Do you know that during emergency braking, automobile brakes can absorb power that is ten times greater than engine power? Or they may not absorb anything at all - a tiny bubble of air wandering through the system pipelines … Engineer Valentin GRIGORIEV talks about some of the subtleties of the device and the operation of the brake systems.
As a rule, passenger car brakes have a hydraulic drive, the main components of which are the main and working cylinders of the brake mechanisms. It is extremely important to ensure their reliable sealing in order to exclude any (including internal) leakage of brake fluid and penetration into the air system. Hence, there are very small gaps in the interfaces of a number of parts, high requirements for the cleanliness and accuracy of their working surfaces, because the sealing rings (cuffs) of the working cylinders must remain fully operational at temperatures of 170-190 ° C, and the brake fluid must not boil at temperatures below 200 ° C. Therefore, hydraulic devices are usually manufactured by specialized enterprises.
So, abroad, brake system elements are made by such companies as Bendix, Girling, ATE (ITT) and others, whose products are bought by many well-known car manufacturers.
The operation of the hydraulic brake is determined by the laws of hydrostatics. When you press the pedal, the same pressure is transmitted to all the working cylinders, and it becomes possible to simultaneously brake all the wheels. But - regardless of the diameter of the working cylinders - an increase in fluid pressure becomes possible only when all pads are pressed against the drums and discs. Hence the main drawback of the hydraulic drive: a violation of the tightness of the system leads to its complete failure. To increase the reliability of the brake system, the latter was divided into two circuits.
Sometimes these two circuits are made completely independent (LuAZ-969M) - two main cylinders located side by side are controlled by one brake pedal. It happens that there is only one main cylinder, but the circuit has a so-called circuit drive splitter, the purpose of which is to disconnect a faulty circuit. Such, for example, is the Volga GAZ 24 scheme. The most widespread are drives, in which one (structurally) cylinder in fact combines two, located coaxially, in tandem. In this case, one circuit "manages" the working cylinders of the front wheels, and the other rear wheels (as on VAZ 2101 … 2107), or the circuits are made "diagonal": one includes the cylinders of the front left and rear right wheels, the other vice versa (VAZ 1111, 2108, 2109, ZAZ 1102). And here is a more complex scheme: one circuit includes one cylinder from the front wheel brakes, and the other includes the second front wheel cylinders and the rear wheel cylinders (VAZ 2121 2, AZLK-2141).
The service life of the hydraulic actuator depends not only on the quality of the manufacture of parts, assembly, maintenance, but also on the magnitude of the working pressure in it. And it is largely determined by your physical capabilities - there are known “talents” who can press the pedal so that it either breaks or hoses and pipes burst. As you know, sitting comfortably, a person is able to exert force on the pedal, 10–20% higher than his own weight. Some studies show that during normal braking, the pressure in the brake system is 20–40 kgf / cm2, but during an emergency or emergency it can exceed 80–100 kgf / cm2.
Modern cars, even of a particularly small class, as a rule, have a brake booster in the hydraulic drive of the brakes - most often a vacuum, working from rarefaction in the intake pipe of a gasoline engine or from a special diesel vacuum pump. The efforts of the legs and the amplifier are summed up, the pressure in the hydraulic actuator, respectively, rises. In fact, this means that you do not need to be an athlete for an emergency stop of the car, driving becomes less tiring, and the ease of “dosing” the braking forces increases the safety of movement.
So, the hydraulic brake is a system in which the working fluid pressure can exceed 100 kgf / cm2, and the temperature (in the cylinders of disc brakes) is 200 ° C. Simplified, its work can be easily represented as follows: with a foot movement you act on one end of an incompressible fluid column - and move it by a certain amount, depending on the area of the pistons in the working cylinders and on the gaps between the pads and discs (drums). When the pads are pressed against the disks (drums), the movement of the liquid stops; Now it changes - depending on your effort - only pressure, and therefore the brake forces of the pads. Of course, at the same time, the ductility of tubes, hoses, etc., should be so small as not to affect the performance of the system.
The condition of incompressibility of a liquid volume can be easily violated when an air bubble enters this volume. If it is small in comparison with the amount of fluid displaced into the working cylinder, then after a certain pedal stroke the bubble “collapses” under the influence of increasing pressure - and the brake works somehow, although the pedal stroke is increased. A large bubble makes the entire pedal stroke soft - the brake practically does not work.
A bubble in a liquid is not necessarily because air has entered it. Often bubbles form directly in the working cylinders (especially disc brakes) if the brake fluid has deteriorated from long-term use. Indeed, modern glycol-based Neva, Tom, and Rosa fluids are extremely hygroscopic, that is, they readily absorb water, even if it is only atmospheric moisture. The brake system is ventilated, which means that the fluid is constantly in contact with air moisture. In a damp climate, sometimes a year or two is enough to get up to 5% water in the liquid, which dramatically reduces the boiling point of such a solution - and here a few intensive braking is enough to make the liquid boil in the cylinders … and your car is left without brakes.
An unexpected rupture of the brake hose is extremely dangerous, especially with such a scheme as on the Lada VAZ 2101 … 2107. This happens with metal tubes, usually due to corrosion after years of use. If the galvanized tubes have already lost the protective layer of zinc, they must be replaced with new ones, without waiting for trouble.
The front hoses on the Zhiguli are quite short, while the suspension is working, they experience alternating stresses, which causes cracks in the outer (protective) shell near the seals, where the excesses are maximum. Deepening, cracks reach the inner, force layer - and the hose breaks. You may have heard or read recommendations to change hoses in five years or 100 thousand kilometers, whichever comes first. We do not recommend following this convenient theory, because we have not yet met a hose on which the first cracks would not have appeared in a year. Is it worth tempting fate? As a rule, all owners of a Lada, if their hose bursts, agree on one thing: the brakes "disappear" immediately and completely! But theoretically, the pedal should be enough even for the brakes of the rear wheels to work.
Why is this happening? Firstly, the front disc brakes are much more effective than the rear ones, therefore their failure - even with serviceable rear brakes! - is perceived as a general failure. Secondly, precisely because the front ones are effective, many people have been operating the car for years and do not even notice that the rear brakes have not worked for a long time - the pads are worn out, the cylinders are “soured”, etc. It also happens that their efficiency reduced due to an unresolved pressure regulator. Finally, after a long operation (over 50 thousand kilometers) of operation, the middle o-ring in the main “tandem” cylinder is also worn out, and one name remains from the “independence” of the two circuits (see fig.): When the front hose breaks, the fluid flows through the leaking o-ring 9 leaves also from a back contour.
How to prevent brake failure? Obviously, you already understood some things: change the brake fluid in time, check the condition of the rear brakes, and not just the front ones, control the pressure regulator in the hydraulic drive of the rear brakes, monitor the working brake (“parking brake”). By the way, about the latter - use it often to develop the necessary “motility”, a habit. Otherwise, when the hose or tube is broken, they become confused and forget about the “handbrake”. The same applies to the ability to turn on a lower gear in time for engine braking.
In this case, it is important not to panic - otherwise, instead of turning on the transmission (competently, with a "rebase"), some simply broke the gearbox. Once again, training, at least mentally, is useful: losing a dangerous situation in your head, the driver gets used to how to act in case of trouble.
If you decide to replace tubes and hoses, do not buy dubious goods even cheaply. And after replacement, be sure to “test” the system - check for strength by pressing the pedal with a force of at least 50 kg.
When replacing the cylinders, do not rush to install new ones without checking their cleanliness. They often include metal shavings and other dirt. Disassemble the cylinders, flush parts with brake fluid and reassemble. Otherwise, the delicate “mirror” of the cast-iron cylinder will be damaged during the first trip, and the scratches will turn into a leak. Rings (“cuffs”) of high pressure, no matter how good they look when disassembling a working cylinder, replace with new ones.
In recent years, we have received more reports of dangerous brake overheating, usually occurring during emergency braking of a car moving at high speed. Not every motorist knows (which can only be regretted) under what conditions these or other parts of the brake system work. For example, the surface temperature of the pads sometimes exceeds 450 ° C, and the brake disc almost glows. A sign of overheating is that the pedal is habitually “hard” (which means that everything is in order with the liquid!), And there is almost no braking. The fact is that the surface of the pads in contact with the disk instantly melts and after a small, preliminary reduction in speed, the car remains almost without brakes.
There is only one way out of this situation: make yourself momentarily release the pedal, press again, release … and so repeat these steps. The picture is almost the same as recommended for stopping on ice. Of course, the stopping distance will be longer, but this is better than not slowing down at all.
Describing cases of malfunction or complete failure of the brakes, we did not set out to intimidate the reader at all - on the contrary, we would like to convince ourselves that timely and thorough maintenance of the brake system, as well as clear and cold-blooded actions in a critical situation, will help to avoid trouble, sometimes very serious.
VAZ 2101 car brake system:
1 - pistons of the right forward brake;
2 - brake pads; 3 - a brake disk;