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This guy thinks 1.09g for Model S is suspicious because, um, physics

Discussion in 'Model S: Driving Dynamics' started by evp, Aug 25, 2016.

  1. evp

    evp Nerd

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  2. RDoc

    RDoc S85D

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  3. Alketi

    Alketi Member

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    Journalism is dead.
     
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  4. SageBrush

    SageBrush Active Member

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    #4 SageBrush, Aug 25, 2016
    Last edited: Aug 25, 2016
    The author is a Prof of Physics -- give him the benefit of the doubt and read what he says.

    He is pointing out that high performance cars are likely to reach an acceleration limit based on tyres+roads rather than motive force. One thing I am unsure about though, is whether static friction really is greater than dynamic for this use case. Tyres get hot. He could well be right that take-off is under one G in a stock Tesla on regular roads.

    As for actual values, our cars with anti-skid controls provide a fairly simple way to estimate max 0-60: just time how long it takes to go from 60-0 when the brakes are slammed. One G is about 61 meters.
     
  5. whttiger25

    whttiger25 Member

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    3.871 seconds at 199mph HOLY CRAP.

    Anyway it's pretty simple, the ability of the tire to distort under force (both in that the shape of hte tire does not remain perectly round under force but molds to the road and that the tire tread "molds" to the shape of the pavement at a smaller scale) allows greater traction than its pure coefficient of friction would imply

    NEXT
     
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  6. McRat

    McRat Active Member

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    Three things the essay forgot:

    With modern performance tires, even on unprepped tracks, the coefficient of friction is greater than 1.000. The rubber adheres to the surface, especially a surface designed for traction like modern roadways.

    Weight transfer on launch makes the car weigh more. Picture it this. As you rotate the CG upwards about the rear contact patch, this takes tremendous force. So you have the force of gravity, AND the force of trying pull the front tires into the air.

    Third, cars can generate downforce. A lot of it. This isn't a Tesla thing, but at full scream a track car can double it's effective weight via downforce.

    At a dragstrip, it takes a 1.91 second "sixty foot" time to generate 1.0g IIRC. There are LOTS of cars that will do that. My pickup truck has run 1.56s in 4x4. None of our Vettes are slower than 1.75s. The Supergas car I licensed in was 1.3's?

    But, at neither road track or a dragstrip is a Tesla comparable to other modern stock high performance cars. High 10's are old news now. IIRC, the 2006 Z06 (2wd) ran high 10's at a touch under a buck 30, which was 10 years ago. Note, this was an experienced driver, the mag guys are all sort of clueless when forced to use a clutch pedal.
     
  7. chillaban

    chillaban Member

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    With all due respect, he is actually an associate professor of physics at a school that describes itself as "a small, independent liberal arts college committed to integrating the humanities and social sciences with science and engineering in new and ..."

    The "science" presented is little more than a grade school quality Newtonian diagram of the theoretical forces associated with an ideal cylinder rolling on an ideal surface.
     
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  8. SageBrush

    SageBrush Active Member

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    When is the last time you competed for a position like that ?

    Heck, go for broke: tell us your credentials and education. It better be a PhD in physics at the very least.

    And keep in mind that this is an article in Forbes. How much math are you expecting ? LOL
    It is obvious that the article is written precisely to encourage people to apply a little HS newtonian physics to a daily experience.
     
  9. whttiger25

    whttiger25 Member

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    Well.....It was also written to vaguely attempt to discredit Tesla's claim of a 2.5 second 0-60 time.
     
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  10. jimmyjohn

    jimmyjohn Member

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    Static friction versus kinetic friction, no?
     
  11. SageBrush

    SageBrush Active Member

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    I read it as skepticism that a stock car can accelerate over one G on passenger car roads. I'll guess his skepticism is misplaced due to dynamic tyre friction. If you read his article you will see that his reasoning is not directed at Tesla specifically at all; he started from a table of static tyre frictions.
     
  12. daxz

    daxz Member

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    #12 daxz, Aug 25, 2016
    Last edited: Aug 25, 2016
    The Absolute Guide to Racing Tires - Part 1 - Lateral Force
    Many production cars can beat 1g in skid pads.


    Friction - Wikipedia, the free encyclopedia

    my relevant credentials: Google searcher, ex-amateur race car pit crew, mechanic, Bachelors in Physics, Mathematics & Comp. Sc. , Master in Comp. Sc.
     
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  13. SageBrush

    SageBrush Active Member

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    Edmunds has some Model S data. If the P100D tyres are the same these values hold

    200 foot diameter skidplate measured at 0.9 G
    60-0 stopping 102 feet, equal to 1.13 G average stopping force
     
  14. RDoc

    RDoc S85D

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    Give me a break. I don't have a PhD in anything, but do know how to read a timing sheet.
    One of the hallmarks of science, as opposed to things like faith, is that reality is the final arbiter.
     
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  15. whttiger25

    whttiger25 Member

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    Agreed. If your theory doesn't match real observations then your theory is wrong. If every single possible way you could think of to test your theory checks out, your theory MAY be correct
     
  16. SageBrush

    SageBrush Active Member

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    Read the article
    Critique the analysis if you are capable

    Then you might deserve a break
     
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  17. Canuck

    Canuck Active Member

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    I'm no scientist - far from it. But I have been reading interesting information on quantum physics lately and from what I have learned both of the statements above are wrong. Quantum physics tells us that reality is far beyond human perception and intuition. In other words, our rational mind and common sense are just not capable of understanding the true nature of reality. So experiments or theories that don't match real world observations can, and have, been proven correct, and reality is not the final arbitrator (not that faith is either) but our perception of reality can be an illusion in relation to certain experiments. In any event, I've probably just mangled trying to explain quantum physics but I'm pretty sure those in the know would take issue with the statements above.
     
  18. S'toon

    S'toon Knows where his towel is

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    Incorrect. You're mixing up the colloquial use of the word "theory" with the scientific use of the word.

    In every day common usage the word theory means "a wild guess."

    The scientific process doesn't work like that. First a scientist comes up with a hypothesis. Then the hypothesis is tested by means of experiment or observation. If the experiment is repeatable,or observational evidence confirmation is found, then the hypothesis becomes a scientific theory.

    My explanation is kind of muddled, so here:
    Scientific theory - Wikipedia, the free encyclopedia
     
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  19. whttiger25

    whttiger25 Member

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    You guy are all right if you want to get into really deep thought about quantum mechanics and I'm sorry for mixing up the word theory and hypothesis and overly simplifying the scientific method...I get it, but really we are just talking about a much more macro thing here, and that is that the coefficient of friction of rubber and asphalt, say it is 0.9, does not due to REAL observations of acceleration, explain the actual maximum static friction of a tire and asphalt for a rotating, inflated tire. That's all!
     
  20. RDoc

    RDoc S85D

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    Quantum theory, in fact is backed up by a very large number of very exact observations of ... wait for it... REALITY!
    The fact that we haven't yet come up with anything like an intuitive model that matches what quantum theory doesn't mean that it's not based on experimental evidence, in fact, things like the double slit experiment were exactly what lead to it.

    One of the main problems many people have with string theory is that there is precious little if any experimental evidence for it.

    No, he is exactly correct. That's the whole point of science. It doesn't ever claim to know the truth, it only claims to have models based on observations. That's what theories are. However, all theories are subject to modification or complete trashing if a single observation is made that is both confirmed and isn't explainable by the theory.
     
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