In automotive design, the "suspension system" of a vehicle is the collection of parts that connect the wheels of the vehicle to the body of the vehicle. The suspension system components are largely responsible for the road-handling "feel" of the vehicle and contribute to its effectiveness in braking. The suspensions system is also largely responsible for isolating passengers from bumps in the road and reducing the amount of road noise heard in the passenger area.
The main part of the suspension is the steering knuckle, which is the part closest to the wheel. It provides the axle connection point and the wheel mounting points. In the most common small cars, it is located at the bottom of a compression link containing the shock absorber and the coil spring
The coil spring is the mechanical device used to compress and expand as the wheels hit irregular surfaces in the road. The force of the car hitting a bump or hole in the road gets absorbed in the coil spring. It's usually located above the steering knuckle and the shock absorber, and is the one of the mount points to the body of the vehicle
The shock absorber dampens the movements of the spring, effectively absorbing the energy that gets stored in the coil spring when it is compressed by hitting a bump or hole in the road. The shock absorber is normally located between the steering knuckle and the coil spring.
The track control arm is usually located at the bottom of the steering knuckle and connects to the body at a lower point in the body than the coil spring and shock absorber. Its primarily used to control lateral movement of the wheel.
The radius rod is a secondary connection between the steering knuckle and the frame of the vehicle. Its main role is to control the movement of the wheel from in the longitudinal, or "front to back" direction. In some cars, a torsion bar is substituted for the radius rod to provide more weight-bearing capability.
The steering gear or tie rod is a long slender rod that connects the steering knuckle to the rest of the steering system. It has only tensile strength to move the wheels for steering but does not contribute to the suspension system's weight-bearing capabilities. Because it does not contribute to bearing the weight of the car, it's not strictly part of the suspension system, but it is often part of suspension system assemblies.
