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Thread: How to calculate shock absorber constants from known c and k values

  1. #1
    Associate Engineer
    Join Date
    Jul 2022

    How to calculate shock absorber constants from known c and k values

    We are trying to put a spring-damper system on a drone to act as landing gear (very small scale, only about 300 grams total). We created a MATLAB simulation to describe spring and damper constants, taking into account drone weight, free fall height, and stroke length of the spring-damper. The simulation tells us k constants, c constants, damping ratios, and compression lengths for an input maximum acceleration, stroke length, mass, and fall height. Those are the constants we then need to find a spring damper (aka shock absorbers) for. The issue is, the datasheets associated with commercially available shock absorbers don't have these constants. I have linked two datasheets for reference.

    One datasheet we have found uses constants of: Max in.-lbs/cycle (Nm/cycle), Max in.-lbs/hour (Nm/hour), Max Velocity in/sec (m/sec), and max reaction force lbf (N). In this case, what is the meaning of these variables? i.e. is Max Velocity an impact velocity? That would be perfect for our free fall application. Or is it velocity of the stroke of the shock absorber? How do we take c, k, and damping ratios and calculate these values? Datasheet found here:

    Another datasheet uses different variables: Load (kg), Max absorption energy J (kgf * m), Speed Range (m/s), and extension force N (kgf). In this case, is the Load a continuous load? Is the speed Range the speed of the physical stroke? Is the extension force a direct F = k*x relationship? Datasheet here:

    Hopefully you can either give me some advice on how to approach these variables or some equations on how to calculate them. Any help is appreciated. Thanks!

  2. #2
    Technical Fellow jboggs's Avatar
    Join Date
    Mar 2011
    Myrtle Beach, SC
    Why do I have to give the same advice over and over and over again? Call somebody! Talk to a live person. The internet and Google are wonderful tools, but it appears to me that those that grew up using the internet for everything never even think of using any other sources. There are numerous manufacturers of shock absorbers and gas springs of all types and sizes. And they ALL have application engineers, whose only job is to help you select the right component and use it successfully. They can do all the interpretation of your needs to their capabilities. Find one or two sources that you think might fit, pick up the phone, and talk to a live person at the manufacturer or distributor. Tell them you want application assistance.

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