Construction Details 2007

Tank Goodness

Dad did the research (probably too much), but Tyler did most of the work. Attention span is more of an issue in the scout's participlation than is skill or dexterity.

Axles: The axle work used a varible-speed electric drill in a vise. Even though it's a power tool, and must be treated with respect, a drill is pretty safe for even an excited Cub Scout to work with. An adult can check that touching a finger to the spinning axle or chuck won't cause injury.

The pipe clamp around the trigger lets us run the drill at slower speed without having someone hold it.

  • Burrs filed off
  • Back of axle head filed
  • Polished with sandpaper, polishing compound, and pumice

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  • Bores polished (Novus plastic polish)
  • Wheels coned
  • Wheel sprue mark shaved off. We're really doubtful this is even worth the effort. We put the wheel in a mandrel, chucked it in the drill, and used a utility knife to lightly shave the wheel tread. Spinning the drill at low speed seems to work best for this. The sharp utility knife can be moderately dangerous for a scout to handle.
  • Extended wheel base. We did this mainly because Dad wrecked the axle slots, leaving us no choice but to drill new holes. Extending the wheel base is reputed to improve stability (less wiggle on the track), but actually make the car slightly slower.
  • Right front wheel lifted (more easily seen in the enlarged picture). This reduces the amount of energy lost to rotational inertia, because it only spins three wheels. You'd think that it would also reduce friction, but that's wrong -- the same total friction accrues, just distributed among three wheels instead of four.

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  • As close to 5.0 ounces as possible, achieved with cylindrical weights placed in holes drilled in the rear of the top deck. (At weigh-in, the official scale said 5.1 ounces, so we were glad we were able to remove a few grams by just turning the car over to expel the weights. Next time we'll add tape, though: the weights also fell out every time a judge turned the car over.)
  • Drilled out some wood from the front in order to add more weight to the back. Doing this moved the center of gravity (CG) only about 5 mm. [The "CM" (center of mass) line is the original, and the line just aft of it is the new. It's still way forward of the ideal.]

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