Thursday, January 17, 2008

Pinewood Derby over the Years

An exceedingly busy week and a half eliminated any chance of a blog entry here, with a huge project at the office, annual start up for the Waves swim team, and another annual activity that is a highlight in our house - the Cub Scout Pinewood Derby.

This year Alex and I made an ice cream sandwich car. This design was first considered two years ago, and I'm glad we finally built it. In fact, it would have been far easier that what we built those years, with more steps in which Alex could safely use power tools back then. Even though Alex will never win the "most likely made entirely by a scout" award, this year he did more power tool work than ever before, did some of the hand work (which was almost non-existent) and he did 100% of the painting. My contributions fell to a little band saw work (but Alex did half), routing the edges and installing wheels.

Not that this blog is going to ever be a source for Pinewood Derby tips, but we did something this year based on experience, that would help lots of people. We used purple painters tape (the lightest hold, lighter than the ubiquitous blue tape) to wrap the white parts of the car while graphite was being applied to the wheel areas and while wheels were installed.

To those not familiar with Pinewood Derby, it's a model car race, primarily conducted by Cub Scouts. It began in the early 50's in California, when a dad with a young son didn't want to wait until his kids was old enough to build a full size Soap Box Derby car (those are the ones that kids sit in and ride down a hill). He invented a small model - 6" long - and a track, to allow younger kids to get the racing bug. It took off and was quickly adopted by the Scouts. Some other organizations such as YMCA and Awana conduct similar races.

Though I was only a scout for one or two years, I recall attempting to whittle a car with a dull pocket knife. Not only is this not going to lead to much of a car, it also is dangerous for a kid. The majority of cars constructed rely on sandpaper and elbow grease, with maybe a few hand tools. The top cars from a design standpoint are closer to mini sculptures.

[ Note to those who don't know me - I've built furniture as a hobby for 15 years, and have a complete woodworking shop in the garage. Building / sculpting a small car is not too tough in our house. ]

Nearly every kid and the majority adults think that air resistance ought to be a factor in obtaining speed. Mais non, mon amis. The surface area is too small and the tops speed is too slow for this to really matter much. What does matter in Pinewood is friction and weight, not in that order.

Since the cars are started on a downward sloping incline, gravity has a say in the proceedings. Thus the heavy car develops (faulty scientific claims here - beware) the greatest initial momentum since the forces of friction are minimal among these objects heading down a steep hill. The typical weight limit for Scouts is 5.0 ounces. So it's in your best interest to get your car to the maximum limit.

But once the car reache
s the flat of the track, the forces of friction take over. This is where races are won and lost. It is not uncommon to see a lead car get passed on the flats by car that has less friction in play. Especially if the lead car is heavy with poor friction control, and the surging car has excellent friction reduction.

How to reduce friction? It's all in the wheels, axles and where the wheel hub touches the body of the car. First - the axle is a nail, which has a small burr left on the inner surface. This burr needs to be removed by sanding. Using a drill in a vise or drill press, one uses sandpaper to grind off the burr. Most people do this, but for a truly fast car, it needs to go further. I like to us
e the expression "sand it down until it shines like machine tool". Using progressively finer sand paper, keep sanding the axle until it has a mirror finish. This year we used sandpaper in the following grits: 80, 100, 180, 400, 1200, 1500. For best results, those highest grits are used wet.

Next step - and this is a secret step I've not read about anywhere else. since the wheel's hub touches the exterior of the car, we place a dab of cryano acrylic glue right next to where the axle will be inserted into the body. While it's still wet, spray graphite
dust onto the glue spot. As it dries, continue to spray graphite. This will create a surface of hardened glue and graphite flakes - much less friction for the hub to rub against than, say, painted wood, which is what everyone else has.

Final steps include some touch ups to the plastic wheels, but mostly it's about installing the wheels true to each other, not too much play, and a really excessive amount of graphite.

Prior year results:

2007 - "The Texas Penny" - 2nd place overall (about of 70 cars).

2006 - "Banana" - 4th place overall (about 100 cars). As a young up and comer, Alex's car was a surprise. Potential controversy when at the end of the night the car slowed down dramatically in the finals, and I found what appeared to be a small smudge of chewing gum on our tire. No lie.

1 comment:

Michael5000 said...

There are, apparently, things on Earth undreamed of in my philosophy. That banana on wheels is pretty slick! Must be all the graphite...