When a moving car hits a hump or hole in the road, a sharp force is imparted to the tyre: some of this force is absorbed by the compressed air contained within the tyre and is apparent as distortion of the tyre wall (the air in the tyre acting as a kind of spring) and the rest is imparted to the suspension system. The object of suspension is to isolate as far as possible the car chassis and body from such forces, and the suspension has two components; springs and dampers.
Like the walls of the tyres, suspension springs distort when subjected to forces and so absorb those forces. Springs which are compressed (or twisted, in the case of torsion bars) contain energy. which they try to release by re-extending. If a spring is compressed and then allowed free movement, it will extend beyond its unstressed length to a certain point (at which it still contains energy). then re-compress under the force of the remaining energy. The spring continues to extend and compress in this way until the initial energy is dissipated. This is called 'resonance'. Resonance is controlled by damping.
The effects of undamped resonance in the suspension springs of a road car would be disastrous, because every time a spring re-compressed, it would reduce the traction of its wheel, perhaps even lifting it from the road. Four wheels all shifting from full to light traction on an undamped car would reduce road-holding severely. To control the resonance of the spring, dampers (often erroneously referred to as 'shock absorbers' or 'shockers') are fitted.
When a correctly damped wheel hits a bump in the road, the spring compresses to absorb the shock (springs and not dampers are in fact shock absorbers), but the damper limits the amount of its deflection and then limits resonance, so that the spring returns to its unstressed length very quickly. This keeps the tyres in the maximum possible contact with the road.
There are essentially two types of suspension fitted to Beetles. The earlier system comprises torsion bars — bars of steel which work in effect as springs when they are twisted. Cars with this suspension are typically known as 'Torsion Bar' or 'Swing Axle' cars.
The later suspension type, fitted to the 1302 and 1303 (and 'S.) series cars, comprises McPherson strut (a coil spring with a concentric telescopic damper) suspension at the front and, although there are still torsion bars at the rear, the swing axles are replaced by double-jointed drive shafts. with diagonal arms to locate the hub. Cars fitted with this suspension are commonly referred to as 'McPherson Strut', 'Double-jointed Drive Shaft' or sometimes 'Diagonal Arm' cars.