Pinewood Derby ideas

[kfitch42]

My son's boyscout troop is having their pinewood derby soon. They have a "parents and siblings" division. So, I was going to make my own car ...  my first thoughts were:
1) Small Arduino (mini/micro)
2) LOTS of LEDs .

Anybody have any other ideas?

p.s. for those not familiar with pinewood derby, here are the basic rules:
http://www.abc-pinewood-derby.com/pinewood_derby_rules.php

[Psi]

They're going to have to be bright LEDs to see them outdoors in full sun.

Maybe it could make a sound that changes the faster it moves.

[Smokey]

While not really electronics related, the biggest thing you can do to make your car faster is polish the axles AND the hole inside of the wheels where the axle rides.  Then you have to get the axles REALLY straight.  Epoxy the hell out of them to make sure they don't shift.

It turns out the shape of your car makes little difference.  One time I made as perfect an airfoil as a 10 year old could make with a band saw and some sand paper.  All calculated from an aerodynamics text book and everything.  (Gotta love the engineer parents).  Didn't matter.  You can leave the car as a rectangular block if you want.  It all comes down to the friction of the axles on the wheels and their alignment.  If I did it again now, I'd have a car 3D machined from some really intricate CAD model, but just for fun.

If you really want to be high tech, if your track doesn't have an electronic timer and finish line you could make one of those. 

[kfitch42]

Actually the track is set up inside.

How hard is it to play real sounds with an Arduino (haven't tried it, or even Googled it yet)?

How would I measure speed (for the changing sound)?

Accelerometer?

Anemometer?

Paint a white dot on the inside of a wheel where a photoresistor (or similar can see it)?

[kfitch42]

If I did it again now, I'd have a car 3D machined from some really intricate CAD model, but just for fun.

If you really want to be high tech, if your track doesn't have an electronic timer and finish line you could make one of those.

I think the track has an electronic time already.

Hmmm, might have to think about using Shapeways or similar ... A K.I.T.T. with a working 'cylon' might be pretty sweet.

Ooohh, speaking of KITT ... ejector seat! although I am not sure off the top of my head how to accomplish the ejection in the constrained space.

[justanothercanuck]

I built something similar in shop class back in high school, but instead of running them off ramps, they were CO2-powered.  Aside from making the wheels as smooth as possible, another thing to look into is a high quality paint job for aerodynamics.  It's not much, but every little bit helps on such a small object.  Weight and size do play a small role, but because of the height/weight/clearance rules, we're talking fractions of a second at most.

[smashedProton]

Excellent comment smokey, but I have some more input.

• polish the axles by mounting them in a drill press, and polishing
• lift one of the wheels to limit friction
• mount the weights in the back, so the weight has more potential energy compared to the other cars
• the front edge of the car should be high up, so that it can accelerate before the other cars.  This is because the pegs that let the cars go collapse downward.   Definitely worth the cost in aerodynamics.

In short, dont design a pinewood derby car like you are designing an airplane.  Its more like a bicycle.

[SeanB]

Tip for axles is to use ball bearings, and remove the seals and wash out the grease, then place a tiny drop of light oil on each ball. You lose a lot of energy in the seals, and the grease is lossy as well. As you are not going to run for long unlubricated balls ( just a touch to reduce friction in the cages) loses less energy.

[kfitch42]

Tip for axles is to use ball bearings, and remove the seals and wash out the grease, then place a tiny drop of light oil on each ball. You lose a lot of energy in the seals, and the grease is lossy as well. As you are not going to run for long unlubricated balls ( just a touch to reduce friction in the cages) loses less energy.

Great ideas, but all of them are against the rules.

[SeanB]

Then bushes and molyslip in a tube direct ( no other oil). Black, messy but using just a tiny amount per bush makes a very low friction unit. Use hard steel axles as well and polish them to a bright finish.

[Psi]

How hard is it to play real sounds with an Arduino (haven't tried it, or even Googled it yet)?
How would I measure speed (for the changing sound)?

The quickest way i found to play sounds on an AVR/PIC was to take a 8bit 11khz mono wav file and add it to your sourcecode as a progmem array.
Then attach a 8bit parallel audio DAC to one of the MCU ports and use a timer to step through the array data at 11khz and set the DAC value.

But for an Arduino i'm sure someone has made it even simpler. Maybe try searching for an Arduino sketch for playing wavs off a SD card.
That would be really simple to setup.

To measure speed you could try and glue a magnet on one of the wheels. Add a reed switch or hall sensor mounted on the cart so the MCU can detect when the magnet passes by. Exactly like they do on push bikes.
A light sensor could work too but i think a magnet + hall sensor would be the best

[EEVblog]

A vertical strip of LEDs on the side that is synced with the speed.
You could display full dot matrix images as the car passes.
Pointless during the day, but at night, a long exposure camera set up as the car goes by would take some great images.



Dave.

[poodyp]

Then bushes and molyslip in a tube direct ( no other oil). Black, messy but using just a tiny amount per bush makes a very low friction unit. Use hard steel axles as well and polish them to a bright finish.

Have you even read the rules?

mount the weights in the back, so the weight has more potential energy compared to the other cars

Adding to this: to get all weight in the back, cut your car so it's as thin as possible. Leave a little bit around the axles so it doesn't split, but cut everything else away. Everything that's removed can be added to the back.

[amyk]

Then bushes and molyslip in a tube direct ( no other oil). Black, messy but using just a tiny amount per bush makes a very low friction unit. Use hard steel axles as well and polish them to a bright finish.

Have you even read the rules?

From the rules posted above:

They may be polished or lubricated

So you can't use premade bearings but you can make your own... that's how I intepreted it. Depends on the definition of "bearing" though, as prohibiting wheel bearings per the pure definition means a rather lame "car".

[kfitch42]

A vertical strip of LEDs on the side that is synced with the speed.
You could display full dot matrix images as the car passes.

I really like that one! I may have to give that a try.

Kevin

[cwalex]

Ooohh, speaking of KITT ... ejector seat! although I am not sure off the top of my head how to accomplish the ejection in the constrained space.

A car airbag?