At my gym, it depends on the instructor giving the class. For the most part, it's a first-come, first-served basis. We only have 2 spinning instructors - one has a "cult-like" following and her classes will always fill up quickly....the other instructor, is not as "motivating" and enthusiastic...therefore, there is usually open bikes in her class and no need to arrive early. But the "good" instructor, has 1 class on Tuesday mornings that is SO popular, that you have to stand in line 1/2 hour before the class and get a poker chip to reserve your spot. This is at a Ballys Total Fitness...can't speak for any other gyms, as this is the only gym I've been to.