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Damper energy absorption during rebound is different from compression

  1. Mar 29, 2014 #1
    I understand that the purpose of a damper system is to stop the spring from bouncing during rebound. But,technically speaking,I do not understand the matter of energy absorption in the damper during rebound.
    1.The damper is absorbing energy during compression along the bump and dissipating energy during rebound along the bump,except that it(damper) is additionally dissipating the energy of the spring also during rebound.Am I right?

    In other words,in the graph below depicting the motion of a spring-damper system over a bump in a road,the damper absorption work during rebound is different from during compression. Does my explanation in 1. validate this graph?

    2.Also,what about the distance travelled by the damper during rebound? I assume it has to be greater than in compression to dissipate the extra energy of the spring.Am I right?


    http://imageshack.com/a/img811/3572/3jaf.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Mar 29, 2014 #2

    mfb

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    Staff: Mentor

    What do you mean with "additionally"? All the energy comes from the spring.

    During the compression phase, some energy gets stored in the spring, and some energy gets dissipated in the damper. During the expansion phase, some of the energy of the spring gets dissipated in the damper, and some energy is used to push against the ground.

    It doesn't have to be different.

    Just look at the geometry - if the length of the wheel/spring/damper system is the same before and afterwards, can you have different distances?
    Different forces are possible.
     
  4. Mar 29, 2014 #3
    Thanks.I was confused on how the damper worked in the compression phase. I thought the damper itself would be absorbing some energy because of its own compression. But,from what you said,looks like the damper does just the job of 'dissipating energy'. Although you didn't complete one of your explanations on why a damper 'dissipates' more energy during the rebound phase(as seen in the diagram above). Is this how they are designed by default? i.e damper dissipates 20% of the spring compression during the compression phase and then dissipates 40% during the expansion phase. If that is so,the spring should travel a lesser distance during rebound,right?

    So,would it be right to say that the diagram of spring work above in the diagram is just the spring work done after the damper dissipates the energy?

    http://imageshack.com/a/img811/3572/3jaf.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  5. Mar 29, 2014 #4

    AlephZero

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    I don't understand your diagram. The force in the damper is proportional to velocity. So assuming the vehicle speed in the X direction is constant, the damper force going up the bump should be equal and opposite to the force going down, not as shown on your diagram.

    There is no relation between the forces in the spring and damper. The force in the spring depends only on the height of the bump. The work done by the damper depends on the vertical velocity. The damper force depends on the speed of the vehicle. The spring force does not.

    The diagram shows the force in the spring is the same as the shape of the bump. That means the vehicle body does not move vertically. That is only an approximation, but if you include the vertical movement of the vehicle you need to include its mass in the model, and the whole diagram would look different. For example the vehicle body would still be moving vertically after the wheel had passed the bump.
     
  6. Mar 29, 2014 #5
    Yes,the vehicle body movement is not taken into account in the above diagram. But,suppose the vehicle body movement is taken into account,would you then say that the damper force looks like this(see below)? I guess by 'damper force',you mean the force the damper pushes-out with on the tire. [I mean,it could also be the damper resistance force].
    damper_example.jpg
     
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