Went out for a little up & down the street today (loose powder, ice underneath) and was kind of surprised the ABS would simply lock & slide on ice.

Had to feather the brakes to get the to bite into the powder.

Was kind of disheartening.

 

ABS will only activate when it sees a wheel stopped while one or more are still rotating (indicating the vehicle is still moving).

On glare ice it would not be uncommon for all four wheels to lock-up (i.e.stop rotating) at the same time. At this point the ABS computer has no way to know the vehicle is still moving, and therefore there is no activation of ABS.

I also understand that ABS will not operate at speeds below 5-7 MPH (approx). At least that is true on some brands, although I don't know if it is true on all brands. At those speeds the locked-up wheel will remain that way.

 

AH, good point.

You'd think it would somehow tie into some motion sensors like they do with cell phone.

Some bank of accelerometers and gyroscopes hidden in the dash.

 

The anti-lock brake system is locking up your brakes?

 

Have you ever been in the situation in which you are applying the brakes to slow down when you hit a patch of ice?. It actually feels like you are accelerating.

The reason is, that while your brakes are slowing you down, you are in fact decelerating (negative acceleration), which can be measured by an accelerometer. When you hit the icy patch you "increase" from negative acceleration to zero acceleration/deceleration (or at least a very low, almost undetectable negative acceleration). So in fact your acceleration rate did increase and the feeling you experienced was real.

This very small negative acceleration (while sliding)would be too small for the vehicles accelerometer to measure. The reading would be no different than if the vehicle was stopped or cruising at a steady speed. A gyroscope would only sense a change of direction.

Remember that the purpose of ABS is not to shorten stopping distances but rather to keep the tires rolling so that you have the ability to steer around obstacles. In the case of glare ice your steering would still be non existent even if ABS did pulse the brakes.

 

The ABS system will never lock up the brakes. The hydraulic pressure in the brake lines, locks up the brakes.

The ABS system will cut the hydraulic pressure to a wheel if it senses that the wheel has locked up prematurely.

Example: You are applying the brakes when the tires on the right side hit a patch of ice, while the tires on the left side are on dry pavement. Since all 4 wheels have the same hydraulic pressure being applied ( forget about the front/rear bias for this example) , the wheels on the right side will lock up early while the left sided wheels will continue to turn. In the terms of ABS, the wheels are rotating at different speeds.

When the ABS senses this difference in wheel rotation it will cut the pressure to the slower or non rotating wheel, and allow it to roll. It then allows the hydraulic pressure to be applied once again, which locks up the wheel again and the cycle repeats.

In the OP's situation, all four wheels locked up at the same time. With all wheels turning at the same speed, the ABS computer did not see a problem and did not react to the situation.