Q: Auto Emergency Braking The Physics Behind It | FerrariChat
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Q: Auto Emergency Braking The Physics Behind It

Discussion in 'Technical Q&A' started by DoubleD33, Dec 13, 2017.

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  1. DoubleD33

    DoubleD33 F1 Rookie
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    #1 DoubleD33, Dec 13, 2017
    I was watching a you tube video on a Porsche Cayenne Crash test and at the end they demonstrate the AEB feature.

    This vehicle seems to stop amazingly fast and made me question how the vehicle does not go into a full skid and can achieve a stop that quick.

    In reading on how the system works it seems to pre-charge the brake system and make the pedal ultra sensitive and throw a lot of power to the brakes, but I cant see how just this action makes it stop like this.

    The tires only have so much surface area with the road and I would imagine that the coefficient of friction changes by road type (Wet, Dry, Concrete, asphalt...etc...) and even tire type...So where is the starting point for applying enough but not too much braking power?

    I find this system interesting but I am at a loss on how to understand the behind the scenes of how it works.

    Are these systems just maximizing the amount of force that can be applied with the hydraulic brake system with out breaking the tires loose?

    Are these systems using other factors like weight distribution via suspension systems?
    If they are using transmission power to brake does that allow for more braking power with out putting a wheel into a skid?

    If a wheel goes into a skid while in AEB mode how does the system recover as in can it bleed off pressure on only the wheel that locks up in milliseconds and still perform a quick stop? I feel if this system went into "dump" or "flutter" mode like most ABS systems you would not stop that quickly.
     
  2. Mitch Alsup

    Mitch Alsup F1 Veteran

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    #2 Mitch Alsup, Dec 13, 2017
    The vehicle was braking at the limit of traction, you can both see and hear the tires ABS cycling between incipient lockup and rolling.
     
  3. DoubleD33

    DoubleD33 F1 Rookie
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    #3 DoubleD33, Dec 13, 2017
    Since the vehicle is braking at the limit of traction....

    Is it safe to say that the “magic” behind this system is to reduce the amount of time it would take for a normal system to build up enough line pressure to activate the ABS?

    As an aside if you have the ability to brake at the limit of traction by modulating pressure at each wheel individually does weight distribution (nose dive) make a difference in stopping distance?
     
  4. smg2

    smg2 F1 World Champ
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    #4 smg2, Dec 13, 2017
    reaction time, lets say you're going 50 MPH, that's 73.3 ft/sec avg human visual reaction time is 0.25sec, that means on avg someone will have traveled 18.3 ft before reacting and braking. Fast as we think we are, we are ponderously slow compared to a computer, smarter maybe but really really slow. If the system is in a pre-charged state then the vehicles feedback loop will be leagues ahead in reacting.
    This does introduce another issue, for the vehicles not equipped with such equipment and peoples love of licking your bumper in traffic I imagine it will result in getting hit from behind then driven into the object you were trying to avoid. Ever try and drive with a safe gap in traffic, in LA, OC or any big city... good luck with that, might as well drive back words.
     
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  5. smg2

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    #5 smg2, Dec 13, 2017
    Yes, coefficient of friction and contact patch area are the factors. Friction is based on many parameters and since force is a big one, the force due to weight and gravity plays a role. weight shift off the rear tires will result in a change. No different then drag launching, the sign of the vector changes but the equations to figure it are the same. In a drag launch we want to weight shift back to optimize the 'grip' to road, same for stopping, weight shift forward will help in stopping, but not too much as the goal is 4 wheel braking. clear as mud right?o_O
     
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  6. Mitch Alsup

    Mitch Alsup F1 Veteran

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    #6 Mitch Alsup, Dec 13, 2017
    The amount of pressure in the caliper ends up being dependent on the amount of weight on the contact patch.
    What this means, is that you don't slam on the brakes, or you will stop the tire before the weight has transferred forward, causing a skid.
    What you want if for the front end of the car to drop at a controlled fast rate, transferring the weight as fast as the suspension can manage, increasing brake pressure as the nose drops. It ends up this controlled rate of pressure increase looks surprisingly like a inverted COS(x) waveform.
     
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  7. Solid State

    Solid State F1 World Champ
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    #7 Solid State, Dec 29, 2017
    I'm mesmerized by the little dude on the surf board.
     

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