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Automobile shock absorbers

  1. Jun 27, 2008 #1
    I'd like to open a discussion on the ins and outs of shock absorbers on cars. The ability of a device to restrict the extension of the suspension springs works miracles that are not always appreciated.
     
  2. jcsd
  3. Jun 27, 2008 #2

    stewartcs

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  4. Jun 28, 2008 #3

    brewnog

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    The damper's purpose isn't to restrict the extension of the springs; its purpose is to prevent multiple oscillations following an impulse.
     
  5. Jun 28, 2008 #4
    I think this is more complicated than mere damping. A shock absorber will generally not impede the compression of the spring to any great degree. On the other hand, once the spring is compressed, the shock absorber resists any extension of the spring. The effect is to isolate the car or truck from the road. If a shock were just a damper operating in both directions equally, then the ride would suffer.
     
  6. Jun 28, 2008 #5

    Integral

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    Seems to me that the effect you describe IS damping. Who said that it has to operate equally in both directions?
     
  7. Jun 28, 2008 #6
    It is simply dampening. That's why in the automotive industry they're also referred to as 'dampeners'.
     
  8. Jun 29, 2008 #7

    Mech_Engineer

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    What you're describing is a variable-rate dampener, where the compression side his much less dampening than the extension side. Shocks do not have a fundamental quality that limits their dampening in one direction.
     
  9. Jun 29, 2008 #8

    FredGarvin

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    A shock absorber is just that...an absorber. In mathematics it is modeled as a dashpot which is a viscous dampener. The absorber works the same in both directions. It is a hole that a viscous fluid can pass through only so quickly.
     
  10. Jun 29, 2008 #9
    In a sense, I can understand what the OP implies. Modern high-performance suspension dampers use some pretty intricate speed-sensitive valving.

    If you want an example, I can probably find an exploded-view of a motocross bike's fork assembly.
     
  11. Jun 29, 2008 #10
    I think I said it was more than damping alone.
     
  12. Jun 29, 2008 #11
    Clarify please. What is meant by fundamental quality? Does this mean they do or they don't have symmetrical damping?
     
  13. Jun 29, 2008 #12
    Fred, you seem to be saying that the shock exhibits equal resistance to both compression and extension. That is what you are saying?
     
  14. Jun 29, 2008 #13
    pantaz, What, please, is OP? Are you speaking of an over the counter shock or something specialized?
     
  15. Jun 29, 2008 #14

    stewartcs

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    OP generally means Original Poster (that's you) or the Original Post.
     
  16. Jun 30, 2008 #15

    Mech_Engineer

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    Yes, most "standard" automobile shocks have the same damping in both directions. However, there are more advanced shock absorbers that use special valving that changes the damping response based on speed, or direction of travel.
     
  17. Jun 30, 2008 #16
    This has not been my experience. I know of double action shocks that help with a spring that is not sufficient for the job involved. But I'm thinking that what a regular shock does it is release the wheel over a time interval that is large enough to isolate the body from a continuous series of impacts.
    Listen, if this is going to turn into an ugly scene than I suggest that the folks who feel they have a motive bow out know. I know some of my questions have not been phrased in the best manner. But I really don't want to go around the block bickering about semantics.
     
  18. Jun 30, 2008 #17

    Mech_Engineer

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    Shock absorbers are not "timed." In the simplest model, they provide a resistive force proportional to a velocity imparted on them; nothing more. You can basically think of a vehicle's suspension as a mass, spring, and dampener: an undamped system will oscillate indefinitely, so by adding a dampener you can make sure the system response settles. However, settling time is dependent on the input conditions and dynamics involved. Generally, the larger the input (or the closer to the resonant frequency of the system) the longer the system will take to settle.

    We are all answering your questions as best we can; but you apparently have some misguided views as to how shock absorbers actually work, and insist on applying your personal anecdotal evidence as argument against scientifically grounded explanation.
     
    Last edited: Jun 30, 2008
  19. Jun 30, 2008 #18
    I didn't say they where timed. I only asked what you people think. I can see this is going to be difficult. Let's lock the thread and call it a day.
     
  20. Jun 30, 2008 #19

    Integral

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    I do not think that the question is difficult. The problem seems to be that you cannot free yourself of incorrect preconceived notions.

    Fred says that auto shock absorbers are simple orifice devices. I will take that as a fact. Note that pantaz points out that there are more complex shock absorbers, are they typically used in cars?? I don't know.

    At any rate it does not matter. You need to understand that the motion you observe is damped. Uniformly damped motion falls under 3 classifications. Underdamped, Over damped and critically damped. A under damped system will oscillate with decreasing amplitude until the motion stops. Over damped and critically damped systems do not oscillate. The motion is suppressed within a single cycle. IIRC how the system returns to the equilibrium position is the difference.

    To really see just what the shock adsorb er does you need to observe the undamped motion, without that you do not know the actual amplitude of motion.

    Read the Wiki article on damping
     
  21. Jul 1, 2008 #20

    stewartcs

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    I would suggest that you read this link (it's the main page for the first link I posted). It is from a company that has over 90 years of experience designing shocks.

    I think they know what they are talking about and they explain the whole suspension system at a basic level.

    Note that there is more than just one type of shock, hence they behave slightly differently, but fundamentally the same.

    Hope that helps.

    CS
     
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