The wheel speed sensor is located in the hub.
The purpose of ABS is simple to formulate - it prevents the wheels from locking during braking. Is the brake pedal pressed too hard? It doesn't matter - the electronics, comparing the signals of the rotation speed sensors on each of the wheels, determines the moment of a sudden stop of the wheel and, by opening the solenoid valve, relieves excess pressure from the brake line to a special hydraulic accumulator. The brake pads are released, the rotation of the wheel is restored, the valve reconnects the line to the pressure, which is created by the driver's right foot with the help of a vacuum brake booster. And if this pressure is still too great, then the cycle repeats again and again. In the cabin, the inclusion of ABS is manifested by a crackling noise during the operation of the executive unit and jolts on the brake pedal, more or less noticeable depending on the design of the system. In fact, the ABS imitates the actions of an experienced driver who, on a slippery road, avoids blocking the wheels by using intermittent braking. But the ABS does this with a frequency and accuracy inaccessible to humans - up to 15 times per second. As a result, the ABS should not only provide the car with a minimum braking distance, but also make it obey the steering wheel - because the wheels roll and perceive lateral force, as far as tire grip with the surface allows. And this is the main advantage of ABS. After all, in the case of blocking the wheels, the car slides uncontrollably straight, even when the steering wheel is turned all the way.
Concerns
It would seem that everything is already clear. Anti-lock braking systems are recognized by the entire civilized auto world and have already become standard equipment on cars, starting from the middle class. But there are often voices of dissatisfaction.
"No electronics can be compared with an experienced driver, some say. – Give me the same car without ABS, and I will stop earlier!"
Others do not like the fact that the ABS, interfering with the braking process, completely excludes human participation. No matter how hard you press on the pedal, it will be pushed back with an unpleasant vibration. The car slows down by itself, there is no way to influence the course of deceleration, besides, with ABS it is very difficult to calculate the braking distance and predict where the car will stop. It remains only to wait... Unbearable for a driver who is used to controlling the situation!
And still others nod at the rally drivers. Like, it's not for nothing that they are afraid of ABS, like the plague! And they, the athletes, know better...
Disable non-disableable
In order to put everything in its place, we went on a technological crime. We found a plug under the hood that fits the ABS executive unit, and pulled it out of the socket, causing the yellow ABS control to light up and the red brake light in the instrument cluster to flash alarmingly. So we deactivated the non-switchable anti-lock system. The tests were carried out in the following way. We have chosen four main exercises: braking on a straight line - on a uniform and mixed surface (mixed doubles), in a turn and while avoiding obstacles (that is, when performing the maneuver, the rearrangement). Passat was equipped with recording equipment that accurately measures speed and stopping distance. And behind the wheel were our test experts. However, some of the exercises were proposed to be performed by ordinary drivers of very different qualifications - from a novice motorist to an experienced professional. Everyone could practice before the control races, get used to the car and "shoot" for the next maneuver.
Each exercise was performed in two series. At first, more than ten races were made with the ABS working, then the system was turned off, and the car again hurried to the route of the next maneuver marked with bright poles.
How to slow down?
With the anti-lock braking system, everything is simple: if you want to get the maximum possible deceleration - drown, without thinking twice, the pedal to the floor. How do you brake without ABS?
First we tried everything "theory" - brake on the verge of blocking the wheels. Does not exceed! The too sensitive Volkswagen pedal does not allow precise dosing of the braking force - and we are mistaken, both in one direction and in the other. And this is in a calm environment. And try to do it in an extreme situation, when an accident threatens!
Then we applied a tried and tested method - intermittent braking.
And for comparison, they tried, without further ado, to slow down "to the floor", with wheel locks. What is the difference? On a straight line, the main difference is in exchange rate stability. This was especially pronounced when braking on the mixed. There was wet asphalt under the left wheels, and a snow-covered roadside under the right ones.
With the ABS turned on, the car at first starts to divert towards the surface with better grip, but the electronics work properly: the Passat immediately levels out and then slows down in a straight line, requiring almost no steering. During intermittent braking without ABS, the driver had to steer a little, and the braking distance turned out to be longer.
But with the wheels completely locked, the car braked almost as effectively as with ABS. But at the same time, it sharply turned to the left at a dangerous angle - up to 40°! The advantage of ABS here is obvious.
Coverage Surprises
Ice can be as smooth as a mirror and rough as the surface of emery. Snow can be dense, rolled, loose, stuck together, frozen...
Why are we? And to the fact that each coating has its own specifics, and after our tests, we cannot give an unambiguous answer to the question of interest to everyone - under what conditions the braking distance with ABS will be less than without it - we cannot!
Lots of options. For example, when, during repeated stops on the same straight line, the wheels of our car rolled over a track of rough ice crushed with spikes, the braking distance with and without ABS with locked wheels turned out to be almost the same. No wonder our athletes say that the most "tenacious" coating - not asphalt, but striped "nails" ice track. And in the snow - what a surprise! – the car with ABS stopped a little later. The thing is that on the snow surface, the coefficient of adhesion turned out to be lower than on rough ice - the spikes here do not work as efficiently. For ABS, apparently, it turned out to be unprofitable. But at "usual" intermittent braking (pressed - released - pressed) during the period of blocking the wheels, a snow roller forms in front of them, which helps the car slow down more intensively. The work of the ABS prevents the wheel from blocking - and the roller does not work.
Don't be afraid to turn the steering wheel!
Imagine a very real situation: a truck suddenly skids in front of you in a slippery turn, and it turns around across the road. Or another: a car jumps out of a side passage onto a narrow street right in front of your nose and blocks your lane, leaving only the oncoming one free.
On a wave of adrenaline rushing into the blood, the driver panicked, reflexively, that there is urine, he will press the brake. The wheels will lock up. And with locked wheels, the car in the first situation will fly out of the turn, and in the second it will crash into an obstacle. For without ABS, only intermittent braking will help to avoid an accident, and here the driver is required not only outstanding composure and endurance, but also outstanding skill.
And here is our example. The most experienced expert needed to call on all his skills to help, so that, alternating strictly dosed pushes on the brake pedal and precise steering work, he could repeat and even slightly surpass the results of the same maneuvers with ABS.
But so what? After all, even the tester needed to do more than ten sighting runs for this, and in a real situation there will be only one chance! And the electronics here will work reliably and for good.
When braking in a turn, the driver's task was generally the simplest: he pressed the brake, turned the steering wheel - and the car, clearly following the given trajectory, slows down intensively. It's available to anyone. It is only important to remember that during braking with ABS, the car can and should be controlled. Do not numbly squeeze the steering wheel, but boldly act with it!
When driving around obstacles (we slowed down when performing the rearrangement maneuver familiar to our readers) the situation was like this. When the speed of the maneuver approached the limit, after avoiding the obstacle, the rear axle skid began to develop. It was both with ABS and without it. But using ABS, it was possible to extinguish the speed so much that the skid was not so sharp and deep. And the braking distance after the detour was 15 meters shorter. Significantly!
True, in order to stay on the road, when correcting a skid, it was necessary to release the brake pedal even with the anti-lock braking system working. Why? We do not argue, ABS leaves the ability to change the trajectory during emergency braking, and not slide in a straight line with locked wheels. But in the fight against skidding, the operation of the ABS reduces the effectiveness of corrective actions by the steering wheel too much. And even for those drivers whose cars are equipped with ABS, in such situations, you need to be ready to release the brake pedal for a moment, deliberately refusing the services of electronics.