Analysis of unexpected "Low Traction" (ABS) with the "brakes out" feeling
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Thread: Analysis of unexpected "Low Traction" (ABS) with the "brakes out" feeling

  1. #1
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    Default Analysis of unexpected "Low Traction" (ABS) with the "brakes out" feeling

    Some of you have encountered the anti-lock brakes engaging (seeing the "Low Traction" message) in situations where you would not expect. It is scary, as if the brakes are not working, or that the car is accelerating (as it felt to me). A number of forum members have reported this on grooved pavement or going over potholes.

    The concern is that the ABS should not be coming on in such situations, and that it could be defective. If the car is indeed accelerating when it should be slowing down, it could easily cause an accident.

    Yesterday, it happened to me for the second time. I was going about 15MPH in a parking lot -- one I have traveled on dozens of times without incident, and the road was dry -- and it felt like the brakes went out and the car seemed to speed up; the "Low Traction" light appeared.

    Fortunately, I have been recording data from the OBD2 port for most trips since October, and had the data ready to analyze. Going back to the location, I saw that there was a storm drain that was slightly depressed in the road. Clearly, a tire went over it (while I happened to have my foot on the brake, so the ABS knew I wanted to go slower), and the wheel speed changed quickly compared to the other 3 wheels, causing the ABS to kick in.

    After looking at the data for a long time, I found the data for one of the wheels. Although all four wheels were slowing down (since I was braking), this one wheel was slowing down much more quickly (looking at a graph, it is quite obvious). The logs seem to show the ABS kicking in within 16ms after the car reported the tire speed.

    Although both times it felt like the car sped up when the ABS kicked in (and therefore like I was losing control of the car), the data makes it look like that was my imagination. The data shows that when the ABS kicked in, the car immediately started decelerating more quickly (which is good). There was only 1/10 of a second when the deceleration was less than before the car hit the storm drain, but combined with the immediate high deceleration, the car was stopping more quickly than I had intended. Despite the feeling of acceleration, the car did not speed up at all (assuming, of course, the data and my interpretation of it is accurate).

    Brake pedals on cars have "feedback", where the harder you push, the harder it becomes to push them (so you "know" that they are working). My guess is that when the ABS kicks in, the brake pedal loses its feedback (the data shows I pushed harder on the brake right after the ABS kicked in). So when the ABS kicks in, it is easier to push on the brake pedal, making it feel like the brakes are out (and I'm guessing that psychologically it also makes it seem as though the car is going faster).

    So from what I can see from the numbers, it looks like the ABS functioned as designed. Perhaps it is a bit too sensitive and should not have engaged, or perhaps it could have been designed to cause less fear in the driver (if I'm right that the brake pedal loses its feedback). But, there is nothing in the data suggesting the car actually went faster (at no point did it accelerate until after I came to a full stop a few seconds later). And perhaps that panic feeling as the brakes seem to go out may help you drive better under the circumstances.

    [EDIT: Added graph]
    Last edited by RScott; 06-28-2012 at 01:45 PM. Reason: Added graph
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  2. #2
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    Interesting. Thanks for sharing.

    Is there any change in regen happening when the ABS turns on?
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  3. #3
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    This is really interesting. I've never experienced this in the Volt but I have in another car when going over metal plates. It's very unnerving. Great to know that the Volt is in fact slowing down.

    You post seems to refer to a chart which isn't attached. Not sure if that is purposeful or not.

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  6. #4
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    tboult,

    I agree it could be a regen transition. The primary braking force could be comming from the electric motor regen but when the ABS engages it transitions to a traditional braking force from the discs?
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  7. #5
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    I've had this happen once when slowing down, going downhill, and the right front tire went over a manhole cover. My theory is that when you're slowing down normally (not a panic stop), the regen is contributing lets say 60% (guestimate) to the braking force, and the brake pads are doing the other 40%. When the ABS kicks in, the regen braking disengages and it gives the impression of an acceleration because 60% of the braking force just disappeared.

    Just my theory.

  8. #6
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    Quote Originally Posted by DonC View Post
    You post seems to refer to a chart which isn't attached. Not sure if that is purposeful or not.
    It was purposeful -- I was too lazy to add it. :) I've gone ahead and added it to the original post. The graph isn't pretty, but functional (it is also easier to read if you view the full image).
    Last edited by RScott; 06-28-2012 at 01:49 PM. Reason: Clarifying WHERE the graph was added!
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  9. #7
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    Quote Originally Posted by tboult View Post
    Is there any change in regen happening when the ABS turns on?
    Unfortunately, I do not know -- I haven't yet figured out what most of the braking-related ID's are for. :(
    B-2689
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  10. #8
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    I have experienced this. Thank you very much for posting this conclusive data.
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  11. #9
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    I have had "low traction" come on regularly when I go down a hill near my home at about 20mph with L selected. This happens before I apply any brakes at all.

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  13. #10
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    Quote Originally Posted by RScott View Post
    I've gone ahead and added it to the original post. The graph isn't pretty, but functional (it is also easier to read if you view the full image).
    Thanks for taking the time to post it. It's great and easy to see what you're describing.

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