Plastic? Ive never seen metal wheels
no because it will not turn correctly
A skateboard has 4 wheels.
by "curve" do you mean turn with out lifting up the front wheels? to do this use your feet and legs to tilt the skateboard, but not so much that one side lifts up. This will turn the skateboard.
A kick turn is when you turn not by leaning to the side, but by lifting your front wheels and moving them in a different direction to turn.
yes the sizes of skateboard wheels range from 40-70 mm
Bigger wheels make it easier to roll over cracks in the road. softer wheels are good for riding vert because they grip the ground alot better than hard wheels.
It is pretty much a wheely on a skateboard but with 2 wheels.