So I had this basically free defective frame...- Mtbr.com
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  1. #1
    Doesntplaywellwithmorons!
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    So I had this basically free defective frame...

    leftover from one of my ediscountbike ebay sprees. I had won two auctions for these frames, didn't do anything with them when they arrived, and two months later when I started assembling one, I discovered BOTH had a derailleur hanger that was drilled too large (and not tapped for the industry standard size derailleur hanger bolt). Ediscount refunded me all the money for the auctions, plus 60% of the shipping charges (when I got the frames, i'd combined shipping with some other items, which weren't defective) and told me to keep the frames. Traded one to a friend, and kept the other (the one which I'd previously began assembling to find the defect 1/4 of the way thru).

    Well lastnight I finally started to do something with that one I kept... after much trial and error, I found a singlespeed gear combo that worked perfectly (the dropouts angle downwards and forwards, so wheel removal/install is simple), which was unusual in size but I was limited by choosing to run a standard ring crank and being out of 34T rings in that pattern. So I went with a biopace 38T figuring on a SS, the extra torque from the biopace would be a good thing. Matched up to a 26T cog mind out. The bike goes up hills well and the chain didn't skip under load at all even standing up. It did derail after I got onto the peak of the hill and level ground, but I think it has to do with the crank's spider being slightly bent and the chain angling outwards once per revolution.

    <img src="https://206.75.155.18/kmr/bikes/neptune1.jpg">
    <img src="https://206.75.155.18/kmr/bikes/neptune2.jpg">
    <img src="https://206.75.155.18/kmr/bikes/neptune3.jpg">

    As pictured, 27.7 Ibs with magura rim brakes and durable/freeride-ish parts. The frame is straight gauge 6061T6 and weighs about 5.5 Ibs, the fork is extremely butted and gussetted chromoly, suspension corrected, and 3.75 Ibs, and well, the rest of the parts follow similar themes in weight/beefiness. Probably going to sell it in short order thought (I have a cluttered bike collection as is, and need some space in the basement for installing central A/C).

  2. #2
    Ride what you want!!
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    I didn't think biopace would work with a nontensioner SS.

    Hmmmm.

    george
    Trogs: Too Tough for Carbon Fiber

  3. #3
    Doesntplaywellwithmorons!
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    Quote Originally Posted by george_da_trog
    I didn't think biopace would work with a nontensioner SS.

    Hmmmm.

    george
    sure, its always a 38T ring afterall...regardless of where the crank is positioned, you're always engaging the same number of chain links. Just looks weird as you pedal.

  4. #4
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    Cool Bike

    Old School, reminds me of old AlpineStars. Pretty cool, looks like a fun ride.

  5. #5
    Probably drunk right now
    Reputation: Ken in KC's Avatar
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    Answer me this....

    Quote Originally Posted by DeeEight
    sure, its always a 38T ring afterall...regardless of where the crank is positioned, you're always engaging the same number of chain links. Just looks weird as you pedal.
    First, your contention that it's still 38 teeth doesn't make sense: Move the chainring forward by 1 foot and the # of teeth remains unchanged.

    As I understand biopace, it's basically built around an eccentric in order to maximize pedal stroke. Biopace on a SS without a tensioner would change the required chain length depending upon where the crank was in its eccentric rotation.

    What am I missing?



    Ken

  6. #6
    Whiskey Tango Foxtrot
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    I'm not a scientist, but...

    I think it works because the eccentricity of the ring is constant and equivalent.

    As the leading edge of the ring extends forward toward your front wheel, increasing chain tension, the top and bottom of the ring effectively "shrink" and the distance between the two gets shorter, decreasing chain tension. If this happens at the same rate, the variance cancels itself out.

    If that makes sense, I'll be quite pleased with myself.

  7. #7
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    Quote Originally Posted by Ken in KC
    First, your contention that it's still 38 teeth doesn't make sense: Move the chainring forward by 1 foot and the # of teeth remains unchanged.

    As I understand biopace, it's basically built around an eccentric in order to maximize pedal stroke. Biopace on a SS without a tensioner would change the required chain length depending upon where the crank was in its eccentric rotation.

    What am I missing?



    Ken

    Or keep it centered but radically change the shape (triangle or ellipse with large eccentricity) and clearly the path length will change with orientation. The real question is how much deviation matters? Biopace may not be that much different from a centered circle in terms of path length deviation to matter but it can't be zero.
    "The mouth of justice contemplates wisdom."

  8. #8
    Doesntplaywellwithmorons!
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    This was covered in another thread in here months ago, regardless of the degree of the ellipse (older first gen biopace are visibly more oval than second gen biopace), its ALWAYS engaging the same number of chain links and since the space between the teeth is an industry constant, there is no change in the chain tension.

  9. #9
    Probably drunk right now
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    ???

    Quote Originally Posted by DeeEight
    This was covered in another thread in here months ago, regardless of the degree of the ellipse (older first gen biopace are visibly more oval than second gen biopace), its ALWAYS engaging the same number of chain links and since the space between the teeth is an industry constant, there is no change in the chain tension.
    I'm not intentionally being an ass, I simply don't understand this explaination. Distance between the teeth doesn't explain what is in effect, a change in the overall chain length.

    In overly simplistic terms, if you hold a chainring/rear cog/chain assembly in your hand and increase or decrease the amount of space bewteen your hands, the number of teeth remains constant while the overall chain length changes.

    I realize the you may think I'm being obtuse. While I certainly am in most cases, I'm not trying to be difficult, simply to understand.

    Since the Biopace operates on an eccentric, it seems to me that the chain length changes based on where the chain happens to be on the eccentric.

    Help me get my learn on.

    Ken

  10. #10
    try driving your car less
    Reputation: jh_on_the_cape's Avatar
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    Quote Originally Posted by Ken in KC
    I'm not intentionally being an ass, I simply don't understand this explaination. Distance between the teeth doesn't explain what is in effect, a change in the overall chain length.

    In overly simplistic terms, if you hold a chainring/rear cog/chain assembly in your hand and increase or decrease the amount of space bewteen your hands, the number of teeth remains constant while the overall chain length changes.

    I realize the you may think I'm being obtuse. While I certainly am in most cases, I'm not trying to be difficult, simply to understand.

    Since the Biopace operates on an eccentric, it seems to me that the chain length changes based on where the chain happens to be on the eccentric.

    Help me get my learn on.

    Ken

    it's freaky but it works. biopace on the SS! i tried it (old road bike). didnt think it would, then someone on this board told me otherwise and VOILA! it's that the average diameter is constant, i.e. the front half which engages the chain always has the same length. so when the big part of the ring is forward, there are smaller parts on the top and bottom that exactly compensate, and round and round it goes. it's a space age technology developed by a group of japanese gnome-droids partying one night in the hills outside of osaka while all hepped up on goofballs. one of them had gotten his nuts caught in the derailluer of a passing mtn biker named SHizmano and his nards were all bloodied and out of round. but they miraculously felt super comfy and ergonomic when being fondled by the concubine she-gnomes. and biopace was born. but then it died.

    and for that bike: you have the 'TOP IMAGE' now! that thing needs some stickers or a rattle can fast! how about a sticker that says 'I am david hasselhof's secret gnome lover' even that would be better than what you got going on.

    in other news, jet blue is taking me to downieville this weekend! woohoo!
    Only boring people get bored.

  11. #11
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    Quote Originally Posted by DeeEight
    This was covered in another thread in here months ago, regardless of the degree of the ellipse (older first gen biopace are visibly more oval than second gen biopace), its ALWAYS engaging the same number of chain links and since the space between the teeth is an industry constant, there is no change in the chain tension.
    It's easy to see with a simple drawing that as the eccentricity goes to 1, you get different path lengths depending on the orientation of the major and minor axes. That difference will only go to zero as the eccentricity goes to zero, in other words, when the ellipse becomes circle.

    In fact, here's what they are (eccentricity->1, major axis=y, "chain stay length"=x).
    Major axis vertical: path length = 2*sqrt(x^2+y^2) + 2*y
    Major axis horizontal: path length = 2*x + 2*y

    I still maintain that the path length isn't constant but that the eccentricity is probably small enough to not make any practical difference.
    "The mouth of justice contemplates wisdom."

  12. #12
    Probably drunk right now
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    Lightbulb!

    Quote Originally Posted by Mr.Bob
    It's easy to see with a simple drawing that as the eccentricity goes to 1, you get different path lengths depending on the orientation of the major and minor axes. That difference will only go to zero as the eccentricity goes to zero, in other words, when the ellipse becomes circle.

    In fact, here's what they are (eccentricity->1, major axis=y, "chain stay length"=x).
    Major axis vertical: path length = 2*sqrt(x^2+y^2) + 2*y
    Major axis horizontal: path length = 2*x + 2*y

    I still maintain that the path length isn't constant but that the eccentricity is probably small enough to not make any practical difference.
    Thank you. It's now more clear. Basically this is what JH said but using physics and without the bloody nuts.

    Thanks for the help and splaining things for me.

    Ken

  13. #13
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    Quote Originally Posted by Ken in KC
    Thank you. It's now more clear. Basically this is what JH said but using physics and without the bloody nuts.

    Thanks for the help and splaining things for me.

    Ken
    I wonder if the biopace shape is truly constant path length (a pure ellipse isn't) or if it's just close enough. If it is, it's a very interesting shape. Anyone have a mathematical description of it? Yes, I'm a dork .
    "The mouth of justice contemplates wisdom."

  14. #14
    try driving your car less
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    Quote Originally Posted by Mr.Bob
    I wonder if the biopace shape is truly constant path length (a pure ellipse isn't) or if it's just close enough. If it is, it's a very interesting shape. Anyone have a mathematical description of it? Yes, I'm a dork .
    it's not an ellipse, but a "point-symmetric egg curve". check out
    http://www.sheldonbrown.com/biopace.html
    it is a very very constant chainlength. it does not bind at all when i turn the cranks. or binds just as much as a round chainring due to eccentricities in the freewheel, how the chainring is bolted on, etc...
    it's not hard to imagine a shape that will do this, really.
    if you are a dork, you will like this... google 'David Huffman origami'
    Only boring people get bored.

  15. #15
    Tear it all out! SuperModerator
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    It does make sense from a design standpoint as well. When Shimano was figuring this out, they designed the biopace rings to have a consistent apparent chain length, otherwise the derailleur tension would be cycling back and forth 2 times every crank rotation, and I'd expect that would cause premature derailleur tension spring fatigue.

  16. #16
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    In theory, the overal chain tension should remain constant with the biopace ring. As somebody else explained, when the larger radius of the ring is forward, the smaller radiuses on the top and bottom should make up for most of the difference. But, I dont think thats quite accurate. CraigH mentioned the that it would make sense for Shimano to design these rings to have constant chain tension, but my observations on all my Biopace equipped bikes tell me otherwise. When you sping the cranks backwards, the derailleur cage is moving back and forth, adjusting for the difference in tension. Albeit, it was a small movement, it was distinct. All these bikes have the first gen. biopace, so Im sure the Gen. 2 rings dont have this problem, if you want to call it a problem. I dont think the slight amount of un-roundness would really hurt anything. So, that brings me to my next question, what is the smallest size Biopace ring available for a 110mm BCD? I wouldnt mind finding a 34 or 36 for my singlespeed.

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