Last year the Derby Day caught us by surprise. We saw that Scouts were heading into the cafeteria when we went to church. I dropped the family, ran over to check on the rules, pulled my 6 year old son from church and headed home. On the way I gave him the action list. In 45 min we were back. In that time we had silver sprayed the part of the block we had left over from the wedge we planned, hot glued the prepped axles and wheels, added weight (nuts, bolt and a beer faucet ring!), got him into his uniform and zoomed back. He won a few races, but the alignment was curving to the right - a bad handicap.
Last years lineup: Our car was the rough silver one with the hot glued hardware on top.
Last Sunday was our Pack's Derby Day 2011 and our car took 2nd. The hardest part of building the car was keeping my son as interested as I was.
We read through the book that we got last year as a bedtime prep for the next sessions work:
Speed Secrets Book
Great book. It details the tips to make three different levels of cars. Winner, Champion and Ultimate. A good idea is to build two cars so that the scout follows the lead and does all the steps on his car. We may try that next year.
We found out about the rules specific to our Pack. We were limited to a 4 1/2 inch wheelbase. I ordered a set of Derby-Worx tools to square the axle holes, prep the wheels etc. I figured that the tools would pay off over the next four years.
Here is my list of important design and prep issues.
1. Weight - shoot for 5.00000 oz. The scale last year weighed lighter than my own (4.9 oz) We ended up drilling wood out of the front. The judges had a weight standard that they set their triple beam balance to. We adjusted the weight on this years car to be 3 grams lighter than last years and then used tungsten putty into a hole under the center of gravity to bring it up to the max allowed. The putty used was about the size of a pea - dense stuff.
2. Center of gravity - 1 to 3/4 inch in front of rear wheels. This requires a light front end and dense weight in the back. The cars start on an incline and then carry their momentum onto a long flat section of track. Having the weight in the back puts more potential energy to work. The book gives templates for drilling out the tail and placing tungsten cylinders in the holes. We added the gun turret as my son's design choice and he was consistent in explaining to everyone that that is where the weight was. The turret is actually a light 1/2" PVC cap and stem which we sawed to shorten. Wood is so light that the holes up front are more design than function. Although they did prove helpful for carrying the car dangling from a finger. We used body filler to seal the weight holes and it is visually inperceptable - stealth.
Aerodynamics - not really a factor at this scale.
3. Axle prep - you have to remove the crimp burrs from beneath the head with a file. Then we straightened the nails with the Derby Worx press and then wet sanded using a drill clamped to the workbench with 600, 1000, 1500, 2000,2500, 3000 followed by metal polish I used on my Fuchs (tripoli and jewlers rouge).
4. Wheel Prep - The new BSA wheels have a rounded inner hub, no real need to taper it again. Polishing the inner rim and hub with 1000 grit and then graphite is vital. The tread surface can get sanded and graphited too. Using a wheel mandrel and drill is the way to go.
5. Alignment - I avoided the axle slots by using the other surface (top) of the block. Redrilling at the required locations with the "Body tool" which acts as a gig with #45 bit to get a straight hole on either side of the block. The car rolled straight enough after placing the wheels that we didn't want to find the enemy of good. It seemed mostly dependant on what we rolled it on. Our old oak table leaves are all a bit warped it seems.
6. Painting - most important part of getting the scout involved. MASK OFF the hub rub area (3/8 inch circle) so that these remain bare wood. After painting, remove masking, mask off paint and rub graphite here.
7. Lubrication - After attending to the wheel hub/body contact points and the wheel /track contact points it is time to make sure there is adequate graphite in the wheel bore. As the Meade book recommends, a hobby paintbrush works great for getting the slippery stuff in there.
8. Break In - spin those wheels! We used a dremel polishing wheel to keep them moving for 10 min or so. Ideally they should spin for at least 20 sec after a finger spin. That's about all ours did - supposably 35 sec is possible!
Derby day actions - I worked with my son so that he was the one who brought he car to the scale and asked me for weight additions. We had competitors behind us and I didn't want to spook them with tungsten putty and hidden holes.
I dug into my pocket and pulled out an assortment of items and a few were added to the scale until it was darn close. We took those items and found a quiet place. I then consulted the copy I made of the random garage items which were weighed
on an analytical scale 
I have that was my wife's grandfather's. (It is maybe 80 years old and uses a weight set for grams, beam weight for tenths and a venier labeled belt with gold chain to add weight.) We added up the weight found for the items and made up 4 1/2 grams of tungsten putty which we had made into ~1 gram pieces. Returning to the scale we found we could add a schrader tire valve stem cap (0.3785 grams) I used a razor blade to make a 1/3 gram piece which my son added to the bottom of the car. The presumption was that we were adding the weight into the turret. It is at the center of gravity, but there is a 4% advantage to adding weight low verses high. I'm confident we were within 1/100 of an ounce (1/3 gram).
So right behind me in line is my son's Den/classmate with his car which is the spitting image of the "Ultimate" car (#2 in the picture below) from the book linked earlier.
It has an extended wheelbase, a notched front, non-kit wheels - all things prohibited in the rules E-mailed to most. I'm still presuming that this dad didn't get the rules because Emails are haphazard, especially to two household families.
Being cars 1 & 2 they raced against each other for the first race and our car lost by a half car length. We won every other race by 1/2 to 5-8 car lengths and thus took second place. I mentioned the wheelbase to a few parents who were then concerned that the winner would be disqualified at the Division Derby on 26 March. We will wait and see. The funny thing is the judges were gaga over this car and gave it the Judges award. It also won the "Sickest Car" voted by the boys and the Peoples choice award! Nice car - but who reads the rules and is it only weight that matters to the judges!?
Now I'm wondering if we should make some modifications within the rules. Ideas are: Raise a front wheel so that it rides on three, matched wheels from the same mold, work on those axles till we get 30 sec spins, notch the underside of the front to get an advantage on the starting pin (I hear a bananna shape helps!). Maybe we will add more graphite and call it good!