All Mountain Bike Parts Descriptions

#23) Saddles/Seats - Seats or more technically saddles, support most of your body weight when riding a bike.
  They are padded and have springs to absorb some of the bumps from the road. They come in super comfy couch models that are so big and loaded with thick coil springs that you may want to put up your feet and grab the converter. Or, you can go the opposite way with the super "wafer thin" saddle that weighs less than 100 grams.
  The funny thing about saddles happens every spring when someone goes out for their fist ride. They arrive home in agony, and experience sourus buttus for, like a week. The first thing they do is go buy one of those big comfy saddles because they have sore butts. The soreness will go away eventually anyway, so remember that the sleek little saddle that came with the bike is probably just fine because it is better for inner leg clearance, perspiration, and circulation. The big comfy couch belongs on a cruiser.
  Saddles are supported by steel, ti (titanium), chromo, or alloy rails. These rails enable the seat to be pushed forward or back about 2 to 3 inches, to position the seat and rider for the optimum pedaling position over the crank.

Bike Parts Diagram