Rear suspension bottoming out or bouncing

At the rear, the bike bottoms out over bumps (a sharp end-of-travel knock), bounces several times after a speed bump, sags too much two-up, or on the contrary stays hard as wood. The rear shock combines a spring (which carries the weight) and a hydraulic cartridge (which damps the oscillations). When the preload setting doesn't match your load, when the shock's oil is dead or leaking, when the spring has sagged after years, or when the swingarm and linkage bearings are seized, the rear end stops doing its job: degraded handling, a tyre that overheats, discomfort and accelerated wear. Many of these points can be adjusted or checked without heavy dismantling; a dead shock or worn bearings need parts but remain accessible. The key point: a rear suspension that no longer works is a genuine safety issue in corners and under braking.