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Insane / Ludicrous launches and third row seating

Discussion in 'Model S: Driving Dynamics' started by mgboyes, Aug 16, 2015.

  1. mgboyes

    mgboyes Member

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    #1 mgboyes, Aug 16, 2015
    Last edited: Aug 16, 2015
    Hi all,

    Couldn't find a thread on this but sorry if I just missed it.

    I'm due a P90D in October, and have of course read every review and watched every launch video going.

    Lots of people say "warn passengers to press their heads against the headrest" or similar, before you do a full on 0-60 launch.

    So my question is "can you do an insane/ludicrous launch with kids in the third row?"

    Their heads would be pulled away from the seat, not pressed into it.

    My two (age 4 and 6) spend most of the time in the back of our P85 shouting "put your foot down!" and "faster daddy, faster!", whether they're in row 2 or 3, but obviously I don't want to do anything that might injur them or anyone else's kids who we might be transporting!
     
  2. Denarius

    Denarius Active Member

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    #2 Denarius, Aug 16, 2015
    Last edited: Aug 16, 2015
    I don't do insane launches with anyone in the rear faces seats. I hurt a friend's neck and he was in a regular seat. Because of that I always warn people to put their head back now.
     
  3. sorka

    sorka Active Member

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    They should just prevent the third row option from being selected with L mode. Can you say retinal detachment :)
     
  4. Thud

    Thud Member

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    It could cause some injury I think! The experience would be like pulling the rip cord on a parachute.
     
  5. jpet

    jpet Jan P.

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    I'm surprised that Tesla even allows ordering a P85D or P90D with the third row seats option. My advise is don't do launches or even hard accelerations with small children in the 3rd row. No problem with children in the 2nd row if you warn them beforehand. My 5 year old daughter loves the launches but I would never push the pedal to the metal without advance warning.
     
  6. ProSkeptic

    ProSkeptic Member

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    You both realize that the acceleration is traction limited to a hair over 1 g, right?

    In other words, the acceleration would only be marginally more severe than tilting one's face toward the floor.
     
  7. cgiGuy

    cgiGuy Member

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    No physics degree here, but what you're describing is night and day from the reality I experienced during a P85D ride. Not sure if I could have even removed my head from the headrest during the first couple seconds. I, however, have no problem lifting my head after "tilting my face toward the floor".
     
  8. mgboyes

    mgboyes Member

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    #8 mgboyes, Aug 17, 2015
    Last edited: Aug 17, 2015
    Erm, no.

    Gravity as a force acts directly on each atom of your body. When you fall off a cliff (which is the correct analogy for acclerating at 1g, not tilting your face to the floor), there is almost no strain or stress on any part of your body because every part of it accelerates at the same rate, in the same direction, in perfect unison. Your bones and your skin and your blood all move as one so there are no forces between different parts of your body.

    When you are in a rollercoaster or ludicrous electric car, you are pushed from behind by the seat, which applies a force to your back, which in turn pushes your spine and ribcage forward. But all your internal organs stay where they are so they're squeezed backwards until they catch up. Your head rolls backwards and puts enormous strain on your neck which is connected at one end to a moving torso and at the other end to a stationary head. Your blood all pools at the back of your body, and is pulled away from your eyes and frontal brain which can lead to vision loss or blurring. Your eyes are distorted from their usual shape and squash slightly flat. Etc etc etc.

    It is completely different.
     
  9. ProSkeptic

    ProSkeptic Member

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    Erm, yes.

    You realize that gravitational acceleration and motion aren't the same thing, right? When you're being pushed forward at 1 g (such as by a car or an amusement park ride) you're simultaneously being pulled toward the center of the Earth at 1 g, for a net total force on your body of 1.4 g at 45 degrees down and back. The horizontal component of that acceleration is exactly the same as what you experience lying flat on your back - you're just perceiving a greater acceleration since you're not used to experiencing 1 g forward accleration in a car.
     
  10. mgboyes

    mgboyes Member

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    #10 mgboyes, Aug 17, 2015
    Last edited: Aug 17, 2015
    You're muddling force and acceleration. And missing some forces from your example. And confusing the directions the forces act in.

    Allowing for the common simplification that the surface of the earth is an inertial reference frame (which I realise in fact it is not, but this is not an orbital mechanics question so let's go with it) then while gravity exerts a downward force on you, equally the floor (or whatever you're standing on) exerts an equal upward force on you. The net result is 0 acceleration [in the reference frame of the surface of the earth]. The same is true of your eyeball - it is pulled down by gravity, and pushed up by your eye socket (which is pushed up by the rest of your head, and then your neck, and skeleton, and socks, and shoes etc).

    In your example of the accelerating car, there are 3 forces - the car tyres pushing horizontally against the road, gravity pulling downwards, and the road pushing back upwards to match. The net of these is a single force in the direction of travel. Also your directions are wrong - the force due to acceleration of the vehicle points forwards, not backwards. If it pointed backwards you'd move backwards, and if it pointed downwards at 45 degrees that's where you'd go.

    The reason you perceive that you are being pulled back into your seat when in an accelerating car is precisely because of the way the force is applied to you through the seat. The seat pushes forwards, which pushes the rear of your body forwards, which due to inertia compresses your body so that on average your body moves nearer to the seat, which you perceive inside the cabin to be "moving backwards".

    Now you talked about "severity" of acceleration. "severity" is not an SI unit, so I assumed you were really talking about how the forces acting between components of your body are perceived (i.e. how you "feel" them).

    As I already said, if you accelerate 1g vertically due to gravity [again, static inertial reference frame at earth's surface] such as when falling off a cliff then there is no "severity" because the force that acts to accelerate you towards the planet acts uniformly and equally across your whole body - the force between your eyeball and cranium is 0; between your head and neck is 0, between your liver and intestines i 0, etc etc. You don't "feel" anything at all.

    However if you sit in a car and accelerate at 1g then you do so because the seat pushes your torso, which tends to leave your head, limbs, organs, and bodily fluids slightly behind. Which feels quite severe.

    Put it another way, if you believe that two forces each cable of accelerating your body at 1g, but applied via different methods, are equally "severe", then presumably you'd be OK with someone tying your little finger to the back of a P85D and doing an insane launch? No part of your body would ever accelerate at more than 1g so it would be fine, right?
     
  11. siucity

    siucity Button Pusher

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    Insane/Ludicrous launches would be a great experiment in a high school physics class.
     
  12. Trustno1

    Trustno1 Member

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    I cant really see the difference between launching with someone in the back facing seats, or do a hard break with someone in the forward facing seats. The forces expirienced by the body will be the same.

    I guess there is much more research to be found on hard breaking than backward facing seats in a >1g car.
     
  13. ProSkeptic

    ProSkeptic Member

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    If I am, in fact, muddling force and acceleration, then I apologize. I was attempting to explain in layman's terms what is happening during a launch, and I'm not always completely adept at such explanations.

    I never mentioned severity... I'm not sure where that arose, but I hope it wasn't your attempt at a strawman argument. Had I intended to mention the severity of any part of the launch, I would have mentioned that the jerk (the semi-techinical term for the third derivative of position with respect to time, or the rate of change of acceleration with respect to time) seems to be pretty high at launch. Perhaps this is the source of some of the sensation of severity.

    Regardless, the analogy of tying a finger to the back of the car is a very poor one - it would be the equivalent of attempting to support a person's weight by the tip of said finger, and obviously would result in injury.

    - - - Updated - - -

    Bingo.
     
  14. mgboyes

    mgboyes Member

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    An excellent point :)

    Looking around it seems that deceleration around 1g is fairly normal for full on dry weather emergency stopping, even in relatively mainstream cars, so in that sense it's reassuring to know that what the rear facing seat occupants would experience is something within the realms of "normal" and "safe". But equally it's not like you'd do repeated emergency stops in a car just for the fun of it...
     
  15. ProSkeptic

    ProSkeptic Member

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    Speak for yourself. I'm not afraid of acceleration, no matter which way its vector points.
     
  16. mgboyes

    mgboyes Member

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    :)
     
  17. AWDtsla

    AWDtsla Active Member

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    You've got it wrong. When you jump off a cliff you are in freefall, you feel no acceleration. However if you lay down, you are feeling the force of 1G of acceleration in the inverse direction of the gravitational field. A force that is indistinguishable from being strapped in a car acceleration at 1G. Last time I checked, humans tend to live no matter in which way they are oriented towards the earth.
     
  18. mgboyes

    mgboyes Member

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    True.

    False, unless you and the car were in freefall before it started its 0-60 launch, which typically is not the case.

    For a car on the ground you experience 1g horizontally and also 1g vertically, which is not equivalent to the force you experience by orientating your body in any way at the surface of the earth.
     
  19. AWDtsla

    AWDtsla Active Member

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    Well now you're getting technical. If you split the planes, you are in freefall in the XY plane, moving at close to 1000 mph when you car is "stopped". If you could free yourself of friction to the ground you would start moving according to the Coriolis effect. It's perfectly valid to consider effects in this plane while ignoring the Z axis.
     
  20. Thud

    Thud Member

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    Not quite that simple.

    Jerk (physics) - Wikipedia, the free encyclopedia

    It depends on how quickly the driver slams the accelerator down. A 3rd row passenger shouldn't have issues as long as the pedal is pressed slowly enough, giving the passenger's neck muscles time to adjust to the changing forces. But that kinda ruins some of the fun for the forward-facing passengers.
     

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