The ornamental design for the ski suspension system, as shown and described.
FIG. 1 is a left side perspective view of the ski suspension system, showing my new design;
FIG. 2 is another left side perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a left side perspective view thereof, shown with environmental structure.
The broken lines showing a ski illustrate environmental structure. All other broken lines illustrate portions of the claimed design. The broken lines form no part of the claimed design.