Co2 Car Shape

[serfboy87]

I realize that a Teardrop shape is the most aerodynamic shape. For this car, I must have a large area in the rear of the wood to hold a CO2 Cartridge. I was wondering,if I were to reverse the teardrop shape (so that the large part is in the rear)... Would I accomplish the same reduced wind resistance? The same reduced resistance if I were to leave the large part in front as on an airplane wing. Any help appreciated...Have a great day!

 

[swertel]

No, you wouldn't get quite the laminar flow. The reason that a tear drop shape is so aerodynamic is not because it cuts through the air so nice, but because it puts the air back together at the end of the vehicle very well.

Why not just create the tear drop shape and then drill a hole in the back point of the tear drop to fit your CO₂ cartridge?

 

[jimkeel]

Did the requirements specify Wood as he build material?
Are the requirements only max min length, C02 placement etc?
I had a very good experience using bits of brass tube and square tube which I found at ACE hardware(sorry for the plug but that was the place)and epoxy. My biggest problem being the car was so fast it still had alot of C02 left at the end of the track and blew the engine mounts off the vehicle. with basically a stick frame there is negligable resistance from anything but the wheels on the ground (use four front wheels filled in with hot glue and ground into a disk shape, as they produce no forward motion the logic behind fat tires is invalid and no contact patch is needed for steering), the axle in the hub(smaller contact patch with ground less rotation and drag+ a little graphite), and the teardrop shaped co2 bottle. Again, there will be lots of co2 left at the end of the track so brace the bottle accordingly, build two vehicles, and make sure the timing device is ON. Also let your competitors race first so that they can enjoy their day at the track before you dominate it.
Happy Trails
JK
P.S. my technology teacher in high school was a very generous person for allowing me to strech his rules so far.
Two things if it must be wood.
1. make your car as long as possible, everyone starts wuth the back of their car in line, if your car is longer you are winning before you begin the race.
2. lighter is better, I believe the aerodynamics are only slight factors although if the nose is too thin it will snap at the end of the track. Smoother paint might help reduce turbulence along the surface?
3. Try to use the four front wheels idea. It saves weight and drag. This is a definite plus.
JK

 

[maddmaxx2]

I have built several 1st place Pinewood Derby cars for my sons. Although they were gravity driven, some of the folowing principles apply:
1) Aerodynamics are of much less importance than trying to get the rolling resistance and friction to zero;
1.1) We used Boy Scouts of America "nails" for axles that needed to have ridges and structural imperfections filed off, then progressively sanded, and pumiced down to a mirror finish all of which were perfomred by mounting the nail in a drill or a lathe chuck;
2) Minimize the amount of structural material and maximize the propellant (in my case lead weights). For you, build just enough structure to mount the CO2 canister and mount the wheels;
3) Pinewood rules prevented modifying the wheels at all; but if is not against the rules, you could turn the interiors of the molded wheels on a lathe to minimize the mass at the periphery (circumference) thus minimizing radial inertia; i.e., concentrate the mass at the hub rather than at the circumference ... just like the Cane Creek (Chrono) bicycle wheels which place the spoke nipples at the hub instead of on the rim;
4) Narrowing the the tread width will reduce the contact patch, which will reduce the amount of rolling resistance (Note pro bicycle racers race on 20mm - 23mm wide tires while we mere mortals ride on 25mm tires);
5) Friction is your worst enemy. Stiction is a secondary concern, i.e., reduce contact patch.

 

[rnordquest]

My son built one (without my help; what good would an aero engineer father be) that significantly beat everyone else in his high school competition. I'd ask him.

Roger