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Vertical acceleration due to car going over speed bump

  1. May 14, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the vertical acceleration of a car going over a speed bump a foot wide and 4 inches tall.


    2. Relevant equations
    Acceleration = dv/dt


    3. The attempt at a solution
    http://imgur.com/55t2WCQ


    I am getting an answer that is around 25*g
    I know this is way too much.
    neglecting the shocks on the car, what assumptions am I messing up that would allow me to do this problem correctly? This isn't homework, rather a small part of a larger project where I need the forces to do failure analysis.
     
    Last edited: May 14, 2013
  2. jcsd
  3. May 14, 2013 #2

    SammyS

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    attachment.php?attachmentid=58711&stc=1&d=1368514163.jpg
    Was that 20 mph speed given ?

    Was anything else given, which you have not included here?
     

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  4. May 14, 2013 #3
    Nothing was given other than the thing I am designing should be able to withstand the force of going over a speed bump. I originally had it at 30mph but my number was way higher so I lowered it hoping that would be my problem. I looked up online the higher end dimensions of a speed bump and got a smaller width of 1 foot and a height of no more than 4 inches. Thanks for the reply.
     
  5. May 14, 2013 #4

    CWatters

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    Last edited: May 14, 2013
  6. May 14, 2013 #5
    Yes, but speed bumps and speed humps are different. Also I was assuming I should use the bump where the force would be greatest. maybe I was going too high with 4'' but http://www.innoplast.com/sbs6.html was closer to what I was imagining going over.
     
  7. May 14, 2013 #6
    r = (4² + ¼*12²)/2*4 =6.5
    Yes, but I see naught where you are going with this?
     
  8. May 14, 2013 #7

    CWatters

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  9. May 14, 2013 #8

    CWatters

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    I was going to work out the acceleration another way but the small radius caught my eye. If curved as per the original drawing it would have very steep sides - perhaps not as bad as a vertical kerb but still.
     
  10. May 14, 2013 #9

    CWatters

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    Article here suggests humps are typically designed to reduce speeds to 15mph where as bumps target around 5mph.

    http://www.dmgov.org/Departments/Engineering/PDF/FAQ9_Speed_Bumps_and_Humps.pdf

     
  11. May 14, 2013 #10
    Thanks. This has been helpful.
     
  12. May 14, 2013 #11
    attachment.php?attachmentid=58728&stc=1&d=1368554212.jpg
    Fixed it and I got 5.4g as my maximum acceleration vertical. Thanks again.
     

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  13. May 14, 2013 #12

    haruspex

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    Sure, but acceleration of what? The tyres, certainly, but a bit less for the wheel as a whole and much less for the vehicle as a whole.
    Also, passenger discomfort is not just a matter of peak acceleration. It's a compound oscillator problem. With a short bump, a low speed can produce a much larger amplitude rise in the vehicle than a higher speed.
     
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