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Resonance danger

Old 04-12-24, 11:03 PM
  #51  
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Originally Posted by asgelle
So many words, so much wrong. Read up on Hopf bifurcation. https://silca.cc/blogs/marginalgains...marginal-gains or if you prefer https://en.wikipedia.org/wiki/Hopf_bifurcation
Uh, yes, I was sorta going in that direction. (And by the way, in the podcast, the titanium bike that cracked at the downtube at the shifter bosses, seemingly due to bifurcation, matches another recent thread with that exact same failure mode location, but no mention of any such event.) I had university classes where the prof would just launch into the math of things from day one without any explanation, losing a lot of folks. So in my professional career, where I considered my job as a manager was also to teach, I try to enter a subject slowly, with lots of analogizing in terms that most mechanical engineers would understand, I don't go systems engineer from the get-go. I had worked up to an example of divergent oscillation, but seemingly from a forced input alone. In Hopf, it crosses the boundary to where the system has sufficient undamped elasticity to it, to sustain the oscillation even if forced input is reduced. I knew the problem on bikes but not conclusive causes, nor solutions, other than a steering damper. I did not know the suggestions they made to reduce speed wobble, that's great info. I'm also glad my Cannondale racer, very rigid (to a fault in ride quality), was before top tubes started to get really skinny, and I've topped out the speedo pedaling at max on a mild downgrade, so I think 64 kph (40 mph), no wobble. My current bicycle, 20" wheel folder, in particular has a mild speed wobble that I can steering induce and pull back quickly on fast descents, behavior opposite to what they describe, worse when unseated and thighs not clamped around seat, so perhaps that is not a true speed wobble. I improved stability in general via mass positioning on the bike, easy with front and rear panniers and racks, and a lot of added mass on the handlebars (clip-on-aeros, drink bottle, bar-ends, heavy L brake handles), plus the cheap wheels, tires, and flat liners are especially heavy for their size. However the above (podcast) may call into question the folding joint in the main tube, which has been tightened up considerably via my improvised "Deltech" (Dahon's new thing, I improvised using kevlar line that triangulates the frame). The smaller tires spinning faster, fatter tires, may be more susceptible than my 700c racer.

There's been a great study on bike stability by folks from Delft and Cornell et al, they made an excellent simulation model, but I think it's only valid for basic stability (steering into a fall), and not wobble dynamics. But very good if you can find online, I myself have the PDF but can't really post that without a link.

EDIT: Reading again the above study, I found this: "All known bicycle and motorcycle designs lose self-stability at high speeds because of gyroscopic terms." (Whereas at lower speeds, gyroscopic forces increase stability.)

EDIT: Excellent article:
https://velo.outsideonline.com/road/...-to-stop-them/

Last edited by Duragrouch; 04-15-24 at 07:44 PM.
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Old 04-13-24, 02:46 AM
  #52  
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Originally Posted by Duragrouch
Yes, dampen is very often used in those contexts, such as to dampen enthusiasm, analogous to "raining on your parade", drowning a good idea, extinguishing a fire (or idea) with water.

Similarly, dampener is something that can make something wet or quell enthusiasm like noted above, whereas damper is a device to reduce motion or energy, or flow, like a furnace damper.

No worries. I was one of those people who referred to casual cotton twill slacks as "khakis", until I heard the enlightening quote I mentioned. I hope you took my comment in a friendly way. But I can definitely understand issues with language translation; Though I have an ear for accents, I have a terrible mind for vocabulary in a foreign language, that has prevented my achieving any level of fluency in any of them.

And listen, the USA has had leaders in government who pronounced "nuclear" (new-clee-ur) as "nucular". So I'm very impressed with your knowledge and command of english, compared to my zero knowledge of your language (from your profile, Serbian?).
I lived in the USA long time ago and had a hard time convincing people to correct me at least when they thought they should (but didn't because they were very nice and tolerant). So no, I have absolutely no problems with you, or anyone else, correcting me or explaining the nuances - I like to learn.
By the way, in these parts of the world people will say things like "Toyota jeep", because jeep is a synonym for a terrain vehicle or SUV, probably since WW2. Things became a bit uncomfortable when the actual Jeep brand arrived on this market...
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Old 04-13-24, 04:04 AM
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Originally Posted by cyclomath
I lived in the USA long time ago and had a hard time convincing people to correct me at least when they thought they should (but didn't because they were very nice and tolerant). So no, I have absolutely no problems with you, or anyone else, correcting me or explaining the nuances - I like to learn.
By the way, in these parts of the world people will say things like "Toyota jeep", because jeep is a synonym for a terrain vehicle or SUV, probably since WW2. Things became a bit uncomfortable when the actual Jeep brand arrived on this market...
Not surprised about the jeep reference. Since the takover of Chrysler corporation by Fiat, a model was added to the Jeep line that is derived from a small front-wheel drive Fiat chassis, and purists scoff at it, "THAT is not a Jeep." I like the design of the Jeep (Wrangler) - based pickup truck with the 4 doors, but they are ridiculously priced, especially given the terrible quality ratings; While biking, I helped a nice woman whose Jeep Wrangler Rubicon (expensive), still under warranty, had died at an intersection, some major fuse or relay blown that I couldn't diagnose without a multimeter. She had just had the whole engine and instrument panel replaced under warranty. But I don't drive much these days, 99% biking.

In the USA "south" (meaning southeast), some refer to all carbonated drinks as a "Coke" (from Coca-Cola). So it is common for someone there to request an "orange coke" or "lemon coke" or "rootbeer coke", etc. And actual Coke is "Co-cola".

Just in the news tonight, a very renowned USA journalist has died, Robert ("Robin") MacNeil, originally from Canada, among his many books he had written one on the history of the English language, and then later one about all the different regional dialects in the USA, titled, "Do you speak American?"
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Old 04-13-24, 08:19 AM
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Originally Posted by 13ollocks
?? If you knew what it was, why didn’t you just say so, instead of the song and dance about that vibration at higher speeds and higher power into the pedals when in the 12 or 11 sprocket, that you get sometimes etc etc? FFS 🙄
Because I was describing it to the OP and trying to verify whether what was felt was actually in the front of the bike or something in the drivetrain. If I just ask if he was experiencing chordal action, then I'm not certain that he'd know what I'm asking about. I wasn't talking to any else that knew about it.

FFS
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Old 04-13-24, 08:44 AM
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It seems to me that the mathematician that introduced the cycling world to Hopf bifurcations didn't even look at the dynamics of a bicycle, so it bothers me a little that people talk about it like it's something that ends arguments about speed wobble/shimmy. I hate to have to listen to a podcast to learn if what the silca guy said shed any light on the matter. If you don't tie it back to a physical model with predictive power, it doesn't really settle anything.

There seems to be an international split in the vibration engineering community over "damping" vs. "dampening." In the U.S., vibration engineers generally assert that "dampening" is the process of getting something wet. But outside the U.S., "dampening" is often used to mean something that removes energy from a vibrating system. Both words have the same root, so it's difficult to make an argument either way based on linguistics.
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Old 04-13-24, 04:41 PM
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(above) Vaild on both points. Please see my edit at the bottom of my post #51; I wish I could post that PDF here. I may try to look for where I originally found that and post a link. Good study.
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Old 04-13-24, 06:24 PM
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Originally Posted by unterhausen

There seems to be an international split in the vibration engineering community over "damping" vs. "dampening." In the U.S., vibration engineers generally assert that "dampening" is the process of getting something wet. But outside the U.S., "dampening" is often used to mean something that removes energy from a vibrating system. Both words have the same root, so it's difficult to make an argument either way based on linguistics.
Are you sure you have that split the right way round? IME it is US engineers who usually refer to “dampening” and certainly UK engineers (like myself) refer to “damping”. Both are technically correct so it doesn’t really matter.
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Old 04-18-24, 05:17 AM
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Old 04-18-24, 06:49 AM
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Originally Posted by Duragrouch
So in my professional career, where I considered my job as a manager was also to teach....
WHOA! What an original concept! [/sarc]

And all these years I'd been thinking I was the only one who bothered. "Managers" mostly seemed to behave as though it was their job to leave making mistakes to others then claiming credit for what worked.

BTW thanks for posting links to that Silca stuff. Never stumbled across that before, lots to absorb!

Last edited by spclark; 04-18-24 at 07:08 AM.
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Old 04-18-24, 08:21 AM
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Originally Posted by spclark
WHOA! What an original concept! [/sarc]

And all these years I'd been thinking I was the only one who bothered. "Managers" mostly seemed to behave as though it was their job to leave making mistakes to others then claiming credit for what worked.

BTW thanks for posting links to that Silca stuff. Never stumbled across that before, lots to absorb!
A perusal of technical manager job postings provides an clue as to why. Goals say things like recruiting, retaining, mentoring, building teams. Skills required will be an alphabet soup of technologies, with leadership skills and aptitude hardly mentioned. Results are unsurprising, given the hiring decisions and priorities.
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