Push Rod Flex

[strad]

Those who watched practice on Speed saw this video of the push rod flexing..
click photo

On top of everything else, would not having this kind of feedback be very fatigued to the driver?

[DaveW]

Thanks for that. An example of poor installation stiffness caused by poor design. The fact that the pushrods didn't fail (presumably) suggested that they were subjected to large bending moments through poor alignment. The bad alignment might be static (unforgivable) or dynamic caused by, for example, rocker flexing.

p.s. I wonder if their simulator models the effect....

[Lycoming]

I would think that the objective of pushrod design would be to have it stiff enough that its deflection is negligible. If I was writing a simulator, I wouldn't have bothered coding it in.

Interestingly, the motion of the pushrod is time dependent...

[Tim.Wright]

Looks to me like its vibrating at a natural frequency rather than being just plain to compliant. Its a problem that is not so easy to spot in the design phase.

Not sure if this would cause a change in the effective wheel stiffness or not. The vibrations are not pointing in the same direction as the load, so perhaps its not causing many handling problems.

Though for sure its not good for durability. This could easily be repaired overnight though.

Tim

[mep]

Somehow I don't see anything when i click on the photo.

[DaveW]

Looks to me like its vibrating at a natural frequency rather than being just plain to compliant. Its a problem that is not so easy to spot in the design phase.

I suspect that the frequency is in phase with the kerbing inputs (the natural frequency of the suspension links should be in the hundreds of Hz).

Not sure if this would cause a change in the effective wheel stiffness or not. The vibrations are not pointing in the same direction as the load, so perhaps its not causing many handling problems.

To get any significant change in push-rod length it has to buckle, so the observed motion is the signature of a change in length, I believe. Pure compressive buckling is non-linear & usually terminal, with implies a bending moment applied to the push rod, which, in turn will make the load/deflection curve (more) linear.

The change in length will be undamped, & therefore uncontrolled, which would be expected to affect Contact Patch Load control everywhere (not just when kerbing). That is quite separate from the steering shimmy caused by the kerbing event. I suspect a driver would not like the idea of a spring placed in series with his steering wheel....

[DaveW]

Somehow I don't see anything when i click on the photo.

What I did was: Right click on the link, select "Save Link As", then play the saved .mpg with VLC (a free media player).

[DaveW]

I would think that the objective of pushrod design would be to have it stiff enough that its deflection is negligible. If I was writing a simulator, I wouldn't have bothered coding it in.

And that illustrates one of the major issues I have with testing by simulator...

[hollus]

Somehow I don't see anything when i click on the photo.

What I did was: Right click on the link, select "Save Link As", then play the saved .mpg with VLC (a free media player).

Thanks Dave, that worked for me. I had this issue before and had forgotten the fix.
Strad, maybe you'll want to add a "save as" next to your usual "click picture"?

[Tommy Cookers]

Looks to me like its vibrating at a natural frequency rather than being just plain to compliant. Its a problem that is not so easy to spot in the design phase.

I suspect that the frequency is in phase with the kerbing inputs (the natural frequency of the suspension links should be in the hundreds of Hz).

a persistent train of perturbations at/near (the right) sub-harmonic of the natural frequency would do that ? (ie rumble curbing)
but would the tyre be stiff enough to transmit this torque runble (the tyres Fn wouldn't be high enough ?)

anyway, would the natural frequency (of the transverse vibration) be hundreds of Hz ?
the ends of the rod are unconstrained and it must have some relatively huge added mass effects under load
natural frequencies of hundreds of Hz are about as common as unicorns ?

[marcush.]

brings back memories of Massas loss of suspension components after rumbling over the kerbs last year persistantly...

[Jersey Tom]

Looks like definite buckling to me. Wow! With it being carbon I'm amazed it didn't shatter right then and there.

[mzivtins]

Shape it like a wing = MOVABLE AERODYNAMIC DEVICE

TBH i think that although the movement is extreme in this case, all of those components must have some form of flex built in, otherwise snapping would occur much more often. Plus the whole unit seems to move as one rather than bend as such. Interesting though, and funny how the comparison to the red bull is on a different corner of the track what if the speed is 10mph higher or lower? thats enough to change the vibration frequencies considerably

[Lycoming]

Everything flexes, but for the most part suspension components should be stiff enough to have negligible deflection under normal operating conditions (such as going over kerbs). You don't want to have to think about a bunch of springs in series, you want to be able to think of them as infinitely stiff and ignore them. It's not like an airliner's wing, which is supposed to flex visibly, to a certain extent of course.

[mep]

Thanks for the tip Dave.
Actually, when I saw the oscillation of the part I wondered if it’s because of bad installation slackness.
That amount of movement is really not normal.