The autobus

Zabel

Zabel (Image credit: Sirotti)

The tough mountain stages split up the peloton every time into a lot of smaller groups which eventually combine to the gruppetto or, if you are in France, the autobus. It's a large group driven by a common interest, arriving at the finish inside the time limit, which is a percentage off the winner's time. The percentage value used varies with terrain and difficulty as well as the winner's average time.