Originally Posted by Stang70Fastback
There are really three core systems you need to understand. I'll try to make things as simple as possible, but still thorough.
ABS: This system is designed to prevent wheel lockup under hard braking, or braking on slick surfaces. The basic way it works is that each of your four wheels has a wheel speed sensor, which measures the rotation speed of the wheel. The car is always comparing the rotation speeds of all four wheels, and it uses these sensors to detect when a wheel has "locked up" (stopped spinning) due to a loss of traction; either you slammed on the brakes too hard, or one of your tires hit some snow/ice/oil/etc... When it detects this, the car can individually cycle the brakes on the wheel(s) that have lost traction. This is done VERY quickly - many times a second - and is basically the car releasing and re-applying the brakes over and over. (That's the buzzing you feel in the brake pedal when ABS is operating.) This lets the wheel keep rotating, rather than simply letting it lock up and skid, which in turn allows it to maintain some degree of grip on the road, and therefore allows you to maintain control of the car. ABS actually increases stopping distance, since you're basically cycling the brakes on and off, but in exchange, you retain the ability to steer the car, which is generally more critical in an emergency situation.
Traction Control: This system is basically the opposite of ABS. Rather than monitoring for wheels that lock up, it uses those same wheel speed sensors to detect TOO MUCH rotation, which is a result of applying too much throttle for the situation. Doing a burnout, for example, or accelerating and spinning your wheels on a wet/snowy/icy road. Traction control resolves this situation in two ways. First, it cuts engine power. You can have the throttle pedal completely buried to the firewall, but the car will basically step in and say, "You're being an idiot. You're just spinning the rear wheels. Let ME manage this," and it will reduce engine throttle until the rear wheels stop slipping. Remember modern cars are drive by wire, so there is no mechanical connection between the gas pedal and your throttle body. So even if you have the throttle pedal fully depressed, traction control allows the car to override that command and say, "He wants 100% throttle, but he is on ice, and we are only going to allow 7% throttle, because any more than that just spins the rear wheels." In addition, Traction Control can also leverage the car's ABS system, to individually brake either one of the rear wheels if they are spinning too quickly.
So far, we have two systems that work in tandem, doing their best to basically ensure your wheels are always spinning at the same speed that your car is moving.
Stability Control is the most advanced system. Its job is to keep the entire CAR pointed in the direction that the driver intended at all times. At its most basic level, it is designed to monitor for two things: understeer and oversteer. Understeer is when you turn the steering wheel, but the front wheels lose traction, and the car continues in a straight line, or turns maybe a little bit, but not as much as it should. For example, if you've ever driven in snow and tried to make a left turn and the car just didn't want to turn and you almost slid straight into a curb... that's understeer. Oversteer is the opposite of that. Basically the car turns TOO much, and the rear end steps out. Think drifting. Or if you've ever tried to swerve and wound up spinning the car around. Or doing donuts!
Stability control's job is to prevent either of those things from happening. It does this by monitoring a whole host of things. It knows how fast you are going (wheel speed sensors), it knows the angle of the steering wheel, throttle position, etc... Based on those things, it knows exactly how much lateral (side) force, and how much rotational (yaw) force it should be experiencing. The car has accelerometers on board that allow it to detect all of these forces. If you turn the steering wheel, and the front wheels lose grip, that will be immediately identified by the car, as it won't "feel" the car turning as hard/quick as it should be. Basically the car says, "We are going 30 MPH, and he just turned the wheel XXX degrees. My math tells me that should result in YYY G's of lateral force, and we should be turning at ZZZ degrees per second. But my sensors are reading much lower numbers, so clearly the front tires aren't turning us as much as they should be." So it responds by doing a whole host of things. It can cut throttle, which helps shift more weight back to the sliding front tires. It can also brake your inside rear wheel, which helps to "drag" the car around the turn, to help it turn better. It continues to do this until everything checks out, and the car is pointing where the driver wanted it to point.
If it detects oversteer (the rear end coming out too far and starting to slide) it will also cut throttle, and it can brake various wheels to help control that slide and bring the rear end back under control.
In normal mode, these systems are SO sensitive, that they generally step in almost before anything even happens. You won't get a massive burnout going before the car steps in and stop it. Instead, you simply won't get more than maybe a slight chirp from the rear wheels. Even if you rev the engine all the way up and dump the clutch, as SOON as the rear wheels start to slip, the car will cut all engine power, and brake the rear wheels. Same goes for stability control. In normal mode, it will detect the rear sliding even really before the rear tires fully lose grip. They are VERY good systems for someone who doesn't know what they are doing, though more skilled drivers often find them too overbearing. However, between those three systems, it's VERY difficult to completely lose control of the car unless you do something monumentally stupid. Generally speaking, there is no solution for too much speed. If you plow into a corner going faster than your tires have grip for, even the most advanced driver aids can't do anything to help you. These systems help you regain traction, but they don't magically give your tires MORE traction. If you are stopped on ice, and you floor the car and swerve violently, the car will manage all that stuff and keep the car under control. If you are stopped at an intersection, on a snowy road, and you want to make a right turn, and you just bury the throttle and turn the wheel, the car will cut power, and manage your wheelsping, and you will make the turn as fast as the car will allow you to. However, if you plow around that turn at 60 MPH, the aids will do their best to help you complete that turn without losing control, but your tires won't have enough grip to do so. So the result will be a nice, controlled, accident where the car just doesn't turn enough and you slam straight into the curb on the far side of the road.
Hopefully that makes some sense...?
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