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Stirz007
| | Posted on Tuesday, November 30, 2010 - 04:16 pm: | 
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Let's say you only had one "hinge" that went from one side of the swingarm to the other (full width, continuous "hinge pin"). Now, chop out the middle section, resulting in two hinges with a common rotating axis. If you assume the hinge itself only works in one plane and does not twist (much), and that the hinges have characteristics that allow them to act as torsion springs, then this is the general idea that I think is being floated.
Why wouldn't it just twist? - the simplistic answer could be "for the same reasons conventional swingarms don't twist" (much). The hinge (torsion bar, bearing, whatever) only allows one degree of freedom (up and down motion perpendicular to hinge pin orientation). For one side to move up and the other down, twisting would have to be possible at both hinges and in the swingarm itself - this does happen, but only within a small tolerance, depending upon the stiffness of swingarm, bearing design, blah blah blah.
There's lots of ways to skin that cat, but the conventional wisdom seems to be with the hinged swingarm and coil over shock arrangement we all know and love - of which there are lots of variations (mono shock, two shocks, torsion bar/shock, one sided swingarm, etc.). Personally, my take is that this string seems to have evolved into an exercise in solving a problem that does not currently exist - the epitome of engi-geekness (AKA the Dilbert Effect).
The original post piqued my interest, but then I decided it was just an idea. Mufflers are a replacement item and have weight, so if the design is implemented the swingarm just got heavier (which would affect handling), you got that funky flex thing going on, and now when your muffler rots out, you get to replace the entire swingarm assembly. My vote is "if it ain't broke, don't fix it" in this case. Nonetheless, ideas often give rise to other ideas (that may be a better solution).
As technology evolves, the torsion hinge may be do-able. Add things like electroviscous damping, or even materials that can change stiffness or energy absorption characteristics via microprocessors, then maybe the elastic hinge will be common. After all, how many solutions have been developed for other relatively simple problems? (like catching mice?) |
Hootowl
| | Posted on Tuesday, November 30, 2010 - 04:56 pm: |
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So the hinge material is spring steel or something similar? For some reason I was thinking something much more flexible. Probably why I saw problems that aren't there. |
Blake
| | Posted on Tuesday, November 30, 2010 - 05:02 pm: |
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Jeff,
Think of it as a leaf-spring clamped between two solid parts. It offers springiness in bending about one axis only. The relatively short length and solid section with high torsional stiffness would hopefully (I'd have to run numbers and know design loads to verify) prevent significant torsional flexure. If not, it seems like some simple mechanical stops or torsionally stiffer design or the like might be employed to augment torsional stiffness, if required.
With the bike sitting at rest or cruising easy, I can't see the "Z" issue you ask about. Maybe it could occur under hard acceleration or braking. Not sure it matters. Damper location and orientation might be critical. |
Blake
| | Posted on Tuesday, November 30, 2010 - 05:15 pm: |
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Stirz,
>>> Now, chop out the middle section,
Why?
>>> If you assume the hinge itself only works in one plane about one axis and does not twist (much), and that the hinges have characteristics that allow them to act as torsion bending springs
Good description with editing. 
>>> conventional wisdom seems to be with the hinged swingarm and coil over shock arrangement
"Seems to be"?
>>> this string seems to have evolved into an exercise in solving a problem that does not currently exist - the epitome of engi-geekness (AKA the Dilbert Effect).
The idea of the flex hinge was to eliminate the bearings to allow high temperature exhaust a less obstructed passage from engine into swingarm.
>>> so if the design is implemented the swingarm just got heavier (which would affect handling),
While the exhaust system got lighter. An overall weight reduction is likely, given that one part is performing two functions.
>>> when your muffler rots out, you get to replace the entire swingarm assembly.
What if the muffler never "rots out"?
>>> That is probably an accurate statement
Thank you for fact-checking my commentary, but "probably" is a weak endorsement.
>>> Old as mouse traps, anyway.....
Door hinges were made of leather in some ancient cultures. |
Hootowl
| | Posted on Tuesday, November 30, 2010 - 05:40 pm: |
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The Z thing was when I was thinking of a much more limber material. I don't know why but when I saw your drawing, I immediately thought about a rubber material. Sometimes I wonder how I dress myself in the morning. Carry on. |
Blake
| | Posted on Tuesday, November 30, 2010 - 06:08 pm: |
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You make great beer. All the rest is thus forgivable.  |
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