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# A-arm question

## A-arm question

(OP)
Hey everybody!

Could anyone shed some light on this phenomenon?
I've been taking accelerometer measurements of the A-arms on my minivan (front suspension). Why is it that at higher speeds (65mph) the peak frequency of vibration does not increase with speed, but instead the nearby frequencies will increase. Is this mainly the result of the A arm structural design, or response of the spring and dampers at higher speed?

Any insight would be appreciated.

### RE: A-arm question

(OP)
Yes you are correct, one big peak around 12 that distinguishes it from the rest.
I've read literature that automotive suspensions are designed with frequencies under 20 Hz for ride comfort.
Is there another reason that driving on the freeway would increase the 10, 15, 18 Hz peaks and keep the 12 Hz peak the same or lower?

### RE: A-arm question

The 12 Hz mode is almost certainly wheelhop. Its excitation on a well balanced wheel is from bumps in the road, and its frequency is more or less constant. However there are other excitations that occur at multiples of wheel rotational speed, so the frequency of those peaks changes with speed.

You'll probably need to read up on campbell's diagrams, waterfalls, and rotating machinery analysis.

Cheers

Greg Locock

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