Pinewood Derby 2022

[LCHIEN]

Finished the two personal cars I was tinkering with, since I had all the stuff and worked out techniques and measurements.

Some progress pictures

And some finished pics

"Let Go My LEGO" car perfectly proportioned to LEGO blocks.



and "Naked Speed". Chassis so streamlined I had no internal places to put weights.





The green LEGO car is slower than a dog for some reason... Aerodynamics and weight distribution I think. Not worried because it was not meant to be real fast. But its not nearly as "dense" or heavy feeling as the other cars with integral weights even though they are virtually the same weight, the maximum 5 Oz total weight. The body is wider than the standard block of wood supplied but within the physical width allowed including the wheels. I had to hollow out most of the inside to get the weight to the allowed max.... three 1.5" diameter holes about an inch deep to remove wood, compared to the 2.5-4 ounces of added weight on the two left cars.

[Jim Frye]

Love the creativity!

[LCHIEN]

Success!

I think we carried home 5 awards for the four cars.
The What-a-car won 2nd place racing and a Craftsmanship award.
The Tiger car got a Larger than Life Award
The Naked Speed car got overall 1st place racing in the Family Division (Adult entries) beating out the perennial winner, they called mine a dark horse.
The Leggo my LEGO car was slower than a Dog due to its shape and weight distribution but got 2nd Place for Cool Design in Family Division.

[leehljp]

CONGRATULATION to all of your family for those awards! Well done!

[capncarl]

I have to ask. How much does Aerodynamics make on this small of an object? What determines which car is faster than another?

Are you limited to wood only for your build materials? Are there other classes for different building techniques? Can you 3D print a car body?

I have an old book, “Making Tin Can Toys” published in 1919, that I purchased when my children were wee tots thinking we would someday learn to make some tin cans toys. I did try to use it when they were in the pinewood derby age but quickly learned that kids that age seldom have the patience or attention span for anything much longer than making a peanut butter and jelly sandwich.

Could making a racer out of a tin can be a possible entry?

[capncarl]

Bump.

[LCHIEN]

I have to ask. How much does Aerodynamics make on this small of an object? What determines which car is faster than another?

Are you limited to wood only for your build materials? Are there other classes for different building techniques? Can you 3D print a car body?

I have an old book, “Making Tin Can Toys” published in 1919, that I purchased when my children were wee tots thinking we would someday learn to make some tin cans toys. I did try to use it when they were in the pinewood derby age but quickly learned that kids that age seldom have the patience or attention span for anything much longer than making a peanut butter and jelly sandwich.

Could making a racer out of a tin can be a possible entry?

Yeah, I suppose so.

The rules are attached. which summarize the rules included with the kits. The emphasis is on using the 1-3/4" W x 1.25" high x 7" long Pine block they give you that has two grooves cut in the bottom for guiding the nail axles. They also provide in the kit four plastic wheels and four nails for use as the axles and there is great emphasis on using those parts.

Pretty much as I read it they allow cutting away the block and attaching stuff of other materials to the block as you wish as long as you stay in the envelope. Overall it has to be less than 2.75" wide to not hit the other cars which means you can add fenders and fairings.

I read the rules that I could discard the block (100% cut away) and "attach" my own block which was 2.5" wide) to have recessed wheels (see the green LEGO car). I suppose you can make the body of plastic or tin. Using a 3-D printer or a bandsaw is probably no different. They key thing is the weight limit the whole car must come in under 5.00 ounces so material makes little difference. I usually weight mine to 4.99+ or so ounces tuned with a couple of small screws to be added or removed at weigh-in time since you have to use a uniform scale. .05 ounces extra (a bit more than a 1/2" #6 screw) is 1%. Given that the energy imparted to the car is M x G x H and we all have the same Gravity and starting Height a 1% mass advantage translates to a 1% speed advantage all other things being the same. A thin tin car body you will probably have to add a lot of weight.

Aerodynamics makes some difference. A large blocky car is definitely slower than a streamlined car as I found out. The fast cars seems to be thin wedges. You make it too thin and you can't make the 5 Ounce weight so you lose out. So maybe a denser material helps. The cars cover 45 feet dropping 4 feet and do it in 3.1 to 3.5 seconds... the scale speed is claimed to be about 190-195 mph according to the display. I haven't worked out if the scaling applies to aero drag. My car was tapered and only 1/2" thick at the center except for the disk weights half-recessed in its back. That's about 14.5 ft/sec or 9.8 miles per hour actual speed.

What I have found to help a lot besides mass and aerodynamics is

• wheel alignment, if the car veers to one side or the other it will lose energy rubbing the wheels against the guide rail - for most people its a matter of how straight the nails are and its hard to change but there are some tricks for adjusting wheel alignment with toe-in. I used
• Camber of the wheels slightly positive so the wheels don't rub against the body and they run on the edge of the wheel instead of the flat.
• nail treatment - I polish them through at least four or five grit levels to a high shine and inspected with a microscope
• Lubrication - dry lube like graphite is the only stuff supposedly allowed and helps a lot
• wheel mass - it takes energy to spin up the wheels and that rotational energy (1/2 MRW^2) steals energy from the dynamic energy 1/2 MV^2 that comes from the initial MGH energy. Less wheel spinning mass is better.
• car center of mass location - farther to the back increases the initial potential MGH energy product since the car is inclined 45° at the start. Extra half inch of elevation is 1% extra energy out of 48 inches.

The difference between my car which ran 3.103 to 3.129 seconds and the second best cars which ran 3.12 to 3.16 seconds was 10 milliseconds which is about 0.3%. So given the limited bag of tricks you can take no shortcuts and must eke out every 0.1% advantage you get. Because the difference was consistent. They made sure each car ran on all 6 tracks and used the best 5 numbers to get your average time. There's not a lot left to luck, just good solid engineering and testing and good craftsmanship. In the past I was doubting that for a while but I put a lot of effort into this and it worked out, beating the guy whos cars, and kids cars, were consistent winners the last two years.