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Valve Shim calculation of a damper

  1. Apr 8, 2014 #1
    How can one calculate the dimensions of valve shims in Gas pressure shock absorber?

    The valve shims are orifices to the fluid flow in the hydraulic damping system. The dimensions play a very vital role in the adjustment of compression and rebound of the damper.
    I have been working on trial and error method, taking some reference dyno graphs, but can not manage to find a formula to calculate the exact dimensions of valve shims.
    any help would be appreciated.
  2. jcsd
  3. Apr 10, 2014 #2

    Ranger Mike

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    how high is the sky? a flippant answer but seriously...you are asking a way inside baseball question without any other input.

    Usually we use the shocks ( ok dampers for the rest of the world) to fine tune the chassis set up. Starting wit ha telephone call to the shock application technician at the shock manufacturer, we find the correct size shock for the race car. The car weight, type of race course, (oval, dirt asphalt, degree banking), all come into play. Once we have decided on the shock model we properly mount it (there is a proper angle and location to determine motion rate).
    Next we spend a lot of time at the track hot lapping the car and use a stop watch and drive input to see if we are in the ball park. We really have to get dialed in on the proper spring package to get this far. The specific shock shim package will do the fine tuning as will the pressure we use for the nitrogen. The psi can vary from 150 psi to 250 psi if memory serves me?
    We can make the shock act as a helper spring with a lot of increased compression or use it to “tie down” the front or rear corner depending upon what we need to do at that track at that time in that particular weather condition.
    Regarding shock dynos..they are only good for making sure all the shocks are performing the same when at proper temperature. Heat is a killer of shock performance. that's why we have to rebuild them yearly. The shock oil breaks down after a time. Shock dynos will not show you how the shock will perform on the car at any particular corner.. there are too many variables. You can use the dyno chart to see measure certain settings and verify compression and rebound settings but that is about it. typical dyno chart is force vs. velocity. On a typical compression graph We can see the rate of bleed until the shim stack pops open and the rate drops. This can be set to come in at any point of the compression cycle and makes things really confusing until you apply it to a certain application. We are concerned with low velocity (under 5 inch per second) for a cars roll pitch handling and high velocity damping ( over 5 ips) for hitting sudden barriers like the berm on dirt tracks or those pesky tar strips. Say we have a car handling real good ( low velocity shock setting working great) but car gets squirrely when hitting the tar strips . Here we may want to kill off compression to keep the car from feeling skatey.

    I like the force displacement charts better but that is another matter. Anyway we can gain insight into handling with a good shock dyno but it will only get us close.. we still need that stop watch.

    The best way to calculate the shim thickness is with a stop watch!
    Last edited: Apr 10, 2014
  4. Apr 11, 2014 #3
    Hi Kapil,
    I think you are working in shock absorber industry.
    You are talking @ pyramid valving in shox which used in pistion & base valve.
    Mostly first disc (nearer to valve face) is 0.152 thk as this is smallest thk provided by supplier.0.152 Thk propely seats on valve & provide properly sealing.this is not creat lag in compression & rebound.
    And reamaning thk in sequence 0.2,0.25,0.3......after seeing graph u can see blow off...according you have to arrange discs....
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