Found only on full-suspension mountain bikes, rear shocks are robust, sensitive and tuneable. Their job is to increase comfort and traction over roots and rocks. Having the right rear suspension can also mean the difference between victory and defeat in competition. Many modern rear shocks allow you to adjust rebound and choose between low- and high-speed compression. Whether a shock is compatible with your frame depends on installation length and stroke travel. Fine-tuning the spring stiffness may mean installing a different, separately-available spring.