No, we do not want to discuss the lack of tread depth in the rain, for example. We simply assume that the tires are not adhering to the existing road surface. This can also be a very grippy surface, in which nevertheless the slip is too big by very hard braking. Incidentally, this could also be caused by unequal braking effect blockage.

That's the practicality of the technique of ABS, you do not look for the reasons. Actually, you only notice speed or better speed differences. Wheels do not have to block completely. By comparing the two of an axle or front and rear in case of a two-wheeler you can filter out differences.

It is amazing that a system like ABS is content with the information of the sensors on the existing wheels. While others get more data through networking, ABS is pretty self-sufficient. On the contrary, it still gives off data to the on-board computer, e.g. the driving speed.

Incidentally, scanning over ground would be much more appropriate here. Since the influence of tread depth would be excluded, which sometimes allows a truck with newer tires 2 km/h more speed than that with worn ones. The other road users must then undergo miles of overtaking.

The teeth for determining the rotational speed can be arranged on the outside of the drive shafts (see picture above). In the chapter 'Drum Brake' you will find the ring inside the drum, which also includes some protection against road dirt. Of course, disk brakes also have such rings with teeth.

It should be emphasized that these are the simplest possible so-called passive sensors. ABS also works together with more complex ones. Here, the iron core of an ordinary coil is extended by a passing tooth (picture above), which significantly widens the electric field to the left and produces a voltage increase.

Conversely, the tooth gap causes a reduction of the magnetic field, which has a voltage drop result. Both registers the ABS control unit and determines by counting the passing teeth the respective wheel speed. And now we now start from certain differences.

All sensors are connected to the control unit. At the same time, the electrical outlets for eight valves are visible, four for each for the two brake circuits, two per line for one wheel. The valve for return is closed by spring pressure and that of the supply line is opened by spring pressure. If ABS fails, the connections from the master cylinder to the wheel brake cylinders remain open.

A failure of ABS does not affect the basic function of a brake system. You can continue driving with the corresponding indicator lamp on. Only the wheel brake solving function is missing when overbraking,. The picture above shows, a wheel whose speed does not harmonize with that of the other, is temporarily excluded from the common braking.

However, at an unbelievable pace, its brake is reactivated and looked to see if it's better adapted now. In this case, the control unit knows two modes: In the first, only the valve is closed towards the wheel brake cylinder, so that the brake pressure does not increase there further and only in the second step, the valve opens to the return and the pressure is lowered.

And of course you can not just let the brake fluid flow back into the appropriate container above the master cylinder. Because if the driver stays on the foot brake, which is very likely, brake fluid would be stolen at each control cycle, which is urgently needed between the main and wheel brake cylinders. The brake foot would approach the bottom plate with concern.

Therefore, there is the so-called 'return pump'. It ensures that the 'branched' brake fluid is exactly pumped there, where it comes from, namely between the master cylinder and the closed valve. This naturally raises the brake pedal slightly. This is precisely what gives vibrations and noise development, at least for the first ABS systems.