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Shock absorber problem

  1. Nov 12, 2011 #1
    I am trying to design a bicycle cargo rack that uses shock absorbers to protect sensitive equipment (i.e. a laptop computer). I have data for the maximum height from which laptops can fall without damage as well as the maximum acceleration to which they may safely be subjected. I am planning to do an experiment that measures the change in acceleration of a bicycle riding over a bump at ordinary travel speeds. I would like to use this data to select appropriate shock absorbers for my purpose. My problem is that I am unsure how to identify the appropriate quantities (i.e., force, energy), and relate them to the appropriate parameters of a shock absorber(damping coefficient etc.).

    My specific questions are:
    1) (Assuming force is the appropriate qty) How can I relate the fall height of a computer to the force it sustains on impact?

    2) If I can measure the change in acceleration of the bicycle, how can I relate this to the force applied specifically to the cargo load (as opposed to the cargo load, bicycle and rider)?

    3) Once I determine the force to which the load is subjected, how do I relate this to the appropriate parameters of the shock absorber?

    Any insight you may be able to provide is greatly appreciated! Sorry for the long post.
     
  2. jcsd
  3. Nov 12, 2011 #2
    Just because a computer can fall from height X and still work does not mean it should be subjected to continuously repeated forces equivalent to falling from 1/2 X. I think your design objective needs to be to carry it with as little shock as possible.

    That being said, the term "shock absorber" is a misnomer. Shock absorbers do not absorb shock, springs do that. Shock absorbers prevent harmonic oscillations of the springs by converting kinetic energy into heat.
     
  4. Nov 12, 2011 #3
    That gave me a laugh as you are the frist person I have ever heard say such a truthful profound statement. colloqual english is very bad at times.

    To the OP - in analysis it is called a damper.
     
  5. Nov 12, 2011 #4
    Thank you for your replies.

    Re: "Shock absorbers do not absorb shock, springs do that," I guess my conception of a shock absorber includes both a spring and a damper. It seems that they are usually both included in the same unit. Ideally I would be able to find the optimal spring stiffness as well as damping coefficient, but I don't know how to calculate either without determining the force exerted on the cargo load.
     
  6. Nov 14, 2011 #5
    Hello,
    I am narrowing down my question in hopes of generating more of a response. Could anyone help me understand how to determine: 1)the energy that would be transferred to an item stored on a cargo rack from a rider going over a bump at a certain speed and 2) the acceleration that the cargo would experience. I know this is an extremely complex problem, but any information you might be able to provide to simplify it would be greatly appreciated.
     
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