Subject: Ideas on the small cars
Date: 05 Oct 1998

1. Stress building them with "found" materials, rather than purchased
   commercial parts. This is hard at first, but gets their creativity
   going. For example, instead of wheels from a Lego set, try:

   - pop cans
   - Pringle's potato chip can lids
   - two paper cups taped mouth-to-mouth
   - a plastic ball
   - a disk carefully cut out of cardboard

   Axles can be things like:

   - a straightened out paper clip
   - pencils or pens
   - coat hanger wire
   - safety pins

   Bodies can be almost anything that is moderately rigid:

   - cardboard box
   - chunk of wood
   - carefully folded paper
   - soda straws and tape
   - plastic bottles or containers

2. I'd first measure the car's performance by rolling it down a ramp.
   Judge the merits based on how straight and how far it goes.
   Things to look for (to learn by experimentation and discovery):

   - weight doesn't make much difference (more weight also adds more friction)
   - the fewer the wheels, the farther it goes (more trouble with wheel
     alignment, i.e. getting all the wheels to point in the same direction)
   - the skinnier and smoother the wheels, the better it goes (that why
     fat bumpy toy car wheels work poorly)
   - streamlining doesn't matter (speeds are so low)

4. Then the best designs can be motorized.

   - direct drive (wheel right on the motor shaft) doesn't work. The motor
     spins too fast and has to little torque ("twisting power"). You have to
     gear it down one of the following ways:
   - friction drive: put a rubber band around the wheel (or piece of rubber
     tubing on the motor shaft) to provide friction. Rub the motor shaft
     against the wheel. Slips if there isn't enough pressure between them.
   - belt drive: put a rubber band around the motor and the wheel. May need
     to have guides or flanges to keep the belt centered or it falls off.
     Likes to slip if the belt isn't tight.
   - gear drive: scrounge gears out of some toy, old mechanical clock, etc.
     Hard to get spacing between gears exactly right, but works well when
     you do.
   - chain drive: like a belt, but with bumps to catch so it won't slip.
     Use a piece of bead chain, or long spring with its ends hooked together.
     Make pulleys out of soft material, like styrafoam, that chain can be
     pressed into to make teeth, or something moldable that sets, like play
     dough. Probably the hardest system to get working unless you can find
     the parts pre-made.

5. Test the motorized cars by running them across the floor, and UP the ramp.
   Time how fast they go, and how steep a hill they can climb as figures of
   merit. Things to look for:

   - The ratio (of motor shaft diameter to wheel diameter) is very important.
     Too large, and it crawls. Too small, and it can't start itself or can't
     climb the ramp. There is an optimum ratio that works the best.
   - The heavier it is, the worse it works. Motor power is fixed, so more
     weight slows it down.
   - Attention to detail is important. The differences between types of
     drive systems (friction, belt, gears, chain) are small compared to how
     well it is made.

Hope this helps!

Lee Hart