Why does ABS go crazy over sharp bumps?

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1,902
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cali
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abs has saved me many times. in my moms 99 villager i was on a turn and then traffic stopped hecka fast. so then i slammed the brakes and it pulsated and i can turn and control the car on that turn. in my accord which has no ABS when i turn on a sharp turn and slam on the brakes then the car would do one thing...go forward in the direction the car was going and it would be almost impossible to control the turn. i learned to counter this though by steering in harder and braking. hehe at least i dont fly off the rail! there are some cars that have bad abs imo. my friends chevey corsica has one sucky abs system. one day i disabled it cause i blew the fuse and she asked me to fix it. then i fixed it and test drove it and it didnt feel like abs...it was more like braking on and off at 1 second intervals. i felt i can control it better without the abs. i dont know if its possible to change out brake systems and not have the problem. i was thinking that abs is computer controlled. or maybe it just needs to be re-calibrated...or get hard stopping brakes. hehe i wish i can help but hopefully u will get it fixed.
 
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4,478
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Southern California
Since ABS cycling is a response to a pending or actual wheel lockup, I suspect that at the moment one or more wheels momentarily lost adhesion just enough after maximum compression and rebound began that it/they locked, thereby triggering the ABS cycling process unexpectedly into operation. Whether this is a defect of your particular car, the pads you use, or something else is one big question mark. Could be it's just a quirk (albeit a potentially dangerous one) of the current technology.
 
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229
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Louisville, KY
Good shocks will help a lot. My Silverado's ABS (same as a Sierra) is excellent unless the rear wheels hit a very sharp bump. A lot of it is due to the cheap factory shocks which lack proper rebound damping. [ April 09, 2003, 06:09 PM: Message edited by: RobZ71LM7 ]
 
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2,095
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IL
Nearly got into an accident the other day. Traffic came to a quick stop ahead of me, so I braked hard but the pavement was cracked and rising up creating a large and sharp bump which then caused my ABS to cycle like crazy, allowing almost no line pressure. Increasing pedal pressure was ineffective, ABS still pulsing even after passing over the bump and I was coming up fast to a stopped car. Luckily I have experienced this before and found the quickest solution is to jump completely off the brakes and then you can get back on them if there are no more bumps. I did this while swerving to the side. Luckily I was able to stop BARELY in time. Not a good feeling. I searched the web and found another example of this as well on the Edmunds site in a 99 Sierra Review:
quote:
"On Monday morning, as I descended the ramp to Santa Monica Boulevard, the Sierra traversed the spot where the fresh blacktop of the ramp met the old concrete of the service drive. A new, insignificant bump at this juncture sent the Sierra's ABS into spasms, nearly hurtling me headlong into a Toyota slowing to a stop in front of me."
I noticed it on the 2000 Jimmy we had as well. Haven't been under right conditions to test it on the GTP, but also experienced it on my Dad's '96 Grand Cherokee years ago. I tried to remove my ABS relay but I don't know if it is the car simply not designed to be non-ABS or my super high friction Hawke pads or my Yokohama tires but this alternative was far worse. Brakes locked up very quickly with little warning. The balance feels off too, probably relies on ABS. Any quick braking and I just get a quiet whooshhh from the front tires and a big cloud of smoke. Even slowly increasing pressure there was no warning, just all of a sudden I have smoke pouring out a front wheel well. So much for that idea. (yes I have driven many cars without ABS before) I doubt there is any solution short of replacing the entire braking system. I don't even know if it is possible to avoid it with current ABS tech?So I guess I am just wondering why.
 
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2,095
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IL
quote:
Originally posted by Ray H: ...I suspect that at the moment one or more wheels momentarily lost adhesion just enough after maximum compression and rebound began that it/they locked...
That is what I figured as well, but I couldn't figure why it continues well after all tires are in firm contact with the pavement.
 
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874
Location
Pacific NW
Jason, my first experience with this behavior was in a Jeep Grand Cherokee. While stopping, with the right sequence of bumps, it would cycle the pump and the pedal would fall away. ABSolutely hated it. I later learned to release the brakes briefly which seemed to reset it. Not comforting when closing on something. Changed axles & lost my tone rings and now ABS is but a memory. Actually stops much better, so long as I'm paying attention. [Smile] Edit: BTW, this isn't unusual depending on the ABS type, shocks, tires, inflation, etc and how they cause tires to temporarily leave the ground. [ April 10, 2003, 08:49 PM: Message edited by: OneQuartLow ]
 
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308
Location
Houston, TX
jason, based on your description of how the car acts without ABS, i'd say something is wrong. and the ABS is over-reacting to whatever is causing the unatural lockup tendency. many models also have a sort of ice mode, but again, something has to happen to make the abs brain think it can't maintain braking force. what type of car is this? (not a '98-up f-body, is it?) edit: just realized it's a sierra. i'd be looking for a caliper problem. -michael [ April 10, 2003, 08:38 PM: Message edited by: Michael SR ]
 
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