Push Rod Flex

[DaveW]

i couldn't get the calculator to open, but if the material used in those calcs is some stiff "ringing ' metal, their results won't apply to a composite pushrod.

The reference is an .xls (Excel) file. It will give natural frequency estimates for a range of materials and geometry. You are correct, of course, but it should be able to convey a "ball park" figure for likely natural frequencies, I think.

[DaveW]

anyway, the coin tap sound you get in composites is a low frequency, well damped 'clunk' sound... if you tap a metal wind chime, you get a much higher frequency, ringing, 'ding' sound.

It's a function of the frequency content of the impact, I think, A coin tap would be a low frequency input that is unlikely to excite the fundamental modes of the push-rod.

[olefud]

Clearly, there is a problem. I don't know what it is, but suppose the outer bearing is constrained so that an end load also imparts a bending moment. Then the inner bearing will be loaded so as to react the end load, but will also have a component reacting the bending moment. This, starting a zero at the inner end an increasing to a maximum at the outer end, will cause the push rod to deflect, and the end load then would further deflect it, until an equilibrium is reached. I think the "shape" of the deflection would depend on the end contraints, rather than any of the push rod natural mode shapes.

If the rod is loaded in compression sufficient to develop bending it seemingly would deflect only in one direction rather than oscillating as shown in the clip –which is what you are saying, I think. Which suggests that the oscillating rod is either in tension or in compression insufficient to cause appreciable bending.

[marcush.]

I think Daves idea is :

the road input is not a flat surface but a series of equispaced upsand downs of say 30or more milimeters amplitude and say 1000mm length...the car moving over this with one wheel ,but the rocker spring assembly not responding / in a locked state
by installation woes something else will give in this case the pushrod bending away...mind you the brake duct also gets involved and affected so maybe there is even more going on than a bendy pushrod...
It could well be the pushrod is then oscillating between the two extremes due to the road(kerb)inputs.

[Caito]

Someone that has a video editor can go frame by frame to estimate the frequency at which the rod is oscillating?

[mep]

anyway, the coin tap sound you get in composites is a low frequency, well damped 'clunk' sound... if you tap a metal wind chime, you get a much higher frequency, ringing, 'ding' sound.

It's a function of the frequency content of the impact, I think, A coin tap would be a low frequency input that is unlikely to excite the fundamental modes of the push-rod.

As far as I know vibrates a structure with its eigenfrequency once it gets excited, otherwise a tuning fork would not work. The coin test is also used for machines but on a slightly bigger scale with a hammer. It does not matter what frequency the input is off because only a single stroke is given to the structure.

Hammer testing is a straight forward method which yields good results under most conditions.
This testing technique makes use of the fact that when a (mechanical) structure is excited by
means of a Dirac pulse, the structure responds with all its eigenvalues (i. e. natural frequencies
and damping). An example of this effect can be demonstrated by impacting a piano with a short,
impulsive blow. If the piano pedals are depressed during the blow, each piano string will clearly
vibrate.

http://www.kistler.com/mediaaccess/20.195e-10.98.pdf

[Caito]

Hammer testing is a straight forward method which yields good results under most conditions.
This testing technique makes use of the fact that when a (mechanical) structure is excited by
means of a Dirac pulse, the structure responds with all its eigenvalues (i. e. natural frequencies
and damping). An example of this effect can be demonstrated by impacting a piano with a short,
impulsive blow. If the piano pedals are depressed during the blow, each piano string will clearly
vibrate.

http://www.kistler.com/mediaaccess/20.195e-10.98.pdf

I guess that would only apply to a linear system, in which by enetering with an eigen vector(all frequencies) you get as an output the same frequency phase shifted and amplified(that is, the eigenvalue).

Would this apply to a CF push rod?

A tuning fork is excited in it's natural frequency, it just has a big gain in that particular frequency(and low damping) which does not occur on other frequencies. The hammer test relies on the fact that a dirac delta has a flat frequency response, hence it excites all frequencies.

[GSpeedR]

Someone that has a video editor can go frame by frame to estimate the frequency at which the rod is oscillating?

Another way would be to estimate the vehicle's speed and curb spacing to determine an input frequency.

Neither a coin tap nor a hammer impact at the suspension is analogous to the curb striking the tire, as the tire stiffness is quite low relative other system stiffnesses (and it provides an independent DoF) so the curbs would certainly not excite the entire frequency band which occurs with an 'ideal' hammer.

That video is too grainy to make any real conclusions from.

[olefud]

I think Daves idea is :

the road input is not a flat surface but a series of equispaced upsand downs of say 30or more milimeters amplitude and say 1000mm length...the car moving over this with one wheel ,but the rocker spring assembly not responding / in a locked state
by installation woes something else will give in this case the pushrod bending away...mind you the brake duct also gets involved and affected so maybe there is even more going on than a bendy pushrod...
It could well be the pushrod is then oscillating between the two extremes due to the road(kerb)inputs.

The entire system is rather complicated and may well include the damped rocker spring and, for that matter, the car itself jolting. But if the push rod bends under load and remains bent it’s not going to oscillate. However, if it hit a bump and bends under the sudden load, unloads and relieves the deflection, and then hits another bump as the rod passes the null point, and keeps repeating the cycle, a good bit of energy will accumulate in the rod even if reasonably designed.

Perhaps some thought should be afforded the spacing of the bumps. Irregular spacing should solve the problem.

[strad]

Then the question becomes...How come the Red Bull and others....doesn't flex like that?

[Jersey Tom]

Then the question becomes...How come the Red Bull and others....doesn't flex like that?

Because they're properly designed with sufficient stiffness. Pretty simple. Don't think this has anything to do with bump spacing or any crap like that. Just a poorly designed part.. went too far on weight saving.

[strad]

Ignore the sound...it was supposed to be muted,,the second part is one third speed..and I cheated and used the same picture for a link...
click photo

[DaveW]

Another way would be to estimate the vehicle's speed and curb spacing to determine an input frequency.

My guess would be the "video time" speed of the vehicle is rather less than 9 m/sec. The clip was taken during the exit from a corner (not sure which), with would probably have a "real time" speed of somewhere between 100 & 120 mph (44 & 54 m/sec), which would imply that time has been "slowed down" by a factor of between 50 & 60. I suspect that here must be a standard for high speed clips, but I have no idea what it might be. Hence the question.

[olefud]

Then the question becomes...How come the Red Bull and others....doesn't flex like that?

Because they're properly designed with sufficient stiffness. Pretty simple. Don't think this has anything to do with bump spacing or any crap like that. Just a poorly designed part.. went too far on weight saving.

Maybe. Chapman was known to push a little too far. Or maybe the Red Bull has a different natural frequency or speed over the bumps. Harmonic resonance took the wing off an airplane built way over strong for static loading.

[Jersey Tom]

I still don't think it has anything to do with a "natural frequency" or whatever. Looks pretty clear cut to me. But that's my gut feeling.

Not the first time a F1 team has been caught out by pushing a little too far. Who was it several years ago.. Red Bull? Had suspension components failing by clipping kerbs too hard. Nothing too fancy about it.