Pinewood Derby track question: Engineering advice wanted

[Roger Feeley]

I help the neighborhood kids build their cars every year and every year I wish I had a test track. The track will be easy to build but I don't want to mess with a timer. I think I can build a timer, I just don't want to. I would probably make it by hacking the on/off button on a cheap stopwatch. The alternative would be something more expensive than I want to mess with. Anyway, it's just a test track and a timer is just one more thing to store and keep working.

It occurs to me that if I make the slope on my track so that it runs slow, I can substitute how far the car goes for how fast it goes. I simply make the starting slope low enough so that the car doesn't build up enough speed to reach the end of the 32' track I will build. A kid can run the car, note the distance and make adjustments. Run it again and see if it goes further. I will build a two lane track so kids can run one car against another. I figure, I need at least 12" of width so why not make it two lanes?

Additionally, running the car slower means that we would have a better chance to observe the cars behavior. Is it going left or right? That sort of thing.

I've read that aerodynamics really aren't much of a factor in Pinwood Derby cars so I don't see that making the track slow would be a bad thing.

Comments?

[Tom Bender]

It's a shame that kids can no longer build their own cars and can't get any help from their parents. Glad you are helping out. A few years ago I helped my neighbor build his son's car. Said son not present.

Rant over. Yes I think you are on the right track (sorry)

[Bill Carey]

Helped my son build his and now help my son help my grandson build his. I agree with everything you said Tom. In my experience THE 2 most important things are center of gravity and letting the car run on 1 rail or the other. Which is why your test track is a great idea. When they bounce from one to the other rail they lose a lot of speed. So your test track can help them keep their cars just barely kissing one rail all the way down. It's a small advantage but worth it. Well done Roger - you're one of the good ones.

[Roger Feeley]

It's a shame that kids can no longer build their own cars and can't get any help from their parents. Glad you are helping out. A few years ago I helped my neighbor build his son's car. Said son not present.

Rant over. Yes I think you are on the right track (sorry)

Rant fully justified! And thanks for the validation. I'm not a physicist.

I have a number of rules to which the parents must agree before I will help:
1. I accept no remuneration of any kind. I don't even want cookies.
2. I want one parent there the whole time to generally observe
3. I will not touch the car. Only the child will touch the car.
4. While in my shop, the parent will not touch the car.
5. We will be using power tools and power tools can be dangerous. I keep plenty of car sized scrap around and I do a lot of practice and pre-visualization with the kid during which I will touch the work. When it comes down to actually working on the car, see rule #3.
6. In the interests of safety, I reserve the right to take measures to avoid accidents which may include touching or even striking the kid. At this point, I tell the story about how I clocked my wife once to save her thumb.
7. Parents are allowed and even encouraged to intervene if things are going south. What they aren't allowed to do is 'help'.

I've helped quite a few kids over the years. We generally use a metal lathe to turn the little bits off of the nails. We use a band saw. I really don't draw the line anywhere. When using the band saw, I generally whip up some special push sticks to insure the 3" rule. We talk a lot about body position and balance and "Where do you want your hands to go if the work slips?" I probably overstress the notion of not trying to save the car if something catches it. "Just let it go", I say.

All that preparation and focus on safety is really necessary because mine is probably the only shop for miles. I'm in the DC area inside the Beltway. Finding any kind of unfinished basement around here is a miracle because land is so expensive. These kids have never seen anything like what I have.

I usually spend 3-4 hours per kid. I've never had a kid lose interest but, in spite of all those rules, it's really the kid that drives the work. I just ask them what they want to accomplish and then I teach them how to do it.

My oldest grandson is 2 1/2. I ought to be pretty well tricked out by the time he's ready. He can already tell the difference between a ball-peen hammer and a dead blow. Can't say the names but I know what he means. He's a bright kid but aren't they all?

[Lee Schierer]

What I learned from several years of car building with my son.
Most important, let the child decide the type of car he wants and let them do everything that they possibly can. Don't sweat perfection.
1. Build the car to the maximum weight. Make provisions to add or remove weight because your scale may not match the"official Scale" the night of the weigh in.
2. Trim all flash off the wheels and nails
3. Once you have the nails where you want them secure them with super glue.
4. Streamlining does help with speed
5. If allowed squirt powdered graphite onto the axles just before the race.

[Roger Feeley]

What I learned from several years of car building with my son.
Most important, let the child decide the type of car he wants and let them do everything that they possibly can. Don't sweat perfection.
1. Build the car to the maximum weight. Make provisions to add or remove weight because your scale may not match the"official Scale" the night of the weigh in.
2. Trim all flash off the wheels and nails
3. Once you have the nails where you want them secure them with super glue.
4. Streamlining does help with speed
5. If allowed squirt powdered graphite onto the axles just before the race.


I agree with you on the streamlining. But aerodynamics can be judged visually. I don't see any need to tweak aerodynamics using a test track. The big difference in performance is in wheel friction and alignment. That gets me back to my original question of whether or not I can use distance on a slow track vs time on a fast one.

This may be digressing a bit. I read about a pack that had a rule where the kid that built the winning car had to tell how they did it. Also, they had to host a build the next year and offer the same advantages to all the rest of the kids. It helped keep things on the up-and-up. I don't know how they could enforce me helping though. I'm certainly not willing to help 50 kids. I guess if they instituted a rule like that here, I would just have to withdraw.

[Doug Garson]

First I'd like to say well done, I think what you are doing is great and will likely make a big difference in the kid's lives. To answer your question I don't know anything about Pinewood derby but I assume these are miniature cars running down an inclined track. So the driving force is gravity and the resisting forces are friction in the wheels and bearings and air friction. I think your "slow track" will enable the kids to optimize the cars to eliminate friction in the bearings and wheels run straight (shortest distance) and to a lesser extent air friction just fine. A full speed track with a timer might be marginally better but as you said, more complicated. I'd love to see some photos of the cars if you have any. I'm a retired mechanical engineer but not an expert in this field.

[Bill McNiel]

Roger,
I could not agree more with your approach. Your shop safety rules and no touch rules are dead on as well. Kudos Sir!

I believe that the distance vs speed evaluation is a valid concept.

Your participant rules are exactly what all should embrace. Thank you for bringing some faith into a trying time.

[Eric C Stoltzfus]

This guy has some great tips on making pinewood derby cars as well: https://www.youtube.com/watch?v=-RjJtO51ykY

[Stan Calow]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

[Pat Barry]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

Its called Pinewood derby. LOL

[Mitchell Ristine]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

Hitting the max weight is important. But equally important is the placement of the weight. Having the weight of the vehicle equally spaced over the whole vehicle loses potential energy. Get all that weight as low as possible over the rear axle.

[Ole Anderson]

Boy this thread brings back old memories. My dad helped me build my car, I helped both my son and grandson build theirs. Even helped build a track once.

[Roger Feeley]

Its called Pinewood derby. LOL


Pine is probably ok. If you look at the design of most cars, there isn't much wood left. If the kid wants to keep the car, a better wood might be a good idea. Most kids paint the cars so there wouldn't be any way to tell the wood species.

Man, you ought to see all the little gizmos for tweaking these cars. There's a whole industry around making them fast. And a lot of physics. The local Boy Scout supply is next door to my local Woodcraft so I dropped in there to pick up a couple of kits to have spare parts for my little trainees. I was amazed to see all those tuning gizmos for sale at the BS store. To me, those resources are a bit shady (as am I). Somehow it rubbed me wrong that it was being sold in that store. I asked if they were directly affiliated with the local counsel and they said yet. I guess what I think of as sort of cheating is sanctioned by the Scouts. Frankly, I see no way that your average Cub Scout could get their heads around most of that stuff.

Check this out: https://www.derbymonkeygarage.com/pinewood-derby-car-tools-s/1827.htm

[Roger Feeley]

Technically, you don't exactly have to have a track to adjust some things. You can go a long way with just a 4' flat rolling surface like an old countertop. Draw a line down the middle to serve as a center line and then parallel lines about an inch on both sides. Incline the surface a bit, say 10 degrees. You want the car to roll but not very fast so you can see what's going on.

Now, find the biggest ball bearing you can. I have a 2" one sitting on my desk. Place the bearing on the center line and let it roll to the bottom. Adjust the surface so that the bearing follows the center line exactly. Now you have your surface ready to test cars.

Set up a car at the top so that it aligns exactly with the center line and release it. What you are looking for is for the car to veer off about 1 to 1.5" either way. This is the most basic setup. If you are doing a 'rail rider', you need the car to veer in a certain direction.

My track idea is not so much for adjusting alignment but for tuning rolling resistance. I realized that If I do the slow speed track, any little defect could affect the speed of the car by a lot. I will be careful to sand everything very smooth.

Interestingly, the local Cub Scout pack has a track with a hump in the middle. You go down the initial incline, flat for a bit and then up and over a hill. I noticed last year that some cars went a bit airborne on that hill. In thinking about that, I realized that they are essentially imposing a speed limit. Imagine an extremely fast car. It hits that hill and just takes off and never finishes. There would be a maximum speed that you can't exceed. Or maybe you would need some sort of front spoiler. Maybe make the ground clearance of the car really low in front. I don't think I like that hump....

[Bill Orbine]

I heard a story about the local cabinetmaker whose son was in the cub scout preparing a car for the pinewood derby. The cabinetmaker felt it wasn't fair that his son had all the advantages of having a shop where many of the other kids were at a disadvantage. The cabinetmaker staged some free time to invite all of the scouts to come and use his shop to build their cars. Of course, with some limits to what machinery and tools they could use for safety reasons.

[tim morris]

Roger it's awesome to see someone else doing this. I'm a scoutmaster and cut cars for all of the girl scout troops in our town as well as the cub scout dens in 2 packs in town. Around 150 cars a year. The level of youth participation depends on their age. I agree it has to be their design 100% their involvement beyond that is what is allowed by scouting rules. As far as your track design I like lowering the slope like you said. To slow them down at the bottom and keep them from leaving the track make your center rail that the cars ride over slowly elevate (get taller like a ramp) so it picks the wheels up off the track as it nears the end of the run. Most of the new aluminum tracks are build this way and it works well to slow the cars to a stop without causing damage or allowing them to shoot off the end of the track.

[Phil Mueller]

Roger, I applaud your set of rules. I was the Cub Scout leader for my son’s pack in the Detroit Area. As you can imagine, we had a few parents who were automotive designers, engineers and the like. One year at weight in, a young scout brings up a car that appeared to be an exact duplicate of an open wheel CART racing car. It weighed exactly the right amount and had a spectacular paint job.

It won the race. I went up to congratulate the son and father (an automotive designer). During the exchange, the father mentioned his co-workers enjoyed running it through the wind tunnel test.

It’s a shame when it’s very likely the scout never touched the car until the moment he walked it up to be weighed.

[Brian Elfert]

Years ago, I participated in a no rules Pinewood Derby meaning no weight limits. The winner's car weighed over 2 pounds. It literally had a bag of bolts strapped to the top of the car. It was surprising it didn't destroy the track as the bumper at the bottom was never designed for that much weight.

The moral of this story is that more weight wins over polished axles and aerodynamics. A real Pinewood Derby with a weight limit means everybody is going to have cars weighing about the same so some of the "tricks" might help.

[Andrew Seemann]

Interesting to see this thread now, I just agreed to let my son's former Cub Scout pack come over to my shop in January for the annual pine car derby build, even though I don't have a kid in Scouts anymore. I usually cut out the cars on the bandsaw and then take the saw marks off on the sander. I know they are supposed to learn character and stuff like that from spending hours sanding their cars, but I find that making them do all the sanding just makes them learn to hate woodworking.

Back to the actual question. I found that the same car will run differently on different tracks. One year I had one kid win at the pack and the other one at council. The pack winner was faster on the pack track, and the council winner was faster on the council track. We did everything the same for both cars and both races. The thing that consternated me was that my oldest son had the faster wheels and the faster running car in our tests (it was his brother's first year so we gave the oldest one the fastest stuff since it was one of his last races), but my youngest's car was faster on the pack's track. The different tricks seem to work better on certain types of track configurations, some cars did better on sharper vs shallower inclines, some did better than others on long vs shorter straightaways, sometimes 3 wheels was better than 4, sometimes 4 was better.

My general advice for making a fast car is to make several bodies, test several wheels, and test several axels, and take the fastest combination. Some cars just seem to go faster for no obvious reason compared to what appears to be an identical car. Use random chance to your advantage:)

[Tom M King]

Somewhere, someone is using carbon fiber, and spent uranium. When I was a Cub Scout, in the 1950's we had to make our own cars, and Dad's weren't even supposed to help.

[Anthony Whitesell]

I saw an uncut derby dressed as a firetruck (fire trucks are blocky, I thought it was clever) win the pack level. Aerodynamics matter very little. Friction and weight are the keys.

[Charlie Velasquez]

Somewhere, someone is using carbon fiber, and spent uranium. When I was a Cub Scout, in the 1950's we had to make our own cars, and Dad's weren't even supposed to help.

I gave my sons two rectangular pieces of cardboard, a template of the top of the car blank and one of the blank's side.
They drew their designs on those and cut them out with scissors.
They traced the side profile and scribbled the waste.
I clamped my belt sander with 50 grit on my bench and told them to sand till they got to their lines.
Repeat with the top profile
Pine is soft, less than 30 minutes and they were done.
Rest was hand sanding and painting.
Added lead in the driver's seat to get correct weight.

They still have those racers 30 years later.

[Larry Frank]

It is very important to let the kids do the work. As a parent it is difficult to keep your hands off.

I also see value in having dads make cars and participate in their own contest. It would be fun to make one and race it.

[Andrew Seemann]

For one of the later Cub Scout years, about 4th or 5th grade, there is a "build it" patch, where the kids have to identify tools and then use some basic tools to make something. For our den meeting, I would show the kids how to use a hammer, brace and bit, plane, chisel, and coping saw (no power tools, Scouts under 18 technically aren't allowed to use power tools). I tried a crosscut panel saw once, but the first kid jumped the saw and nearly cut his finger, so that was the end of that. Most of the kids struggled to make even small cuts with the coping saw and very rarely could one of them drive a nail straight.

Over the years, I found that many of the kids, especially the younger ones, don't really have the strength, skills, or coordination to cut out and do the whole derby car themselves. And for whatever reason, the wood that comes in the kits these days is some kind of hard Ponderosa pine which can be surprisingly difficult to work. The other thing to remember is that we as woodworkers have the tools and skills and knowledge to show a kid how to do a car and likely kids who would want to try to do some of the woodwork. Most parents don't know the difference between a coping saw and table saw and have neither the tools or knowledge to share. For the average family, it takes all the skills between the kid and parent to make a car. That said, I don't care for the parents that do the entire car themselves to the point of not even letting the kid touch it. It's a block of wood with plastic wheels folks, not the Indy 500.

Once my own kids hit about 5th grade I did make them start bandsawing out their own cars, but at that point they had already been using the saw for a while.

[Roger Feeley]

Years ago, I participated in a no rules Pinewood Derby meaning no weight limits. The winner's car weighed over 2 pounds. It literally had a bag of bolts strapped to the top of the car. It was surprising it didn't destroy the track as the bumper at the bottom was never designed for that much weight.

The moral of this story is that more weight wins over polished axles and aerodynamics. A real Pinewood Derby with a weight limit means everybody is going to have cars weighing about the same so some of the "tricks" might help.

I think a no rules competition for adults would be fun. I have this idea for a car that comes apart using a .410 shotgun shell (blank). Basically, the shell goes off when the car hits the flat part and launches a weight backwards to propel the car forwards. The trick would be a trigger that's twitchy enough to sense a magnet mounted on the track. The whole thing looks like a good way to blow your hand off.

There are videos of cars with CO2 cartridges. Those look like fun. Estes rocket motors would work but lack reliability.

I wouldn't consider doing anything like this unless there was a safety cage over the track and maybe firefighter/paramedics standing by.

And there is the problem of our local pack having a track with a hump in the middle. That thing bugs me. I've seen the front wheels of cars go airborne coming over the hump. Then they lose tracking and fail. All because they are too fast. To truly compete on that track, you need a vertical fin and air tunnel tests to insure that it goes straight if the front wheels raise up too high. Or you just need a slower car.

[Alan Caro]

Roger Feeley,

While doing a Pinewood Racer, may as well optimize it as much as possible.

The important considerations:

1. Gaining the maximum possible potential energy stored at the starting point.

__ This involves putting as much of the allowed 4 oz. weight as high as possible. This helps because the higher position is storing more energy, the same mass has further to fall.

2. Minimize friction

__ Devise minimal contact with the track.

= Run the car with one wheel lifted, that is run on three wheels.

__ Devise minimal friction between the axle (nail) and the wheel while maintaining relative position /stability

= Configure the end of the axle for separate wheel guides of small contact area.


3. Devise minimal friction between wheel and track

= Run each wheel at a transverse camber so that only the outside edge of the tire surface touches the track

4. Minimize tracking problems.

__ The track will have defects and no car can expect not to contact the guide rails.

= Run the car as a "rail rider" that has a deliberate, controlled period contact with the guide rails

5. Aerodynamics

__ Quite low speed, but don't throw away any possible advantage. A bullet is the ideal form.

Below is a quick model of these principles:


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Has a bit of wheeled submarine running in reverse,..

Showing the blank, the high-mounted weight, which would be mounted and filed to the maximum allowable weight. This is the maximum 7" long, 2.75" wide immediately under 5 oz., and the main body is 1" in diameter. The weight is slightly ahead of the rear axle for stability by lowering polar inertia- the tail wags the dog. Those who follow car racing will recognize the famous tendency of rear-engined Porsche 911's to spin in the wet. This weight position makes this racer a "mid-engine: racer.

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The axle mounting creates a pocket for the axles to be bent for wheel camber and to correct tracking. The two small projecting blocks are about 1/8" square and drilled to guide the wheels with minimal continuous contact. The drilled areas might fill up with Graphite,...

Have a look into the strange world of "rail riding"- all the National winners apparently use it.

Anyway, that's how I'd do it. If you ever need a racing harpsichord let me know!

Alan Caro

[Morey St. Denis]

Great job with the 3D rendered & shaded graphics, Alan. I'm not following your reasoning on placing the majority of extra mass high as possible. Rigid body mechanics dictate that potential energy of position can be converted to kinetic energy based upon the overall change to the center of mass of a rigid body from start to ending position further down the track. Also, with guide rail contact there could be other detriments to stability and time period of increased friction in placing the center of mass unusually higher than point of contact... Now if you could put that extra mass at the end of a tall pendulum mechanism that trips following first motion and stops somewhat beneath the axle, you would have effectively lowered the center of mass from start to finish along the gravitational vector and could well realize some actual benefit...

[Alan Caro]

Morey St. Denis,

Thank you for your comments.

The placement of mass was in consideration of the very tiny introduction of angular momentum introduced by the fact of the track being curved. If the track did not have a variable slope, the mass would make no difference, the acceleration for a mass of any weight would be the same, if the drag was the same. The radius of the curve in combination with the radius of the arc between the front and back means that the center line of the mass changed ever so slightly more (by the difference of the relative centers of mass on that radius). Perhaps equal a few thousands or hundreths of a second advantage, or perhaps it's a fallacious argument and the mass could be anywhere.

The idea that drag is increased when contact with the track is made is an interesting idea, and true, and that is why the idea of the rail-riding tracking is important as a more constant contact means the distance to contact is always minute. The cars that are designed to track with few contacts move quite far and there is much more transverse reactive force by the larger distance; a lot of forward energy is converted into bouncing off the tracking rail.

It's a very interesting subject. A friend of mine that worked on the Pioneer and Viking projects at NASA was over for dinner a couple of evenings ago- I'll ask him how he would do it. I was looking at a photo of a single rocket he had worked on and asked how he calculated the center of mass- an worked out to stabilize it as the center changes as the fuel is expended, and he said he gotten it to within 2cm. He'll know.

The more I think about the Pinewood Derby competition the more I like what it teaches kids about science, technology, and craft. I've learned something from it myself- or have I?

Alan Caro

[Andrew Seemann]

Bear in mind that the car needs to make it down an actual track, not a theoretical one, and that most tracks are much less than ideal in terms of flatness, straightness, and smoothness. Having the weight high up will make the car quite unstable, and the car will jump the lane on most tracks. On ours, we required each car to have a test run down the track at least once prior to the race to verify that it wouldn't jump the track. If a car did, or if it appeared it wasn't sufficiently stable, we required the car to be modified prior to racing. It wasn't fair to the other cars to have to keep rerunning a race, as it can affect the performance of them in later heats.

The thing is, different track artifacts require different compromises in design, so what works well on one track may not work well on another. Before we finally found a replacement, our pack's track was a 30 year old contraption made of old paneling and lumber that somehow we managed to coax back to life each year. Cars ran differently down it than the aluminum one at district level which never seemed to be set up quite right. And often a different car was fastest down the district track than the pack one. The council's was another beast entirely. It had a higher drop than the others, so that completely changed how cars ran down it compared to the others.

[Morey St. Denis]

"it's a fallacious argument and the mass could be anywhere." Believe lower is likely better for conservation of energy and stability considerations.

I have no particular expertise with Pinewood Derby races, nor have I competed. I am however often invited to advise and judge regional science entrants, and certain STEM competitions such as FIRST. Figure it's how I can assist in my own small way to furthering the educational opportunities of youth...

Agreed that the change in pitch position of the vehicle is a miniscule factor as it undergoes a positive pitch displacement. But by deliberately attaching a mass concentration distant from body center, some energy of the system must be spent in raising that extended mass portion against the force of gravity as the body pitches up. The work thereby extracted is proportional to that optional mass concentration, its distance from a more neutral polar position and the sine function of the change in slope of the track from start to timed finish position. I fail to see how the principal of conservation of momentum could be employed to advantage in this positive pitch rotation. Perhaps if the pitch displacement were negative such as in an outside parabolic loop with gravity assist, but with useful loops you eventually pass full circle and likely must eventually pull out, canceling most all but the vertical change in height from beginning to end and trading potential energy of position for kinetic energy and momentum.

Agreed that random trial and error can sometimes deliver some useful empirical insights in many "real world" or field trial situations. Whenever time, effort and expense are potential considerations you are well advised to employ the scientific method; which is to form a educated hypothesis, then devise a series of tests to confirm or deny that theory. It goes without saying that you are well served to first analyze the situation using the best scientific tools at your disposal, such as the basic physics & applied mechanics in this particular example.

[Jeff Heil]

My son placed in the top 3 cars of his den every year in scouts and was 2nd overall in the pack his last year in Cub Scouts. He designed and did most of the work on his cars (he was afraid of the bandsaw the first two years) every year and he took ownership of how successful he was in the pinewood derby. I researched and provided guidance. I always made him sketch out his design on the block.

Here's what we found worked best for us to be competitive:

I agree with the others that a car that rides against the rail runs straighter and faster. We always set one front wheel up 1/16th to not touch the track, (approx. 25% less rolling friction). We balanced the car on a steel rule to get the center of gravity (COG) about 3/4" forward of the rear axle. We found using tungsten weights allowed more control of the COG than less dense weights. We would bore a 3/8 hole down the center of the block before cutting out the shape of the car and slide the round weights back and forth as needed.

My son spent lots of time with a small file taking the burr off the nails and emery cloth and wet sandpaper polishing the axels (just chucked them in the drill press). We always put a dab of super glue on the ends of the nails to keep the wheels attached to the car. Saw many cars over the years loose their wheels if they jumped the track. We made a short test track from scrap 1/4 plywood to test cars to make sure they ran straight and true without wobbling.

Great lesson for my son was learning the more effort and time he put into his cars, generally the more successful he was. Also fun shop time together. Our pack had pretty clear rules against not shaping the wheels or using anything but the pine block and stock wheels to keep it fair and not allow someone to buy aftermarket machined wheels.

Our pack gave every kid that built a a car a medal for building a car and entering the race and trophies for the top cars. I am not a fan of participation awards, but we made sure to frame the context as a "car builder award" and making the effort to enter the race.

I always asked the kids about how they built their cars at check-in and on race days, and could always tell the pride of participation and appreciation of the process of getting a car ready to race vs the kids that had parents that built the car for them or did not allow them to touch the car. If the dad carried in the car to weigh in and the son was not allowed to touch the car until they got to the scale it was pretty obvious who built it.

I still have my pinewood derby cars from 35 years ago that I built with my dad. Good memories.