One of the really neat things about America being such a large country is that seasonally, February and March look very different depending on where you live. Here in Boston we’re still looking at feet of snow still on the ground but down in Florida (where everybody in Boston wishes they were right now :)) they’re prepping for flower festivals at Disney and throwing on jackets if it dips below 60. No matter where you live though, in the Scouting world February and March mean the same thing: Pinewood Derby season. And Pinewood Derby season means the need for speed.
With Camp Sayre in full prep for STEM Camp this April vacation (that’s Science, Technology, Engineering, and Math in case you were wondering) and NASCAR driver Scotty Lagasse bringing his race car to the camp in order to teach scouts about automotive science, I thought this might be the perfect time to talk a little about the science of building a faster and furiouser pinewood derby car. P.S.: If you’re not into the sciencey part skip ahead to the cliff notes. 🙂
The Sciencey Part
If you want to start tweaking your car to get the most possible speed you first have to know what makes the car go fast in the first place and the name of that game is energy transfer. When a car is sitting at the top of the track, ready to go down, it’s full of potential energy (or stored energy). Technically potential energy is mass times gravitational force times height (Ep=mgh) but essentially in pinewood derby speak it’s the weight of the car times the height of the drop. The higher up on the track the weight is (ie the more towards the back), the greater your potential energy.
Once the race starts and the car begins to move the potential energy is transferred into kinetic energy, 1/2 the mass of the object times the speed. Of course, the optimal word in that sentence is speed. The more energy transfers, the faster the car goes. That means that the more potential energy you start with the more you have to transfer to kinetic energy during the race. Now that means that all you really have to do is put all of the weight in the back of the car and you’re a shoo in to win, right? Nope. Let’s talk about center of gravity.
A car can be super speedy but if it’s unstable it won’t make it down the track. You’ll crash, you’ll flip, stuff will happen and even if it doesn’t somebody who is smarter about their weight disbursement will still be faster. All the science stuff above tells us that we want the weight towards the back but if it’s too far back you’ll pop a wheely somewhere on the drop and then it’s all, “Good try maybe next year.” You need to place most of the weight far back enough to make a difference but forward enough to still be stable. To help with that and translate the rest of this sciencey stuff into useful tips. Let’s check out the cliff notes.
The Cliff Notes
To build the fastest car possible you’ll want to do these specific things:
1. CHECK THE RULES!!! BSA has official rules but a lot of Packs add their own and you don’t want to get disqualified because you did something that you thought was OK but your Pack doesn’t allow. That would stink!
2. Max out the weight of the car. That’s 5 oz in the official rules and tends to stay standard.
3. You want the most weight somewhere between 1-1.5 inches from the rear axle of the car. That will keep you stable and make you fast.
4. Create a streamlined profile. You might not think aerodynamics would matter but this crazy physics professor out in California did a huge set of experiments with pinewood derby cars and found that an aerodynamically cut profile will beat the standard block of wood by over 5 inches (you can watch the whole thing here: http://pinewoodphysics.com/index.html).
A low profile might not look exciting but it takes a lot of the weight of the wood out of the car and that allows you to control more of the weight and center of gravity while also reducing air resistance. If you’re competitive enough to be reading this post, it’s totally worth it to put some thought into how you cut the car .
5. Graphite. Graphite is a smoothing agent and reduces friction. By now we all know what that does. The cool think about graphite is that it doesn’t really matter what type you get, they all do the same thing, so you don’t need to bust your wallet on special stuff, you can just get the cheaper kind and it will still work.
6. Raise one of your wheels. In a perfect world all of the potential energy converts over to kinetic energy during the race but unfortunately we lose some of it to friction, that’s the wheels turning on the track. As an example think of a regular train verses one of those crazy Japanese high speed trains. Those high speed ones work using electromagnets that suspend the train above the track, eliminating friction, that’s why they go so fast. When you raise one of your wheels you reduce the friction points from 4 to 3 and that means less energy is lost to friction.
7. Straighten and polish your axles. The straightening helps you keep a straight alignment so that your wheels don’t wobble as they go down the track and the polishing, again, reduces overall friction.
Now there are a whole bunch of other things you can do in order to increase the speed of your car but really that depends on how crazy you want to get. The most important thing though is to have fun with it.