Anti-Lock Brake question

Was driving in icy weather yesterday and had the first bad experience with
the ABS system. I have gotten so accustomed to just putting my foot on the
brake and having the ABS doing all the work when required. Thankfully there
were no other cars arround me as I tried to slow for a turn. I stepped on
the brake and there was a solid feel to it, no ABS action and the car kept
going straight. I assume that all four wheels locked up and with no way for
the vehicle to know that it was still moving, the ABS never engaged.

Has anybody ever experienced that and is there a way to overcome it? I would
assume that the answer is to go back to pumping the brakes when you get the
solid feel of the brake pedal or just to use a lighter touch on the pedal to
avoid four-wheel lock up.

Thanks for opinions
Oppie

 

ABS can't creat traction where none exists. On glare ice, ABS isn't
going to help.

Ed

 

Very true on that but once the wheel stops spinning and becomes a plow, it
has actually less tractive force. Guess the next time I drive in icy
conditions, I'll just tread *much* more lightly on the brake.
I got a bit cocky (always a dangerous thing) when I came up a long hill with
no problems. In retrospect, the hill had been sanded and the sanding ended
somewhat before my attempted turn.
Oppie

 

ABS can't creat traction where none exists. On glare ice, ABS isn't
going to help.

It most certainly will helpl. ABS may not give you much more traction in
that situation, but it will do a lot more for maintaining steering control
than locking up the brakes would - and that's big in my book. I have ABS on
both my SC & SC2, and have felt it in action quite a bit during WI winters
for the last 12 years.

Oppie - it sounds like either of 2 things may have happened.

* The ABS engaged, but could not do much because of conditions, and the
turning wheels didn't have enough traction on the ice to actually turn the
car.

* The ABS did not engage due to a problem with the braking system.

Usually there is an audible sound when the ABS engages that most people with
it have heard and should be able to recognize (I've heard people refer to it
as 'marbles rolling around in a coffee can' or 'a slurping sound'. Did you
hear it?

If you did not hear that sound, or the ABS or Brake idiot light came on,
then there is likely a problem. Get it diagnosed and fixed, rather than
compensating for it with behavior you'd use in a non-ABS car (pumping the
pedal).

Good luck,
Lane [ lane (at) evilplastic.com ]

 

I was more interested at the time in avoiding other cars than looking at my
dash indicators <grin>.
ABS definitely did not engage so I assumed that all wheels must have locked
up and gave no velocity information to the controller. We are expecting
more snow this week and I'll take the car to a parking lot and experiment a
bit. The ABS/ traction control function perfectly on wet or uneven pavement
where you get one or more wheels slipping. Just trying to learn
capabilities. To quote Clint Eastwood in Dirty Harry, "A man's got to know
his limitations."

Oppie

 

I live at the other end of the country, but we do have heavy rains and mud,
causing the ABS to function. I often wondered if it would be effective to
have one "aggressive tread" tire which might work as a baseline for the ABS
under conditions like this. True, its not really practical, but just a
thought ....

 

Interesting thought. The front tires should be matched so possibly on the
rear put the odd tire. Would make tire rotation a bit confused though.
I was thinking that they might eventually add a sonar/radar type doppler
speed meter to detect actual vehicle speed. Would need ones front and rear
that read out in two axes each to show when vehicle is skidding not just
straight but at an angle (yaw). Is it worth the expense? Maybe on a high end
vehicle with stability control but certainly not on any vehicle that I'll
ever own.

 

Why go to that expense and complexity?

It would seem to me that the computer's program should be able to discern
the difference between 4 wheels locked and sliding forward and a vehicle
that is actually at rest. If the vehicle's speed is greater than some
threshold, say 15MPH, and then within some time threshold, say less than
200ms, goes to zero, and if the brake pedal is depressed/engaged, then it
should be fairly simple to assume the 4 wheel locked condition and activate
the ABS.

Bob

 

You're a genius. Change in sensed wheel speed with respect to time above a
threshold *would* be a good indicator of wheel lock. The threshold could be
made adaptable by throwing in the acceleration sensed by the airbag
controller. Don't know if the accelerometer output can be read by the ABS
system in real time though.
Oppie

 

Thanks for the title, but it just seems like the easiest way ... after all,
to activate ABS, you would think there is already some type of connection to
the Vehicle Speed Sensor output and a minimum speed threshold. Personally,
I'd be surprised if they are not doing this already. You indicated you plan
to test your system when you get some snow/ice, so let us know since it is
possible that your vehicle is not working as designed.

It also might be some type of bug that you discovered in the program that is
unique to that particular vehicle or year, make and model.

Bob