If the car starts from an incline and goes to a flat run out, moving the center of mass to the rear effectively increases the change in height over the run and increases the amount of energy the mass and height convert to speed.

 

Wow, alot of great stuff here. Thanks guys. For the record I started hand carving yesterday and both my boys are enjoying the build process. (They are 7 and 4).

I had high hopes for making a block of wood look just like my 928, but the wood block is too long. So it is looking more like a 928 limo, but that's OK. I found a close color to my OB to paint it Petrol Blue. I didn't use any power tools. A hand coping saw and sand paper.

Next phase will be playing around with all the stuff I can do to the weight, axles, graphite powder, etc. Thanks again to all. Much appreciated. I'll let you know how we do in the race -- in two weeks.

 

Great thread, bringing back many fond memories. I got a box in the basement full of Derby cars that my boys and I made over the years. I think it's time to get them out and share them with the grandkids.
A couple of war stories. One year some kid from a single parent home showed up to the race with nothing but a block with wheels. (Pre-cut) No paint, nothing, just slapped the wheels on. He took second place and had a lot of dads rethinking all their engineering theories. It was a great lesson for me and my son.
The final year for one of my sons was all out. The idea was to make it as small as possible and as heavy as possible. The shape of the car was an "I". Cut off just as close to the axle slots as possible. All weight right down the middle between the axles. I think the block was only about 1/2" or 3/4" thick. All wheels (beveled) and axles polished to perfection, graphite packed. Even the paint was wet sanded and waxed. This thing just looked fast and could not fail. We were going to the city championship. You know where this is going. What a dog. On the first run, the scout master slammed the car down at the gate and jacked the wheel alignment. We tried to fix it before the second run, but this thing was just a dog. My son and I just looked at each other in total disbelief and started laughing. Just another one of those priceless moments.
Enjoy your derby years and good luck to you and your son.

 

If the starting gate is made of ferrous metal, bury a neodymium magnet in the nose of the car so it is pulled forward as the gate rotates down......

 

For the Dad's division, I tried to devise a contraption that would hold a steel ball above the car and release it once the car reached a certain angle. The ball would roll down and toward the front of the car, then hit a bumper at the front while the car was in the flat runout. It would give the car a delayed push where friction and aero drag is worst. Too complicated.

So, I am hand shaping mine to look like a McLaren Formula 1 car and hope it makes up in cool what it lacks in speed. My son wants to win "most humorous", so he is making his to look like a man sleeping in a bed. He'll probably win fastest car.

For the OP: you can build or buy extenders for the sides of you car to make it wider. Just cut two blocks the same height and length of your car, about a half-inch wide, cut out wheel holes and glue them to either side. This is allowed. Our Scout Shop even sells the extenders for like $2 a pair.

Matt

 

My son and I built a 928 derby car a few years ago. We were used to winning, but this was the slowest car we had ever built, but looked really cool. I couldn't find anything in the rules about having to use the narrow block of wood that comes with the kit, so I, I mean we, started fresh with a 2x4. It ended up being too heavy (5oz) limit, so we used a router and took out some wood from the bottom, hollowing it out some, then covered the holes with tape. The wheels were completely recessed in their fender wells, just like the real thing. I'll try to post some pics soon.

Heavier is definately faster, you want to get as close to the 5oz limit as possible, polish the axles, use graphite, and make sure the wheels are aligned right.

 

I think this is dating Rob a bit. This idea was actually used in the national Soapbox Derby several decades ago. A kid had put an electro magnet in the nose of his car which was battery activated. He would turn it on by pushing his head/helmet back to activate a micro switch in the headrest cushion of the car. The starting gates used at all the tracks was steel, so when his car was placed up against the gate he would activate the magnet and the gate fell foreward to open so it literally gave him a slight pull at the start. I think he went to the national champoinship and I think he won before they figured out what he was doing.

Jerry Feather

 

Make sure the alignment shop doesn't lift the car before aligning.

 

Good one Jon!

Here's a couple of shots of the car we built. We couldn't get the shape exactly right, but at least it's recognizable to anyone that knows the car.

 

Very well done!

What a great project for you both, I vote you both winners!

 

Great car, Don!!!!

That's exactly the shark I tried to describe how to build. Your pictures are worth a thousand words.

Jerry Feather

 

Now I'm never posting a picture of mine. Yours is way better then what I carved. Nice job.

 

Thanks, Having a belt sander, router and Dremel really helped. We traced the side profile of a 928 onto the block of wood first and cut that out, then worked on the other angles.

Wish the wheels could have looked better but we had to use the ones that came with the kit, so we just painted the centers silver.

My son wanted pop up headlights, but I had to draw the line somewhere.

 

Lots of memories coming back on this one! Both my sons did this, and I agree that the kid should do as much as possible to make the whole thing worthwhile....we used all the tricks mentioned- rounding the wheels, polishing the axles, sloping the nose, weight in the back... fastest car my youngest son built was actually shaped like an M1 Abrams tank! Won and was on display at the local Scout office for a year...but the fastest car at a larger meet was assembled at a mass assembly meeting for all the kids, using just the parts in the kit- shaped like a dinosaur- ! There were some very pissed off dads with their Super-Swoopy maximum massaged cheater cars, left in the dust by Barney-The-Purple-Dino...

Bottom line- it's about THE KIDS! A Dad division is good idea- but let your son do his own thing, just help him have fun with it....

Look, I own a machine shop- I could have made precision-ground axles, perfectly round wheels, CNC cut bodies, perfect alignment, etc... but it would have been a DAD car, not my scout's....the tank was much more satisfying because HE did it....

Just my .02c...

 

that is so cool! nice job