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Curious brake fade transition

Discussion in 'Prime Main Forum (2017-2022)' started by bwilson4web, Jul 17, 2018.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    This is not a 'hair on fire' problem but sharing a curious braking anomaly. Following a car at ~50 mph (80 kph) using dynamic cruise control, a light changed and the leading car instead of coasting through the yellow came to a quick stop.

    My following car started to brake but not as fast as I wanted. I had noticed the braking force is often weak at a distance and becomes stronger as the distance closes. Higher speeds, 50 mph (80 kph) and above, seems to amplify this effect.

    Not caring for the nonlinear, automatic braking, I manually braked and there was a brief reduction in braking force. A little surprised, I pressed the pedal harder and there was a brief, fraction of a second, strange feel and transition noise but the car stopped quickly enough. It was almost as if the ABS was 'waking up.' There was never any loss of control as much as the system 'jiggled' in an unexpected way.

    Guess it is time to put the dash cam back in the car and see if I can record it someday.

    Bob Wilson
     
    #1 bwilson4web, Jul 17, 2018
  2. Trollbait

    Trollbait It's a D&D thing

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    I've noticed in other cars while using dumb cruise control that the accelerator seemed less responsive when depressed for passing.

    Probably something to do with the difference between where the the actual braking and throttle position is and the position of the physical pedals are while CC is on.
     
    #2 Trollbait, Jul 17, 2018
  3. SteveMucc

    SteveMucc Active Member

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    I've noticed what you are describing. I suspect it's due to some convergence algorithm. e.g. the initial breaking force from the DRCC is different than what you initially had when you pressed the break pedal. They probably don't want an immediate decrease in breaking force at this point (e.g. if the DRCC was at full force and you slammed on the break, there would be a period of time when the break pedal was at almost no force and they don't want to suddenly remove all breaking force). So I suspect there's some decay mechanism in place that drops the car from the initial force to what you're new input is via the pedal. What the decay factor is, however, is anyone's guess (may not even be linear). I didn't, however, notice any "noise" when this happened, just some slight unpredictability in the amount of force applied.

    Depending on how close you are you may also have had the additional input of the anti-collision system deciding to play with the amount of breaking force in order to keep you from hitting the leading vehicle. I have no idea at what distance that system kicks in and/or how smoothly it kicks in. For all I know it may be engaging quite frequently.

    And you may also have had an additional input from the echo guidance system if it was at a place that you've stopped often.

    never really though about how much effort must have been put into just hitting the breaks...
     
    #3 SteveMucc, Jul 18, 2018
    Trollbait likes this.
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