dshepp806

I make my living ridding control rooms of , what we refer to as, modal resonances. Although primarily to do with space, however we always take a look at any "structural" resonances that may be significant (or quite commonly known),..this, being more in line with you fellows' discussion. Sounds like something quite interesting

2 Hz,...? Are you kidding me? Some of the surround spaces are looking for that magic # (capability) in the control room.

Anywaze,..back to discussion...

Best,

Doyle


Still have yet to read the article, but will do.

autobonrun

I'm not sure that determining the natural frequency would tell you the optimum design; it would tell you which design to avoid. If someone was unfortunate enough to select a spring rate at it's natural frequency, any upset (e.g. bump) would send the system into ever increasing swings. Most have seen the old film footage of the suspension bridge that started to sway at an ever increasing rate due to wind load. Everyone has heard an amplifier screech when the volume is set just right; above the natural frequency no problem, below the natural frequency no problem; at the natural frequency big and ever increasing problem. We typically call this overdamped, underdamped, and critically damped.

You would have to be very, very unlucky to pick a torsion bar rate that was at the car's natural frequency. When I increased my torsion bar thickness, I increased both the front and rear such that the car was stiffer but the ratio of front to rear thickness was the same. I was happy with the amount of understeer my car had, I just wanted it to remain flatter on curves. Changing the ratio can affect the weight transfer front to rear and consequently affect over/understeer. In my case, I made the car corner flatter without affecting the turn in.

video of a system that did not account for the natural frequency

http://www.youtube.com/watch?v=j-zczJXSxnw

RecceDG

Although calculation of natural frequency can be used to determine resonances, that is not why a racing engineer does it.

Firstly, natural frequency provides a comparable measure of "stiffness". Unless you are discussing identical cars (in all ways identical, including weight) spring rates are meaningless. Depending on the design of the suspension, a car with a 300lb spring can be equally stiff as a car with a 1600lb spring. Even wheel rate (which takes into account the motion ratio of the spring) doesn't cut it if the cars being compared differ in weight - a 300lb wheel rate has different implications on a Formula Ford and a Camaro.

Secondly, once you have natural frequency, it becomes possible to speak meaningfully about damping forces, both relatively (to what % of critical is the car damped?) and absolute (what force is required to produce a given % critical damping rate?) which is (heh) critical in determining shock damping curves.

More information is at the ATW website at http://farnorthracing.com/autocross_secrets.html

DG

Tom F

Exactly. So, who has gone through the pain of taking the necessary measurements for our old 911s? (Besides Porsche, that is.) A look through the website shows that there's a lot to measure.

irobertson

Excuse the hijack :-)
They are truly insane.
I know of one room that is flat down to 23Hz. It was a huge deal making this happen. I believe that below 28Hz you actually don't hear anything. In order to go below that you need to make the room "move". Below 28Hz, the perception of sound moves from the acoustic realm to the seismic realm.

cheers

whalebird

Fun discussion. constructive-interferance is every engineers nightmare, at least civil/structural. everything is supposed to be in equilibrium - static. When stuff goes dynamic people die.

r911

Brian - I think you may want to cross post on Pelican.

re modeling - one problem is that apparently no one knows the height of the Center of Gravity.