Resonance and Natural Frequency

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Resonance and Natural Frequency

Postby AdvTechTS » Fri Sep 22, 2017 3:34 am

Hello suspension experts, I'm a trainer and trying to clear up an inconsistency. In the PicoDiagnostics NVH Assist files it states that the natural frequency of a vehicle suspension is 10-15Hz and some newer vehicles are up to 20Hz. I have also seen other resources out there that state a range between 1-2Hz. This is a significant difference, can some please verify this value? maybe a link to a reputable resource also? Thanks!
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Re: Resonance and Natural Frequency

Postby Steve Smith » Wed Sep 27, 2017 8:26 am

Hello and thank you for the post.

I am so sorry for a late reply and want you to know I am working on feedback which I will post ASAP

I think we may have a confusion with the term Natural Frequency in terms of spring rate and Natural Frequency in terms of component inherent Natural Frequency that leads to resonance

It would appear 1-2 Hz is the target natural frequency of the sprung mass (vehicle body)which is goverend predominately by the spring rate, whereas the suspension system as a whole has a natural frequency of 10-20 Hz in which it will resonate if a vibration source of 10-20 Hz is applied

I will follow this up ASAP

Thank you for your patience, take care......Steve
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Re: Resonance and Natural Frequency

Postby Steve Smith » Wed Sep 27, 2017 10:41 pm

Hello again and thank you for the post.

Typically T1 vibrations felt in the cabin floor or steering wheel tend to fall around 10-20 Hz which is often between 60 to 80 mph depending on tyre size.

This is no coincidence as let’s look at what is vibrating:

The road wheel imbalance is our source, the suspension is the transfer path whose collective component natural frequency falls within this 10-20 Hz band, the cabin/steering wheel are the responders.

SOURCE TRANSFER RESPONDER.jpg
S-T-R


This is why often between 60 to 80 mph there is a sweet spot of intense vibration (suspension resonance at 68 mph for example) but at 72 mph the vibration diminishes and can often disappear.

The wheel is still imbalanced (source) but we have gone beyond the resonant point of the collective suspension components.

If we take the suspension collectively as one assembly from road wheel (source) to steering wheel (responder) then 10-20 Hz is correct.

Resonance occurs when the frequency of the source vibration (applied to a component) matches the natural frequency of that component

Thinking about our imbalanced road wheel at 15 Hz applying this imbalance energy to the suspension (whose natural frequency is 15 Hz) we experience a dramatic momentary rise in vibration-Resonance

Using the example at 68 mph the steering wheel vibration reaches its peak, but at 72 mph the steering wheel vibration reduces. We have driven through the resonant point of the collective suspension components.

A typical characteristic of resonance is the fact that although we continue to increase the vibration level (source) applied to the component (suspension) once we go beyond the resonant point the vibration actually reduces.

Resonant frequency.jpg
Resonant frequency


The image above uses the suspension assembly as an example to demonstrate resonance.

I hope this helps, take care......Steve
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