Joe Pelonio]

First of all, let your nephew do as much of the work as you can. With my son, I set up a drum sander on the drill press and let him shape the car. He did the design, he did the majority of the contouring and all of the painting.


Good point. Mine did all the work too, I just helped by making suggestions, but he's been using tools since he was about 5. The only problem we had was the mess he made when spraying the paint!

[Lars Thomas]

Here's another reason to let your nephew do the work. . . think of it like this: If you do all the work and lose to a bunch of 9 year olds; that'll be a blow to the ole self esteem! (I'm still recovering)

[john dennis]

Well, in my day back in Brooklyn, NY, we got a piece of 2X4, split a roller skate in half. Nailed on half to one end of the twoby and the other half to the other end. Then we nailed a wooden milk crate on top of the twoby and raced down the street hoping no cars were coming up the street... City life is always just a little bit different than the rest of the world.
Good luck!

[Eddie R Norton]

Many years ago when my son was a Cub Scout we built a car that won our derby. An engineer friend of mine advised us to focus on aerodynamics since that is where 80% of the friction/resistance is. So we cut a shape that basically looked like an airfoil and sanded it very smooth and applied a rubbed laquer finish.

We also drilled a hole in the body that we filled with BBs to get the weight to the maximum allowed (5 oz then, probably the same today). If you do this make sure that they are glued in solid since any loose parts will disqualify it. Some think the CG should be just in front of the rear wheels but I am not sure if this is science or lore.

Mount the wheels in a drill and sand off any bumps in the plastic and you can legally apply graphite (dry lock lubricant) to the axles.

Good luck. And, whatever the result, I hope you have a good time with your child building it.

[Jeff Weight]

As for the design shape & paint, let the kid do most of that work, with you supervising.

For the technical winning stuff....make the wheel base as long as possible, this requires drilling holes for the wheel pins. Weight the car in the back, as this will give you the most inertia at the bottom of the ramp, launching you ahead of the competition. Use the half round derby lead weights if you can find them. Buy the wheel polishing kit from the BSA and follow the instructions. Us the finest powerdered graphite lube you can find. Put the wheels in a zip lock bag with some graphite a few days before the race and shake regularly to coat the wheels. Install the wheels square to the body and use a feeler gauge to adjust the spacing between the body and wheel.

That about does it. Take some pictures of the kid, car, and his blue ribbon and post them here.

Good Luck!

Jeff

[Derek Rose]

I am actually a Wolf Den leader in Cub Scouts, and our Pinewood Derby is coming up in a few weeks. I can remember making pinewood derby cars with my dad and setting those cars by my bed for weeks after the actual races. It's all about the boy's experience with tools and creativeness.

The nice thing about this project is that it can be split into two parts, already mentioned above. He can work on contouring the car, sanding it down and painting it. You can help out with the most important aspects, such as polishing out the burrs in the axels and sanding the wheels. The speed lies in the axels. Keep the tires from wobbling and make sure they don't rub up against anything on the nails.

9 times out of 10, it ends up turning into a fatherly competition. That's why we had the dads do one a few months ago for themselves. :) And the car that won wasn't sophisticated. It was a bulky version of a suburban, complete with roof racks and camo paint. And yes, it met the weight limit.

[Michael Cody]

I am assisting my nephew to make his first (and mine) pine derby car...anyone have tips to make descent one?

They can only use the nail type axle and the approved nails are much smaller than the wheel holes which is causing some wobble...anyone have a good fix for this? I am planning on building it up a bit with thin CA http://www.penturners.org/forum/images/icon_smile_big.gif but not sure if that is a good idea or not.

Planning on using graphite powder lube...is there better choice?

Thanks!

Make sure you know your pack's rules. My pack rules stated you had to use the wheels & nails that came with the kit, you couldn't modify them. Only graphite dry lube was allowed. Additionally you had to fit the size (we have a size box the car must fit in.. and we were at the 5 oz. limit. If the rules let you, route out the slots for the nails and replace with a piece of hardwood that has nail holes drilled square and accurate. The slots often are not square with each other in the kits.

Best cars I've run in the dad's race and based on 10+ years experience with my sons ... is the old lead sled... basically cut the body down to about 1/2-3/4 inch thick -- just a flat piece of wood with the wheels mounted as straight, square & even as possible. You want the sucker to track straight. Then hang all the weight as far back a possible ... center of gravity should be at or behind the rear wheels. Based on the kits we got, we actually always built the cars backwards as the rear overhang we more than the front. So we put reversed the body to get the most overhang on the front. We were allowed to take any flash off the wheel outsides .. and we always polished the nails -- just put them in a drill and polish till they shine.

Lastly the old lead sled can look good too. My youngest son's last car was the sled with a toilet paper wrapper on top with a nose cone ... it said ACME ROCKET on the side and had a small Wyle E. Coyote cutout on the top of the rocket. Came in 2nd. One of my scouts built a sled the looked like a hersey's candy bar, another built a piano keyboard. The last 5 years I was in cubs (my oldest is an Eagle scout now and in college, his younger brother is still in scouts @ 1st class) .. my den finished 2-3 boys in the top 5 each year... including the winner in 2 of them.

[Tom Henry]

You have to go online and get the best axles and wheels...That is the key...

[Dan Mages]

Wow... I remember building many, many pine cars as a kid. Everything from the General Lee to a hershey bar. I might still have one sitting in my parent's basement. The key is to let the kid have fun. You can also use this as an educational opportunity to teach your kid the basics of physics and aerodynamics.

Have fun and good luck.

[David Epperson]

You have to go online and get the best axles and wheels...That is the key...
But illegal in some councels. So know your rules.

Make sure you know your pack's rules. My pack rules stated you had to use the wheels & nails that came with the kit, you couldn't modify them. Only graphite dry lube was allowed. Additionally you had to fit the size (we have a size box the car must fit in.. and we were at the 5 oz. limit. If the rules let you, route out the slots for the nails and replace with a piece of hardwood that has nail holes drilled square and accurate. The slots often are not square with each other in the kits.
We usually sanded the bottom of the car so that the slots were mearly a suggestion as to where the axles went and drilled the holes directly above the slot locations. Axles were between .070 and .085 inches in diameter so a drill slightly larger than 1/16 worked best - real snug, but not enough to split the wood.


Best cars I've run in the dad's race and based on 10+ years experience with my sons ... is the old lead sled... basically cut the body down to about 1/2-3/4 inch thick -- just a flat piece of wood with the wheels mounted as straight, square & even as possible. You want the sucker to track straight. Then hang all the weight as far back a possible ...
All true, till this part

center of gravity should be at or behind the rear wheels.
If the CG is behind the rear wheels the front wheels will be levered up off of the track. I found that right around 3/4" to 7/8" in front of the rear wheels worked best for our set up - long overhang to the front of car and wheelbase/axle locations as in the supplied kit. Any farther back and the front end would lose stability and develope too much side to side wobble, which bleeds speed. I always was of the opinion that the extended wheelbase philosophy was counter to the spirit of the rules and purpose of the game...Just my opinion. But with with several pack championship cars, a district champ car and a councle wide champ car built by my instruction by my boys over a 15 year span of building and teaching, I think I know a bit about it. But if you rules and concience allow it, do it your way.:D


Based on the kits we got, we actually always built the cars backwards as the rear overhang we more than the front. So we put reversed the body to get the most overhang on the front. We were allowed to take any flash off the wheel outsides .. and we always polished the nails -- just put them in a drill and polish till they shine.
We agree on body direction. I used a benchtop drill press as a lathe and taught the boys to use a needle file to take off the worst of the flash, followed by an inexpensive whetstone to take the humps out, followed by a high speed spin with Flitz or Simichrome polish. One other item, when removing the flash I had the boys put an ever so slight (5 to 10°) bevel on the inside of the nail head - just to reduce the amount of area in possible contact with the wheel hub. No disc brakes, more speed.


Lastly the old lead sled can look good too. My youngest son's last car was the sled with a toilet paper wrapper on top with a nose cone ... it said ACME ROCKET on the side and had a small Wyle E. Coyote cutout on the top of the rocket. Came in 2nd. One of my scouts built a sled the looked like a hersey's candy bar, another built a piano keyboard. The last 5 years I was in cubs (my oldest is an Eagle scout now and in college, his younger brother is still in scouts @ 1st class) .. my den finished 2-3 boys in the top 5 each year... including the winner in 2 of them.
Good Job Dad!

[Dario Octaviano]

All the info and advise are great and appreciated.

Thank you very much guys!!!

[David Epperson]

I am assisting my nephew to make his first (and mine) pine derby car...anyone have tips to make descent one?

My main question is where is the ideal center of gravity located on the length of the car? I know it should be as low as possible but not sure if the CG should be nearer the rear or the front. I read that it should be 1/3 from the rear but not sure if that is good advise.
As a mechanical engineer, one year I got the wild hair to do the entire energy balance equation for a pinewood derby car. Some of what I learned from that might surprise a few folks.
First -these cars are going at the breakneck speed of only about 10 miles an hour (Same speed MAX as dropping them from a vertical height as high as the starting gate - and not even that fast by the time all is said and done). So while aerodynamic styling is fun, so long as you don't mount sails or parachutes to the car - it's not going to matter that much. :D

Second - the little wheels are spinning at the break neck speed of over 3000 rpms - so friction losses to the axle and wheels ARE critical. Minimise them at all costs. And the more you take off of the outside of the wheel to smooth it out, the faster the wheel has to rotate to maintain speed. Only take off enough to smooth it off. Then quit. Watch for those folks that narrow the wheel up - this reduces the rotational inertia of the wheel, which reduces the amount of gyroscopic energy stored in the rotating wheel, leaving more energy available for linear speed...which is a form of cheating. And one which is hard to spot and catch. (Which is one of the reasons why wheel modifications are not allowed).

Third. After building and testing several of my own cars I determined that 3/4" to 7/8" in front of the rear wheel was about as far back as I could safely locate the Center of Gravity (the point where the car would balance front to back) before we started losing stability in the front end. I then taught the boys to build them that way.



They can only use the nail type axle and the approved nails are much smaller than the wheel holes which is causing some wobble...anyone have a good fix for this? I am planning on building it up a bit with thin CA http://www.penturners.org/forum/images/icon_smile_big.gif but not sure if that is a good idea or not.

Planning on using graphite powder lube...is there better choice?

Thanks!
Not a good idea. This was a fairly common problem with the later cars the last time we built any, but the wheels only "wobble" when you spin them with no weight on the axles, with the cars weight on them holding the axle to the bottom of the hole, it's not as big an issue, and lets you pack more graphite into the wheel. I used a film can full of graphite and a pipe cleaner in a dremel to work the graphite into the wheel bore. I also tumbled the polished axles in this same can of graphite for a few days before final installation. But we DID mix a 50/50 mix of graphite and silver paint to apply to the body area around the axles...just to prevent the wheels from digging into the wood body quite so badly.

[Scott Loven]

We built two cars exactly the same one year, one won the pack, and 1 came in about 20th or so. This is what we did.
The boys designed, I cut, they sanded, painted etc.
1) polished the Axel and filed off the nib between the Axel and the head
2) stuck the wheel on a nail, sanded it true, and polished the wheel surface as well as the part that touches the car.
3)re-cut the slots so the axles were parallel to each other
4)stuck the wheels on the axles and on the cars, and ran the cars on a plate of glass to see that they all touched the ground. We pushed the cars along the glass to see if any of wheels wobbled.
5)drilled holes in the bottom and back for the lead
6)filled in the holes, weighed the car, smoothed and painted the holes, and waxed the car.
7)checked the wheels again per 4 above, super-glued the axles in place and went to the race.
Scott

[David Epperson]

We built two cars exactly the same one year, one won the pack, and 1 came in about 20th or so. This is what we did.
The boys designed, I cut, they sanded, painted etc.

2) stuck the wheel on a nail, sanded it true, and polished the wheel surface as well as the part that touches the car.

The wheel polishing mandrel (looks like a longer version of the Dremel cutoff wheel mandrel or Dremel buffing wheel mandrel) works much better for this operation.

4)stuck the wheels on the axles and on the cars, and ran the cars on a plate of glass to see that they all touched the ground.
Some people actually advocate letting one wheel stay off the ground to eliminate the rotational energy that gets stored in it. I've always found your way to be more consistent.

We pushed the cars along the glass to see if any of wheels wobbled.
Good practice!


5)drilled holes in the bottom and back for the lead.
Care must be taken here since there has to be more than 1/4" and preferably 3/8" of clearance between the bottom of the car and any flat surface to allow for free clearance around the race track guide rail.


6)filled in the holes, weighed the car, smoothed and painted the holes, and waxed the car.
Waxing the car is a step often forgotten about. And one best done before the wheels are put in for the last time.

7)checked the wheels again per 4 above, super-glued the axles in place and went to the race.
Scott
Here is another area where I've seen over anxious parents mess up. The least little amount of super glue (or Locktite, DAMHIK) in the wrong place here will slow the wheels down and in a worst case will glue the wheel to the axle, preventing it from ever rolling again. Seen that happen unfortunately. I prefer a TIGHT press fit, and modified an old set of needle nose pliers just for pushing and pulling tight fitting axles.

[Al Willits]

Many years ago the neighbor and his kid were over, they had been building one of these cars and did pretty well, dad was smilling and proud of his kid and how well he did.
We chatted for a bit and the dad headed back home, I was lubing the kids bike chain and asked him if he did enjoy the competition, he replied ya, but next time he hoped to build it...

This is a true story, and one I hope proud papa's heed.
Just a thought fwiw
Al

ps...good luck to the kid...:)

[Sam Chambers]

Dario, you've already received some good advise, but here's mine:

1. It's his project, not yours. Help and guide him, but let come up with the design, and do as much of the work as is safe. When my step-son was racing pine cars at our church, I let him draw out the design on paper, cut it out and trace it onto the wood blank. Then I cut it out on the bandsaw. He sanded (and sanded...and sanded...) and painted it however he wanted.

2. If there's an "open" division, build one of your own! I won for best design every year, though mine never raced worth a darn!

3. As others have mentioned, get a copy of the rules that will be in force on race day, and follow them.

4. My step-son's cars always raced better than mine. On his, we de-burred and polished the axles (nails), and trued up the wheels on the drill press. We were only allowed to use powdered graphite as a lube, and we shot some in between the wheels and the car body right before the race.

Have fun with your little guy, and best of luck!

[Jeff Weight]

Scott Loven reminded me of a couple of things.

1. Don't cut a new slot for the axil, instead drill holes. This is much more precise.

2. While drilling the holes drill one slightly higher up on the body than the other 3. This allows the car to run on 3 wheels, decreasing the amount of drag.

[Scott Loven]

http://www.woodcraft.com/images/family/web20013big.jpg
Just saw this in a woodcraft flyer only $12.95
Pinewood Derby Speed Secrets


"If you want to compete successfully your first time out, or improve on last year's finish, this book is for you." - Don Murphy, Founder, Pinewood Derby

This the only book on the market to guide parents and children through the process of building a winning Pinewood Derby racecar. Step-by-step instructions enable readers to build the car body, prepare the axels and improve the wheels with minimal tools. As an undefeated derby racer and parent, author David Meade reveals the inside secrets that will separate any car from the rest of the pack. Inside you'll find:

Winning car patterns
A gallery of pinewood derby cars
Secrets for shaving seconds off your race time
27 diagrams and 200 photographs that make building easy
128 pgs., softcover

[David Epperson]

Been a while...How'd y'all do?

[Mike Kenney]

I agree with the people saying let him do most of the work himself as a child i remember doing my own. And now look at me thousands of dollars later im still a kid working with wood and injoying it.

And on the note of where to put the weight the best results i remember was right behing the front axel