Suspension
The suspension keeps the tyres in contact with the road and the vehicle stable over bumps. When it is worn, grip and braking suffer even though nothing about the brakes has changed.
Signs of wear: the vehicle continues to bounce after a bump, leans heavily in corners, sits low on one side, or knocks over rough ground.
Overloading is what destroys suspension fastest, and an overloaded vehicle also steers and brakes worse. Load within the limit, and spread the load rather than piling it at one end.