Tom F

Has anyone done the measuring and the calculations to determine the natural frequencies of our torsion bar equipped 911s? It looks like a daunting task, but it would help to definitively answer the question of what torsion bar diameters are "best."

theiceman

sounds great .. let us know how it goes

928drvr86.5

How would the resonance frequency help in determining what T-bar diameter is best?

Tom F

Look at this: http://farnorthracing.com/autocross_secrets5.html

May911-85

good common sense reference Tom!

race911

"Best" is ever-evolving. We just laugh when some of us discuss how the cars were set up back in the '80's. But at that point we sure had figured out setup compared to those antiquated notions of the '70's!

My belief at this point, with all the knowledge out there, is that damper (shock) valving is so much more important than 1mm soft or stiff on torsion bar rate. Consult an expert, figure his parts price/shop time is worth a few bucks over the online special, have a superior result whether it's on-road comfort or competitive edge.

Ed Hughes

There's so much experience out there with every conceivable set up, that I question if there is an undiscovered mother lode. I second the thought that custom valving of dampers is often overlooked. It made a huge impact on my setup.

928drvr86.5

I agree the reference is good, and it answers my question perfectly. However, I would hardly categorize it as common sense especially for those without an NVH-specific engineering background. Maybe i am just dumb.

I work with NVH engineers on a daily basis and I am privy to their technical banter, so naturally i was curious to how the natural frequency of suspension components could be tuned to effect better handling without considering road surface. Typically when the subject of natural frequency of a material or component comes up, it's in regards to designing an exhaust system or engine mounts, etc to ensure the natural frequency is not within the frequency modes excited within the given the system.

In my field, test intrumentation, natural frequency (resonance frequency) and frequency response are important considerations for selection of transducers intended for use in experimental testing. You always want to make sure the transducer you select has a natural frequency above that of the highest frequency content of the parameter your are trying to measure whether it be strain, accleration, pressure, displacement, etc.

Unless i missed something, the main component I see missing in all of this is consideration of the excitation frequency working on the suspension. To my thought, resonance frequency of the suspension system provides only part of the puzzle. In my opinion (I'm not an NVH engineer) if you are not considering the excitation frequency acting upon said system, that's a missing consideration. The excitation frequency in this case would come from the undulations or imperfections in the road which won't always be uniform or even similar from road to road. What works well on a smooth track may not necessarily be suited to, or even safe on the typical road with dynamic excitation. It is for this reason that there has been much research done lately on active suspension dampening systems whereby the dampening fluid's flow properties are altered thru magnetization and the suspension adapts within fractions of a second to accomodate whatever surface is currently underfoot.

Ed Hughes

Whoosh!

(the sound of this all rocketing over my head)

Ed Hughes

Whoosh!

(the sound of this all rocketing over my head)

928drvr86.5

Mine too, someone please help me learn.

theiceman

I have done a lot of work in Engineering designs and PFV of components. especially where stepper motors are used and a frequency is inuced into a system.
Personally I think that this is totally irrelavent to suspensions and would have such a minimal impact to anything it is not worth the design analysis time spent on it.

To simplify things everything has a specific frequency that it will begin to resonate .. and the biggest structures can be affected by this . ( this is why soldiers break formation when crossing bridges becasue the in time march can cause actually cause the bridge to sway. ) In more practical terms you want to make sure an exhasut doesn't vibrate as you will get the "drone" through the car , and engine mounts because you will feel the vobration through the car. i

Ever wonder why your car may only vibrate at certain speeds when you have a wheel out of ballance ? that is becasue you are hitting it's peak frequency of vibration.
The idea in general is to engineer it out or have it cancelled out. again i don't think it is an issue at all in suspension and will not affect the performance of the suspension at all .... but then again that is just my .02c .... which incidentally today is the same in US and CAD

May911-85

not very scientific but i like to check out successful builds (tuning) that are set up the way i want my car to perform... no one set up satisfies two drivers exactly the same way... but it might get you close!

whalebird

Cool study. It may not be relevant, but the differential of unsprung weight across the suspension components - say from the ball joint to the control arm mounting axis. There has to be something funky going on in there. Lots of people try to achieve the lightest wheel/tire combination in an attempt to gain something. I have talked with some smart chassis guys and you can easily have wheel/tires that are TOO light. If anything they lock up to easy under braking, but they will also resonate differently with the occilations of the suspension. I know a crew chief from back in the days (late seventies) that would fill their 911/935 rear control arms with lead shot at Daytona. This all seems to relate in my mind - with regards to the OP.

Tom F

As always, posting here or on any of the Porsche related message boards taps into a huge well of knowledge. However, I think that the frequencies referred to at the link I posted (around 2 Hz) are not frequencies that typically concern NVH engineers. I think that the natural frequency, in the context of "Autocross to Win" simply serves as a measure of optimal spring rates for handling purposes, and that the point of selecting a natural frequency as the author, Dennis Grant, suggests has nothing to do with ride harshness or noise.

That's what I believe, anyway.