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Sudden Realization From 50 Years Ago

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Sudden Realization From 50 Years Ago

Old 09-01-12, 04:06 PM
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VeloBrox
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Sudden Realization From 50 Years Ago

I think something just blew my mind, but I need you knowledgable lot to help me understand what I just saw.

I was watching this educational film from AT&T (Bell Labs) on wave patterns from 1959.

At 19:11 Dr. J.N. Shive talks about how wave energy gets reflected when it encounters abrupt impedance differences, and at 22:02 he explains how this reflection can be minimized with a gradual change in impedance.

How is this relevant for bicycles?

All butted tubing I've seen has had gradual butts, where the wall gradually thickens into the butt. I think that if one were to have abrupt butting with no sloping, there would be much more resonating energy going back and forth in the tube under use. This would mean that "perfect" butting would be totally smooth, with as gradual thickening as possible instead of going thin-slope-thick.

In the video, the "fat" part of the wave machine would represent the butt in the frame tube, the "impedance matching device" would be the slope and the thin part would be the tube where the wall is at its thinnest.

I think sleeves or center bulges in handlebars serve the same function - avoiding sudden impedance change between the bar and the larger diameter of the stem clamp.

What do you think? Am I off my rocker or am I on to something?

Last edited by VeloBrox; 09-01-12 at 04:11 PM.
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Old 09-01-12, 05:39 PM
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What? My brain's impedance is butting against your words. Andy.
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Old 09-01-12, 08:50 PM
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I'm with Andy.
I don't get what you're saying the benefit of the different butting profiles would be.
maybe you're over thinking this.... I can guarantee that I if build you a butted frame and a frame with straight gauge tubes you would'nt be able to tell the difference in the ride.
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Old 09-02-12, 01:59 AM
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No, that's not it at all.

What he's trying to say is that almost all butting profiles available right now are a compromised fat-taper-thin. That's just making the mechanical stage of forming the butt as easy as possible for tubemakers, and understandably so. What he's suggesting is that in essence, the perfect tube section should be a reaaaaally graceful parabola tangent to the straight exterior tube wall - no thin-taper-thick obvious changes, but essentially a constant blended taper from the very dead centre of the tube to the ends.

There's only one problem with this - it suggests that we think the loading on any given tube is identical at each end, and we know it isn't. SO while the idea is nice and graceful, it's not representing the real-world conditions.
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Old 09-02-12, 05:21 AM
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Ah, the mighty Falanx! I've read many of your comments before and I know you know a great deal about these things.

Do you agree that in principle, the tapered section of a tube profile is, in essence, an "impedance matching device"? And that an abrupt change in wall thickness would result in much more energy being reflected back through the tube?

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Old 09-02-12, 07:06 AM
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While I think that the acoustic behaviour of bicycle frame materials is far more important than is currently credited, I don't think your concern is actually a problem. A wall thickness change of 0.4mm over a length of 50mm is an angle of about half a degree, that's not an "abrupt" change. At this angle there is no real change of acoustic impedance and in truth there isn't even any appreciable change at the end of the tube.

The acoustic impedance of a typical frame steel is around 45 MPa.s.m^-1 while a typical soft silver brazing alloy is around 25. This may seem like a huge difference but it isn't, the reflection at a flat interface is proportional to the square of the difference in impedance, so in this case it's (45-25)^2 / (45+25)^2 which is about 8% meaning that 92 % of the energy passes straight through the interface.

The intensity of any standing wave is therefore limited by a combination of the energy passing out of the system, the internal damping of the material and the transient nature of the drivers at the frequencies of interest.

Last edited by Mark Kelly; 09-02-12 at 07:40 AM.
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Old 09-02-12, 09:44 AM
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The long and the short is that it's nigh on impossible to make a taper like you suggest in a steel, or any other metal, tube, VeloBrox, and Mr Kelly's math is sound and assertion is entirely reasonable. It's be nice to try in a perfect world. but the truth is across a 600mm tube, the slope from centre to ends would be equal to the average size of metallurgical defects, and therefore impossible to shape perfectly.
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Old 09-02-12, 10:47 AM
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Originally Posted by VeloBrox View Post
All butted tubing I've seen has had gradual butts, where the wall gradually thickens into the butt. I think that if one were to have abrupt butting with no sloping, there would be much more resonating energy going back and forth in the tube under use. This would mean that "perfect" butting would be totally smooth, with as gradual thickening as possible instead of going thin-slope-thick.
Like Tange "Infinity" tubing, you mean?

http://www.equusbicycle.com/bike/tan...ngecat1997.pdf
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Old 09-02-12, 01:12 PM
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I'm not sure that brochure shows what you think it shows...
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Old 09-02-12, 01:48 PM
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You cannot help such matters for the frame. Maybe that's why the seat swap thread is popular.
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Old 09-02-12, 03:38 PM
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What this also means is that the small discontinuities in tubing thickness from the butt-taper-thin transistions are small stress risers; tubes will eventually fail at those points. Kasei makes tubes with a continuous taper from the thick to the thin region. That may help a little bit?
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Old 09-02-12, 05:19 PM
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I didn't make myself clear, apologies. I never meant that the existing, tapered butting is abrupt in any way. The abrupt butting I envisioned as inefficient would be one with no taper at all: Simply thick wall --> thin wall with no gradual slope in between.

The tubing I proposed would simply smooth out the transition in wall thickness even more. Very instructive to see that tube makers have thought about that long before I did.

Even if all this is of no practical use it was very enlightening to think about tubes in terms of acoustic wave motion. It's nice to join one's favorite hobbies - in my case bikes and sound reproduction.
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Old 09-02-12, 11:43 PM
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Originally Posted by VeloBrox View Post
I didn't make myself clear, apologies. I never meant that the existing, tapered butting is abrupt in any way. The abrupt butting I envisioned as inefficient would be one with no taper at all: Simply thick wall --> thin wall with no gradual slope in between.

The tubing I proposed would simply smooth out the transition in wall thickness even more. Very instructive to see that tube makers have thought about that long before I did.

Even if all this is of no practical use it was very enlightening to think about tubes in terms of acoustic wave motion. It's nice to join one's favorite hobbies - in my case bikes and sound reproduction.
I know what you meant :-)
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Old 09-02-12, 11:44 PM
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Originally Posted by tuz View Post
What this also means is that the small discontinuities in tubing thickness from the butt-taper-thin transistions are small stress risers; tubes will eventually fail at those points. Kasei makes tubes with a continuous taper from the thick to the thin region. That may help a little bit?
Not to any meaningful degree.

And.

Really? I can see that being an absolute nightmare to draw.
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Old 09-03-12, 01:25 PM
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the butt transitions on tubes are really smooth. Hard to imagine that there would be any stress riser there. The likelihood that a crack will start at the edges of the heat affected zone (HAZ) dwarfs the chance that they will start at the butt transitions

As far as the OP goes, I have done some work in structural dynamics, and I find the frame structure somewhat uninteresting from that point of view. The bicycle itself is an interesting dynamic system, but I never seem to have the time to get into that too far.
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Old 09-03-12, 06:28 PM
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Haha well good to know.

For the Kasei tubes, I imagine they draw a tube normally but with a longer transition, then cut off the butt?
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