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  1. #1
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    Yes | No

    "unintended acceleration"


    ok this is not an e36 question or even a BMW question, but a friend of a friend recently had an unintended acceleration experience with her car, a modern VW of some kind (2002-ish jetta I think). This was an automatic transmission. Apparently she ultimately stopped the car by slamming it into park, after trying neutral unsuccessfully.

    I always thought that the brakes were capable of stopping the car against the power of the engine, even at WOT. I've witnessed that capability, but not at highways speeds, so I can't really say for sure what the brakes could or could not do in that situation.

    Could the engine really overpower the brakes at highway speed? Assuming we're not talking about an especially high HP vehicle. 1996 BMW 328i
    2003 BMW F650CS - R.I.P. 8/25/2004
    2005 Suzuki SV650S - 3/26/2005


  2. #2
    Steve G
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    Yes | No

    Absolutely impossible without brake failure.

    The brakes are many, many times more powerful than the engine.

    At highway speeds the transmission is in a higher gear and the engine puts less torque to the wheels (not more).

    If her foot was on the brake the car would have stopped. The problem is her foot was somewhere else, and probably on the accelerator. In the panic she didn't realize what was happening. She pressed harder, thinking she was on the brake.

    I'm not being critical of your friend. I had a similar experience once. I was wearing big winter boots and driving a small car that I wasn't used to. It was an automatic. At one point I was pressing the accelerator and brake pedal at the same time. The engine revved up, but fortunately I had enough force on the brake to stop the car and I turn the key off. Then I had time to realize what just happened.


  3. #3
    wicked94pgt
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    Yes | No

    Thanks that's what I thought. Some other ideas...


    Feel free to be critical - I don't actually know this girl, and I doubt anyone here knows her either. I was sort of the guy being critical when my friend told me the story. Not really critical, just trying to figure it out.

    As far as speed goes, the engine puts less torque in higher gear, but aren't the brakes less effective? Or at least feel less effective? And could brake fade come into play?

    One theory I have is people push and the brake and think it's not working because they're going fast and the engine is revving against them. So then maybe they start panicking and trying other stuff (like neutral and park), or just stabbing at everything and hitting the accelerator like you said.

    Or, could they use normal braking force and experience brake fade before they try enough force to really slow down? Wouldn't the brakes still stop the car even under fade? At what speed could fade typically occur with standard factory street brakes? Could it even happen at ~60 mph? A little fade could also just trigger the same panicking condition that leads to stabbing the accelerator, even if the brakes could still have stopped the car.


  4. #4
    Steve G
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    Yes | No

    Brake fade

    Brakes can stop 3000 pounds of car in a few seconds, easily. That's many times more energy than the engine can deliver in the same length of time. I don't think brake fade is a factor.


  5. #5
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    Yes | No

    What if they don't apply enough force soon enough?


    Push, brakes, brakes feel like they're not working, heat up and fade by the time they really get on them hard.

    I don't even believe that theory myself, just trying to rule out anything I can think of. I have to think people would push as hard as they can pretty much immediately, even after finding that they feel like they're not working.

    It has to be like you said, panic and push the wrong pedal, or both pedals.

    I don't generally have a lot of sympathy for large corporations, but I hate waste, and I think a lot of effort was likely wasted in the wake of the hype over Toyota's alleged problems. NHTSA concluded driver error after months of investigation.

    1996 BMW 328i
    2003 BMW F650CS - R.I.P. 8/25/2004
    2005 Suzuki SV650S - 3/26/2005

  6. #6
    Steve G
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    Yes | No

    Maybe possible, but I think it would take a driver

    delibrately "riding" the brakes with just the right pressure so the car doesn't slow down too much. And it would take a long time, maybe tens of seconds, or minutes. It makes no sense when you can stop the car in seconds with the same brakes.

    I don't doubt unintended acceleration happens. The thottle can get stuck open, computer malfunction, etc. But it's a one in millions possiblity. What are the odds of brake failure happening at the very same time? That's what I don't believe.


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