Anti-Lock Brake question.

In the ‘I pit traction control’ thread in the Pit, ivn1188 states that ABS does not need to lock up the tires to test the traction conditions.

Hmmm. But how can it tell what the limit is if it does not exceed it? Does it not have to lock the tire (even for a micro second) or at least slip it to determine the traction conditions?

I wonder if you need to distinguish between various methods of ABS. Old ABS was not terribly good compared to today and my sense is even today you get a variance of how well different systems perform.

In that thread someone linked this video which shows a car on snow with the “EPS” system engaged maintaing remarkable control in snow (even the driver was surprised). I am guessing that traction control and ABS systems are merging to work together more.

I do not know (not an auto engineer) but I am thinking such advanced system might well manage what you are describing by mixing ABS with traction control in a more seamless whole.

Yeah, I’m kind of curious, too, as I thought ABS was computerized cadence braking, but apparently that’s older systems. Still, I can’t see why it can’t be programmed to execute perfect or near-perfect threshold braking. I mean, how do you as a driver know where the limit is and how to stay just under it? I assume the same decision process can be programmed into a computer ABS system.

Whoops! :smack:

ABS uses wheel speed sensors. They don’t measure force. However, the system can react very quickly – surprisingly quickly – to changes in wheel speed that indicate imminent lockup or the beginning of slippage. It would not surprise me if the system could respond to a wheel slowing down (approaching lockup) before actual lockup occurred, nor if it could respond to a wheel speeding up (starting slippage) before the driver could perceive slippage.

Yeah, In my vehicle, VDC control uses ABS, and regulates engine RPM. It also uses ABS to simulate ‘posi’ traction. Braking one wheel to transfer power to the other.

GaryT Thanks for your reply. I guess if it senses all tires slowing down at the same rate, it assumes no slippage and good traction.

I was going to say what Gary said.

Here’s a good link if you want to learn more about how the system works:

In layman’s terms:

At each of the four wheels, there’s a speed sensor, a type of “speedometer” that communicates the speed of that particular wheel to a central control unit. (As you’ll read in the article, some simpler and/or older systems don’t have a sensor at each wheel.)

The controller’s main objective is “make sure all wheels are rolling at the same speed.”

That is, your foot’s on the brake, three of the wheels are going 40MPH, but the right-front wheel is only going 10MPH.

Now, the controller says:

“Hey, that right front wheel is going way too slowly; I’ll bet it’s locking up on ice or snow or something. Well then, I’ll reduce brake fluid pressure to that wheel a little bit so that’s going 40 again like all the others.”

And then:

“No, wait, that’s not right. Now the other three wheels are going 37MPH; I think the car’s owner must be slowing down. Well, I’ll once again adjust braking force on that right-front so that it’s going 37 like all the others.”

Of course, all of that “thinking” is going on very quickly, many times per second.

Enipla asked if the system needs to allow the car’s wheels to lock up, even for a microsecond. I really wouldn’t know about microseconds, but we at least know that after a car with ABS makes a panic stop, it doesn’t leave dotted-line skid marks, so that at least tells us something.

No dotted-line skid marks?


To be fair, that’s in snow, but I believe I have seen dotted-line skid marks on pavement before.

That image came from this page on abs.

Thanks to Chris Luongo for the informative link on ABS systems.
I was surprised to see the insurance industry statistics are not showing positive results with ABS brakes over non-ABS. That sure should change in the future as I am totally impressed with the function, and the reliability of these extremely complicated systems.
Back in the 80’s when ABS was really getting known A good friend bought a new ford pickup with 4 wheel ABS and the thing was comical :smiley: On ice it did not matter how softly one would depress the brake pedal, his truck would instantly lock up all 4 wheels for more than a second and then go into the pedal pumping operation.
He had it back to the dealer many times and was told that was normal operation.
Of course we would razz him about it witch didn’t help:o Like how can you call them anti-lock when they always lock:cool:

Anecdotally, I watched one of those “POLICE! STOP!” TV shows a few years ago that said accident investigators could still get useful information about the “shadows” left by ABS in full operation. However, it is also possible that newer systems have eliminated such shadows.