All Mountain Bike Parts Descriptions

#10) Wheel Rims - Also known as HOOPS, they hold the tires and tubes to the wheel.
  Rims provide shock absorption as well, by buckling during an impact and then popping back into shape from the spokes tension holding its shape together. They were made of wood, then steel, but now aluminum because of its flexible nature and its lightness. The number of spoke holes in a rim vary, depending on the bikes intended use.   Road bikes use less spokes which make the wheel weaker, but using less spokes also cut through the wind better. BMX bikes use 48 hole rims, 48 spokes. Thirty-two holes rims seem to be the standard for most mountain bikes, a good low drag but maximum strength combination. Aerodynamic shaped rims help smooth out the air flow, although I find that they crack easier from big impacts because they don't flex the same way a regular rim might. It would be best to keep them on the road bikes.

Bike Parts Diagram