Fail of the Week: Pinewood Derby Cheat Fails Two Ways

Would you use your tech prowess to cheat at the Pinewood Derby? When your kid brings home that minimalist kit and expects you to help engineer a car that can beat all the others in the gravity-powered race, the temptation is there. But luckily, there are some events that don’t include the kiddies and the need for parents to assume the proper moral posture. When the whole point of the Pinewood Derby is to cheat, then you pull out all the stops, and you might try building an electrodynamic suspension hoverboard car.

Fortunately for [ch00ftech], the team-building Derby sponsored by his employer is a little looser with the rules than the usual event. Loose enough perhaps to try a magnetically levitating car. The aluminum track provided a perfect surface to leverage Lenz’s Law. [ch00ftech] tried different arrangements of coils and drivers in an attempt to at least reduce the friction between car and track, if not outright levitate it. Sadly, time ran out and physics had others ideas, so [ch00ftech], intent on cheating by any means, tried spoofing the track timing system with a ridiculous front bumper of IR LEDs. But even that didn’t work in the end, and poor [ch00f]’s car wound up in sixth place.

So what could [ch00ftech] had done better? Was he on the right course with levitation? Or was spoofing the sensors likely to have worked with better optics? Or should he have resorted to jet propulsion or a propeller drive? How would you cheat at the Pinewood Derby?


2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which celebrates failure as a learning tool. Help keep the fun rolling by writing about your own failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

85 thoughts on “Fail of the Week: Pinewood Derby Cheat Fails Two Ways

      1. The only propulsion allowed is gravity. So, you need to make it streamlined, slick, sleek, and efficient. I built five of them over the years and it was amazing how aerodynamics played a role vs. low friction. Friction always won.

        1. Many, many ages ago, I was in scouts and participated in a pinewood derby, where cheating was not allowed. I had about the least aerodynamic car ever, a block of wood that I drilled out until I met the weight limit. I did, however, work at making the wheel alignment as close to perfect as I could, and used the best dry graphite lubricant I could get my hands on. I managed to get to the state finals with that car.

      1. You probably can do it with one 555 or two, I have a 38Khz beacon for a micro-mouse competition made with an attiny 45 and 5 lines of C to setup the timers to generate the 38Khz carrier and a basic modulation(on/off)

  1. Space constraints? I’d stick a 20ft long stick on the front so it triggers the sensor as soon as the gates open. It doesn’t even need to support its own weight if it can rest on the track.

  2. More weight? Perhaps something spring loaded that shoots a very light car and drops a heavy mass on the track?

    Three wheels? Slight lift to overcome the velocity-dependent term for rolling drag?

    Liquid nitrogen and super-conductors in the car and permanent magnets installed to the under-side of the track??

    It would be interesting to see if a height-changing mechanism (cams and all) could make the center-of-gravity follow a curve of quickest descent because it is likely the track itself doesn’t. That should give the cg the fastest path.

    One mans motor is not another mans motor. A linear motor could cause thrust shown only the aluminum track, but the controller would have to be small, as would the power-supply. Having a motor push air instead of the track, like an airboat (think everglades) might work.

    putting a mousetrap spring and small gears to turn an axle might be good. It is potential energy in the spring instead of in a battery. Elastomers make good springs.

    A gyroscope might allow the car to run on less than 3 wheels. 2? 1?

    Model rocket motor? (Estes)

    Piezoelectric transducers to liquidize the particulates in the graphite/diamond for lower friction?

    Camber in the wheels so contact patch is minimized? Can you machine the wheels to only touch along a very thin line?

    Make the wheels out of something very light and having very little moment of inertia (smaller radius) so the potential energy is transformed into higher linear kinetic energy of the car?

    Red-hot heated leading edges to reduce drag while the vehicle moves?

    Have the fan suck through tubes in the body to give suction along the boundary layer? (https://www.nasa.gov/centers/dryden/pdf/88792main_Laminar.pdf)

    1. Not “more weight”, but rather,spring-eject half the cars mass back towards the top of the track when the car is half way down. If you work it right, the ejected mass would reach the top of the track as the car reached the bottom…

    2. Did year after year of the Pinewood Derby. Three wheels is a great tactic, accomplished by simply putting the axel of one wheel higher than the other 3. Add all the weight at the back, ALL the way back. Polishing the axels works and using a dry lubricant is superior to anything wet. A bit of toe-in on the front wheels and have the back wheels wider apart so only the front ever touch the center guide…

      The above list is quite enough to keep any diminutive participant totally engaged as well as at risk of dominating the races. Then again, I’ve seen kids whittle a bit off the body AT THE TRACK, drive in the nails, and win. Better for you and I to build the track and finish line electronics… leave the fun to the kids.

  3. Punch a pinhole in the back of the lithium-ion battery, but have a gate which covers the hole. When the wheels turn, a mechanism drops the gate. The battery then starts venting furiously, propelling the car forward.

  4. Cut as much weight as possible from the wheels, polish the axle-wheel mating surfaces, and then shave the wheels so the contact area with the track is smaller. Those are the traditional “cheats.”

  5. I’m surprised the LED-laden one passed inspection. The rules prohibit exceeding the standard width and length, as well as a weight limit (height is a bad idea anyway, you’re just increasing wind resistance). The original block of wood need not be used, but the standard wheels and axles (basically nails) must be used, making it hard to engineer in an motor that drives those wheels evenly. Generally the winners use a jig to make sure those axles go in straight and level.

      1. Dropped in a vacuum weight has no effect on the speed at which objects fall. What about when they have friction to overcome such as little plastic wheels nailed to a piece of wood? Doesn’t weight help for that?

    1. +1 – raced a 5# block of steel with machined axles and wheels one year. Took 2nd place in the first heat, but destroyed the car that took first (it was a ducted multistage fan that had been carefully engineered to only weigh in at 4 oz. and had the wood body thickness of 1/8″ of balsa). Overall I took first place in the adult competition and had to pay to have the track repaired as the brake strip and backstop weren’t designed for the sudden load applied to them. Worth every dollar spent!

  6. Machine the wheels to run on a small part of their surface. Rough up the contact area for increased traction. Use the remainder of the wheel surface and machine semicircular turbine buckets. Vent the contents of a C02 cartridge through a thin tube onto the wheels to accelerate the car.

    1. Roughing the contact area would have the opposite effect to machining the wheels to run on a smaller surface. Mind you, doing both would result in wheels with a smaller wind profile while potentially having the same rolling resistance, so it would potentially be worthwhile.

      1. You’d want to keep the traction though if you’re driving the wheels. I know it increases rolling resistance, but if you put enough torque on the wheels you’d just spin them in place.

    1. I didn’t know the “fill your tyres with nitrogen” thing existed until I saw a youtube video link entitled “Don’t fill your tyres with nitrogen”. I immediately wondered why I shouldn’t put air in them.

      1. Yeah, I heard about the nitrogen fill fad a couple years ago. Air is 78% nitrogen anyway.
        It reeked of the same odor as the “oxygen free crystalline copper speaker wires”

          1. Reason to use nitrogen in racing car tires is that it heats up less quickly than straight ‘air’. That means that the tire pressure varies less with the temperature, and the tires become a bit more predictable. So it’s about predictability of the wear and not about speed.

          1. It doesn’t lower resistance, in fact, the oxygen was actually added during smelting to increase conductivity by removing impurities. Spec-wise, oxygen-free C10200 copper is no better than C11000 ETP copper (aka standard copper wire). Even the spec for ultra pure, very expensive C10100 oxygen free copper is only 1% better, and in practice all grades exceed the conductivity requirements for C10100.

            If it was relevant for motors, it would also be relevant in audio – speakers are essentially specialised linear motors.

            Oxygen-free copper is relevant where the presence of oxygen is a problem, which occurs mostly in systems (like particle accelerators) and processes (like sputtering/plasma deposition) that occur under vacuum and/or require very pure copper to avoid unintended chemical reactions.

  7. If you have to play by the rules, the secret is to get the center of mass as far to the back as possible. By doing that, your car starts with more potential energy than the others.

    If not, just make it as heavy as you can, so that a larger fraction of the potential energy gets converted to kinetic energy, rather than the rotational energy of the wheels. (Really just having the car mass >>> wheel mass, with the added benefit of better traction on the track.)

    1. I didn’t now that was acceptable (seems even encouraged by the comments) to bend the rules of the soapbox deby, in that line of thought I like your ideas, I don’t know if taking a longer, diagonal down the ramp let the kart go faster or not, when I was child and I went down a small ramp in the bike seemed like taking long diagonals let to a faster final speed.

      1. It is and is often encouraged, albeit subtly.

        In the Boy Scouts, cheating is often rife in the derby. My troop was one of the very rare troops at the time with a female troop leader. The other, obviously male, troop leaders thought this wouldn’t do and largely rigged the pairings to determine the winners even before my troop (and a small number of others) completed their races.

        My father was furious as my car was _just_ finishing the first or second run as they were handing out trophies. I just walked out and my troop leader handed me a shitty participation ribbon the next week.

        At least this guy’s company doesn’t make any pretense about what’s really going on.

        I actually miss that troop. She owned a lodge which sported about five or so arcade machines. While other kids were learning how to tie knots or make fire, we were learning how to repair, maintain and um… stress test those arcade machines. I got my first real taste of electronics during those years.

        1. Our troop had one kid whose father was a slot car builder. The father took it as a supreme insult to his manliness if his kid came in anything but first. It was actually kind of pathetic, so we just let him win. I’m not positive he didn’t violate the rules in some way, but it wasn’t worth the drama. The kids had plenty of fun anyway.

          My wife ran the troop with another woman. Aside from the reactions of the other troop leaders in town (all male, and of the standard, overweight leader profile), everything was great. They love hiking, so the troop, which had never been an “outdoor” troop, suddenly became one, and applications skyrocketed. They also pushed a lot of the kids through Eagle. The other thing they did, was organize a Christmas tree pickup. The town loved it (saved them from having to do it), we made a huge amount of money in a single weekend. No other fundraisers needed all year, [cough, stale popcorn] and 20 years later, it’s still going..

  8. I just love the conclusion at the end of Ch00ftech’s writeup. Spoiler: after simulating it through all the physics software, he found the car really would have levitated… If only he could have driven the coil at 2700V!

    1. That reminds me of the failed demonstration of the physics teacher in school. He wanted to show, that a capacitor passes AC, but he just happened to have a very unsuitable combination of parts: 12V/20W bulb and 1µF capacitor @50Hz :-) I calculated he also would have needed more than 3000V – if the capacitor would have sustained that voltage. There was no voltage marking on this ancient part with the size of a matchbox

  9. Picture a large L shape running out the front of the car and then up, rigged to fall 90 degrees the moment the car starts moving, you now have a block Infront of the other cars and they can help pull you down the track, so long as your car trips the sensors first you win.

  10. All those solutions depend on your car being faster than the others, there is another way. Put valves on angles on the top, when you click a button on a remote it squirts superglue onto all the other cars. You do this before the race starts. Design your car to be slow; it only needs to be faster than the other cars and the other cars are glued to the track.

    Alternately, you could mount two small pistol barrels to the car, pointing left and right. Carefully load the rounds to they are equal, fire them at the same time, and watch the other cars fly away!

  11. I remember when I actually did win the pinewood derby. My dad was an excellent engineer. We weighted the car to the max limit with shot and also ground and polished the ‘axles’ (tacks). Father/son teams who tried one or the other of these things did well but we were the only ones who did both. Of course the aerodynamic race car shape with the high gloss cherry red paint job (not to mention the chrome exhaust headers i glued on from a dragster model) did’nt hurt!

  12. Friction is the main killer of speed here. As a kid, I won a couple pinewood derbies (and did a science fair on how). The best thing was to file down the nail axles to be super smooth, use graphite powder as lube, and the real kicker, use modified wheels that have minimal contact with the track. If the wheels didn’t spin, the wheels would glide over the track, hardly touching it. Oh, and all the weight in the back, so gravity helps it along just that much more. That little car was a bullet in every race!

  13. I was a Cubmaster.

    For the cubs, the rules are: the pinewood block with wheels and axles must weigh no more than five ounces. You must use the supplied axles and wheels. You may not machine the wheel width. You are not allowed to use any additives on the outer wheel surfaces. You don’t have to use the axle holes, but wheels cannot go past the front or rear of the car.

    There are whole websites that go into the pinewood physics in depth. Most of the proposed schemes here have been tested already. Ideal CoM has already been calculated. There may be some advantage to better aerodynamics, but in general it’s hard to beat a thin wedge or doorstop profile. Three wheels are not only possible, but a must to consistently beat other dads in the real race. Graphite is the most common axle lubricant used. Winners also polish the axles and machine a smaller center profile to capture more graphite. Winners will also cut inner wheel mass and machine the manufacturing defects out.

    Open class racing is really ridiculous, might as well launch them with shotgun shells for all of that. Some restrictions should be in place for open class racing, otherwise it’s just throwing money at a problem instead of showcasing ingenuity and skill.

  14. When my son was in the Cub Scouts he entered his Pine Derby car in his Troop Contest. After some brainstorming, I took each wheel and turned them slightly convex to minimize the contact patch with the surface of the track. Just enough so it would not be evident we were flexing the rules. Axel shafts were polished with rouge, a special graphite powder I got from the lab here at work was used instead of the very course powder you get with the kit. The car was 10% forward biased by using a Mettler Balance and the car was aerodynamically shaped although at that speed, my CFD model indicated drag in the microgram region. Results? We won all heats. It was a fun project and my son learned a lot from the project.

    1. Another similar trick with the wheels (since you aren’t supposed to machine the WIDTH) is to sand them concave so that the contact patch is two very thin edges. Done properly, you can get this down to about a millimeter.

      1. I tried that concept originally but found I could not keep the wheel from wobbling side to side. This caused the car to go to either side, hitting the wall of the track. The convex solution turned out to be the best. I even simulated the smaller contact patch between wheel and surface. A convex wheel allows it to gyrate about the contact patch and recovers much quickly about its vertical axis. The two surface approach you pointed out did not work as well both in simulation and during testing. BTW I had tested two axels with toe in and toe out to see if it made any difference in stability. As I originally thought, it did make a big difference in stability but at the expense of additional friction. The car was not as fast!

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