So for those of you that don’t know, Pinewood Derby is a “sport” where children in Boy Scouts, Awanas, or a variety of other organizations are supposed to build cars from kits and race them down a ramp. In reality, these kid’s parents or their friends generally build them. I seem to have fallen into the latter category, as I’m an engineer and everyone assumes I know something about this or am nerdy enough to help.
Sadly, these people would be correct. When asked, I said I would help a friend’s kid build her car. The agreement was that she would sketch out a profile of the car and I would cut it for her. This has not happened yet, but, thinking about the physics involved, there are a couple things that can be done to give yourself – your child I mean – a good chance of winning.
The first important thing is the concept of energy of position. All cars start at a certain height above the ground. However, the center of mass of all cars does not. The change in positional energy is equal to mass * gravity * height (of the center of mass). By concentrating as much mass as possible towards the rear of the car, one can actually raise this slightly. This means that if you’ve concentrated the mass correctly, you’ll actuallybe starting higher than the other guy.
Apparently there is a set weight limit, but you can actually add weights later if you are under the limit. If you need to do this, you’d definitely want to put these weights as far towards the back as possible.
The second concept that can help you is the principle of angular momentum. Mathematically, angular momentum = moment of inertia * angular velocity. To give an object angular momentum, in this case the wheels, you have to put some sort of force on it. As you only have a finite amount of energy (see above) for your car, it’s best to use as little of it as possible to make the wheels turn.
So, if it’s not illegal, you can sand the middle of the wheels or turn them on a lathe. The further from the central axis that you take material out, the more it reduces the wheel’s moment of inertia, so focus on that. This has the added benefit that this weight can now be distributed farther towards the back of the car.
Besides these two things, there is wind resistance, which probably doesn’t make too much of a difference. If you want to minimize this, it’s best to make your car low and have few sharp edges. Also, there is the actual bearing surface of the wheels. Bearings can actually be overlubricated in some cases, and quite a bit of money has been spent on designing them correctly. This however seems like overkill for Pinewood Derby. Add what you want I say.
Last, but not least, how much your car rubs against the side of it’s track should be minimized. The more it rubs, the more energy is lost to friction. I’m not sure how to minimize this occurrence, but free to leave your thoughts in the comments section. If legal, rubbing some graphite on the outside of the surface most likely to hit the walls may have some effect in minimizing this frictional loss.
In the end though, if you end up doing poorly and your car still looks awesome, that probably means just as much. But if you want to simply try to win, these concepts from physics should help you do it.