There are a few ways to stiffen the rear shock on your motorcycle. One way is to adjust the preload. This can be done by turning the adjusting knob clockwise or counterclockwise. Clockwise will increase the preload, while counterclockwise will decrease it. Another way to adjust the stiffness of your rear shock is by changing the spring rate. This can be done by installing a stiffer spring or by removing spacers from your existing spring. Finally, you can also adjust the damping of your rear shock. This can be done by turning the rebound damping knob clockwise to increase stiffness, or counterclockwise to decrease it.