The ornamental design for a fan for an exercisebike, as shown and described.
FIG. 1 is a front view of the fan for an exercise bike;
FIG. 2 is a rear view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view thereof, shown in environmental structure.
The broken lines in the figures illustrate portions of the fan for an exercise bike and environmental structure. None of the broken lines form part of the claimed design.