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How are automotive engine mounts tuned to behave differently for different frequencies, like they are soft for high frequency low amplitude vibrations and hard for low frequency high amplitude excitations..?

I am aware of active or hydraulic mounts, but want to know how a regular mount is tuned to behave in such a way..?

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    $\begingroup$ What leads you to believe that they have this different characteristics for different frequencies? The motor mounts that I've installed are hunks of rubber with steel bits in them. If they have a force vs. displacement characteristic that varies with frequency at all it's because rubber has some damping, which means that they are effectively harder at higher frequencies. $\endgroup$
    – TimWescott
    Aug 6, 2019 at 19:10
  • $\begingroup$ @TimWescott some rubber mounts have carefully designed voids in them to provide a wider range of behaviour depending on the applied force and amount of movement. $\endgroup$
    – Solar Mike
    Aug 7, 2019 at 6:52
  • $\begingroup$ Thank you for your answer. I wanted to know how engineers overcome the challenge of dynamic hardening of rubber at high frequency low amplitude vibrations which make it poor insulator. $\endgroup$
    – SS4
    Aug 7, 2019 at 14:36

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Before designing any support structures, we must know the loading conditions and the constraints.

Luckily, the loading conditions in most automotive scenarios are extremely well known to auto designers. Apart from this, there are also forces due to vehicle motion, such as hitting a bump, pothole, cornering, braking etc.

All these loading scenarios are used in hand calcs and/or FEA models. Using this, as you noted for actively damped mounts, a response curce is generated for the mount and a table is programmed into it's controller. For passive damping, the loading scenarios are analized, first for safety: Mounts should not have a catastrophic failure in any scenario, and then for NVH/cost reduction. The mounts are usually a complex design, with a dampening material inside them.

Hopefully this can help.

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There are many ways you can modify or design something to reduce resonance, and therefore reduce the risk of it being damaged by vibrations. In order to do so you need to know what frequency range the object will be experiencing, what it's made of, its dimensions, mass, etc., and can then do calculations (which are usually now done by computer software) to determine the resonant frequency of the object. You can then tune the object by changing some of the physical qualities mentioned above in order to move the resonant frequency out of the range of vibrations it might experience.

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