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Frictional accelerations greater than one G

  1. Mar 30, 2010 #1
    I see it all the time; engine powered [not jet] dragsters getting EASILY over 1.5 - 2.0 G's, and then sometimes MORE.

    And their most rapid accel is in the first part of the race. I know they can increase the downward force on their tires because of wind pushing down on them, but at the beginning the gain is zero to negligible.

    [my kids would come home from High School and tell me these "outrageous" times and velocities for the dragsters and I flat out DIDN'T BELIEVE it [1972=1988]...then I saw it on TV with my own eyes, .......... and NOW it's even higher.

    I was taught [College Physics in 1955] that the greatest f [frictional] factor is 1.00000000 and thus greatest accel is 1.000000 G.

    I've got a couple of ideas/opinions, but can't find evidence that I'm right, and even if I AM, I can't see that much gain [from my ideas].

    What is the secret? People that don't even KNOW Physics are out there doing it RIGHT NOW............. WONDERING.

    thx,

    LarryR : )
     
  2. jcsd
  3. Mar 30, 2010 #2

    berkeman

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    I believe it's over 4g:

    http://hypertextbook.com/facts/2007/AnamAhmed.shtml

    I'm not sure of the math (and don't have time at the moment to figure it out), but keep in mind that the dragster's tires are spinning much faster than it is moving. This is likely the source of the extra capability to generate forward force. If the tires were just barely not slipping, then it does seem like the limit in acceleration might be 1g.
     
  4. Mar 30, 2010 #3
    but I was taught [1955 College Physics] that kinetic friction [spinning] is LESS then static friction [tires NOT spinning]................ and yes, I DID see the greater than 4.0 accel but was too shy to post that; it is almost beyond belief for me, ha.

    Opinion; I believe that it is BEYOND "friction"; I have an idea and have relayed it to a friend [for a witness] but don't wanna state it right now, as I value "cold thoughts" from ya'll more then thoughts that you get from MY thoughts, know what I mean? NOT being "secretive"........... I just wanna know; it's academic, but isn't that what we love?

    LarryR : )

    thx,

    LarryR : )
     
  5. Mar 30, 2010 #4
    if the had wheels spinning faster than the motion it would switch to kinetic friction and there would be less acceleration. I assume that they have the exhaust shooting upwards conserving momentum and pushing the car down against the ground. That would be the only plausible source for that kind of friction I think.
     
  6. Mar 30, 2010 #5
    They also could have wheels that create some sort of vacuum under them, im sure its possible.
     
  7. Mar 30, 2010 #6

    berkeman

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    I don't think the downforce from the exhaust or any vacuum effects at the tires are significant. I'm still thinking about it, but Figure 3 in this paper sure shows that the 4g acceleration of a top fuel dragster is near constant from the launch all the way down the track:

    http://www.inds.co.uk/education/ChrisButlin/Vernier%20LabQuest%20Investigating%20the%20motion%20of%20a%20model%20dragster.doc [Broken]

    .
     
    Last edited by a moderator: May 4, 2017
  8. Mar 30, 2010 #7
    Nice chart Figure 3............. but my old eyes cannot read the GREEN numbers; anybody's computer fine enough to read them? I think I got the other colors.

    thx,

    LarryR : )
     
    Last edited by a moderator: May 4, 2017
  9. Mar 30, 2010 #8

    berkeman

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    The green numbers (Hayabusa) are about the same as the red ones (600hp Escort). Here are the green numbers:

    Code (Text):

    MPH      Sec.
    0-60     3.13
    0-100    5.87
    0-150    11.46
    Max      182.1
     
    Last edited: Mar 30, 2010
  10. Mar 30, 2010 #9

    I did a calc on all the 9 pcs of data that I COULD read, and he accel DOES IN FACT DECREASE as the velocity increases [kinduv normal in my opiniion] ............ NOT approx constant as you had suspected.

    Good graph tho.

    LarryR : )
     
    Last edited by a moderator: May 4, 2017
  11. Mar 30, 2010 #10

    berkeman

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    The leftmost graph for the top fuel dragster shows almost constant acceleration, IMO. Air resistance is increasing with speed, but so is downforce from the rear wing.
     
  12. Mar 30, 2010 #11

    berkeman

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    Besides, the point is that the initial acceleration is basically the same as when there is more downforce available. That's the non-intuitive part that I believe is your original question.
     
  13. Mar 30, 2010 #12
    Thanks for doing that so quickly; so I made this chart and if I DID make errors, pls let me know. So yes, the first two DO keep their accel rate up, in fact it INCREASES during the trip, but the second two are more like I'd expect; decreasing accel with progression down the track.

    Published et's
    0 to 60 1.03 2.58 2.87 3.13
    0 to 100 1.72 3.87 6.18 5.87
    0 to 150 2.53 5.78 13.63 11.46

    G's for Figure 3; if errors, pls let me know
    0 to 60 2.6555 1.0601 0.9530 0.8738
    0 to 100 2.6503 1.1779 0.7376 0.7766
    0 to 150 2.7027 1.1830 0.5017 0.5967


    LarryR : )
     
    Last edited: Mar 30, 2010
  14. Mar 30, 2010 #13

    jack action

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    Frankly, I never heard of such a statement that friction coeff can't be greater than 1. I can confirm that even the friction coeff of today's tourism tires reach easily 0.95, high perf tire can go slightly higher then 1. Racing tire vary from 1.3 to 1.7. Dragster tires reach easily 3. One engineer from a tire company even told me about dragster tire that in the dynamic deformation at launching, the friction coeff could be as high as 5.

    Anyhow, I search on the web where this theory of µ < 1 came from and found nothing. But I found http://mathforum.org/library/drmath/view/51493.html", that might help you understand why it is possible to have friction coeff greater than 1.

    All I can think for this theory is that under its own weight, the maximum friction force produced cannot exceed the equivalent of the object's weight.

    But the force present at the tire contact patch can be A LOT more than the weight of the car.
     
    Last edited by a moderator: Apr 24, 2017
  15. Mar 30, 2010 #14
    qqqqqqq
    Yes, but only with SOMETHING adding the downward Force, right? like wind downward force on the slanted surface?
    qqqqqqq

    LarryR thanks ya'll........... does anybody else know about these tires that have f=5?
     
    Last edited by a moderator: Apr 24, 2017
  16. Mar 30, 2010 #15

    jack action

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    First, the dragster tires don't have a friction coeff of 5. It's just when the tire deforms itself at launching. It lasts for a fraction of a second and it won't really show on the car's performance. 3 is a more appropriate value (That's high enough isn't it?).

    The maximum acceleration of a car, in g's, is equal to the tire friction coeff (if it is a 4WD) or less (if it is a 2WD). If you add downforce, then the normal force increases, but not the weight of the car so the maximum acceleration will also increase (hence the 4 g's acceleration even if the dragster tire has f = 3).

    But downforce DOES NOT modify the friction coefficient.

    With that last statement, I was trying to find an explanation for where that theory of f < 1 came from. After re-reading it, it doesn't make sense. Probably because the statement f < 1 doesn't make sense either: Friction coeff greater than 1 have been measured (and not only car tires on asphalt), so it is possible!

    Just re-read Doctor Ian's answer in http://mathforum.org/library/drmath/view/51493.html" I gave you.
     
    Last edited by a moderator: Apr 24, 2017
  17. Mar 30, 2010 #16
    Well, you guys have helped a lot, and tho we don't agree on EVERYTHING, you provide VERY GOOD thought provoking stuff, and it gives me incentive to look even further, so you've got answers AND you're a catalyst. Thx.

    I'd love to see documentation on those tires with f=5 [I believe you, just wanns see more; because THAT is a major part of the answer to my "puzzlement"].

    But here is what I found 20 mins ago and I'll share it with you if you're interested. A 159 page paper prep'd by a student in 1998 that concentrated on dragsters, their tires AND their rear wing. He actually [to me it seems] concentrated on the wing more than the tires, and tho that IS of interest to me, I was wanting to learn about the "phenomena" that I was inquiring about; f.

    But here it is [it's been there all the time waiting for me to find it, and I've been puzzling on this since 1972-ish.

    www.caselab.okstate.edu/pubs/tmb_thesis.pdf

    Thanks again, and don't stop NOW........... if you've still got thoughts, I'm really anxious to hear them.

    LarryR : )
     
  18. Mar 30, 2010 #17
    qqqqqqqqqqqqqqqq
    My response:
    Oh yes, I was typing while you posted this, so it'll seem that my post "ignored" yours, not so, I just NOW found yours after writing mine one minute ago. Yes, that Dr Ians thing was helpful and I DID read it, yes. And yes, I acknowledged that f CAN be more than ONE, but I didn't learn that until yesterday evening; thus my puzzlement.

    I don't know if you saw it, buy MY College Physics was in 1955, and I'd LOVE to go back and see IF they said what I said, becuase I've kinda blamed it onto THEM, didn't I, huh?

    But this is 65 years later, and I'm still learning, and AGAIN, I thank ya'll, but don't stop now if you have more to say.

    LarryR : )
     
    Last edited by a moderator: Apr 24, 2017
  19. Mar 30, 2010 #18
    Perhaps you have already implied the answer...

    More explicitly, the tires expand at such a high rate (fully expanded within a fraction of a second I expect) that they act as springs accelerating the CofG of the wheels and eventually the car upward.

    Accelerating your car upward from a standstill by spinning your tires will increase the normal force on the contact patch, making it appear as though the coefficient of friction is much higher than it is.

    Therefore it is advantageous to spin up your tires quickly, and maximize the diameter difference (change in potential wheel centre height) to maximize the normal force (effective car weight).

    Obviously drag racing isn't only about horizontal acceleration.
     
  20. Mar 30, 2010 #19
    qqqqqqqqqqqq
    For a very SHORT time indeed; yes, but the normal force you refer to is for, as you say, a "fraction of a second", so the moment the dia stops changing, now we don't HAVE that "additional normal force", as the rising axle has STOPPED accelerating upwards; no acceleration upwards, no force downwards.

    And your LAST sentence, I certainly agree with wholeheartedly.

    LarryR : )
     
  21. Mar 30, 2010 #20
    Time for vertical accel must be greater than the time of wheel spin up. Chassis mass must also be accelerated.
     
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