Pinewood Derby track question: Engineering advice wanted

[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:


Pinewood Racer_1_FR_Above + Blank_10.29.18.jpg

Pinewood Racer_1_L Elev_10.29.18.jpg

Pinewood Racer_1_FL_10.29.18.jpg

Pinewood Racer_1_RR_10.29.18.jpg

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.

Pinewood Racer_1_FL_under_10.29.18.jpg

Pinewood Racer_1_FL_Axle Det_under_10.29.18.jpg

Pinewood Racer_1_FR_Axle Det_no WHLS_above_0.29.18.jpg

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.