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I've been building and racing Pinewood Derby cars for over eight years now.  And that's not counting the cars that I built and raced as a cub!  Over the years I've learned a few things about building a competitive car.  Here are just a few of the things that I've learned.

It's All About Physics

  1. A Pinewood Derby car is propelled by only one force: gravity.  It has a certain amount of potential energy which is equal to its distance above the finish line times its mass.  As the car drops, potential energy is converted to kinetic energy (speed).  More potential energy means more potential velocity so it's very important that you maximize your potential energy.
  2. A Pinewood Derby car is impeded by only one force: friction.  Friction causes heat and that causes energy loss.  Energy lost to friction means that there is that much less energy to convert into kinetic energy.  All potential energy that isn't lost to friction will be converted to kinetic energy so it's vital that you reduce friction as much as possible.

You should always be thinking about these two principles when you build your car.  Everything you do should maximize potential energy and minimize friction.

Here's one other thing that I think is interesting.  If you do the math, you will find that in the absence of friction, the amount of kinetic energy (speed) that your car can obtain is actually NOT dependent upon your car's mass.  A 4 ounce car has the potential to be just as fast as a 5 ounce car.  HOWEVER, added mass DOES help overcome friction.  So the reason you want your car to be exactly 5 ounces isn't necessarily so it can go faster, it's so it can better overcome friction.  Also, the more weight your car has, the more flexibility you have in positioning that weight and, as we will see, where you position the weight can be very important.

So lets talk about positioning your weight.  Think about the configuration of your car at the starting line.  The car starts on an incline which means that not all parts of the car start at the same distance from the ground.  So where do you think you should put most of your weight?  At the back!  That's because the back of the car is higher off the ground so it contributes the most potential energy.  But be careful not to get your car's center of gravity too far back or your car may become unstable and wobble all the way down the track, creating more friction.  Or worse, it may jump the track entirely.

The fastest cars I've seen have a center of gravity at about 1/3" in front of the rear axle.

Overcoming Friction

Think about where friction comes from in a Pinewood Derby car. 

Friction is a resistive force that occurs whenever two objects slide past one another.  In this case friction is principally caused by three things: the wheels rubbing against the side of the car, the wheels rubbing against the axles, and the wheels rubbing against the track.

There are several things that you can do to reduce friction (without being labeled a cheater) and all of them are equally important.

  1. Polish your wheels and axles until they shine.  Carefully remove all burrs with files and then sand, sand, sand, sand, sand with ever increasing grits of sandpaper.  But be very careful not to melt the plastic wheels!  Wet sandpaper is great for this and you can buy polishing kits in the scout shop and at most hobby stores.
  2. Make sure your car tracks perfectly straight.  The straighter your car travels, the less often it will bump the track which means fewer course corrections and less friction.
  3. USE GRAPHITE.  It's amazing how much of a difference dry graphite makes.  Use a small paint brush to push graphite into your wheel's axle holes.  Spin your wheels over and over again to ensure smooth operation, long spin times, and even distribution of the graphite.  Then apply more graphite right before you check in your car on race day.

One other tip that I've found useful is to raise one of your front wheels so that it's about 1/8" off the track.  Three wheels touching the track means only three wheels creating friction and it makes a big difference.  You want it high enough that it doesn't touch the track but low enough that it will still bump the center rail and keep your car on the track if it starts to drift a little.

And one final thing about friction: wind resistance does contribute some friction but at these speeds it's so minimal that you can pretty much forget about it.  While aerodynamics might help a little bit it isn't really a major factor so if you want to put that cool Lego driver on the top of your car, you go right ahead.  It's not going to slow you down.

Miscellaneous Tips

Hobby shops like Hobby Town are a good source for Pinewood Derby weights, decals, and decorations but STAY AWAY FROM THEIR WHEELS AND AXLES!  Not only are they illegal in most races but they absolutely stink.  They are poorly made, their little plastic hub caps which secure the wheels are always popping off, and they are SLOW SLOW SLOW!

One little trick that I've used with some of my cars is to add a small bolt sticking up out of the back of the car over the center of gravity.  Permanent weights in the car body provide all but the last little bit of weight.  Then I use washers to bring my car up to the exact weight.  This way I can make tiny adjustments as necessary, my weight is always securely attached to the car and there's no way it will come off during the race.

The best and most complete source for additional Pinewood Derby tips I have ever seen is a book called Pinewood Derby Speed Secrets by David Meade.  This book is available at the scout office or on-line by following the above link.  While some of these "secrets" are a little bit over the top, it's an excellent resource and I highly recommend it.

But the most important tip that I can offer is: HAVE FUN!  Remember that this is a cub scout event and everyone should have fun just building and racing their car.  Coming in first is not even remotely the most important goal.  And it's a good thing too since only one car can be first.  The most important goal is for cubs and dads to spend time together and to have fun building and racing something cool.

Good luck and happy racing!

Steve Blanding

The Ubercar II by Steve Blanding