Rear Suspension, no pivot point - How does this work???- Mtbr.com
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  1. #1
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    Rear Suspension, no pivot point - How does this work???

    I realize this is a fundamental question that must have a simple answer, but I'm not seeing it at all. Here's an example of the type of suspension I'm talking about....


    This one happens to be a Dean, but I've seen the same thing on other bikes. It seems to basically be a fixed triangle rear, except there's a shock up top. I can't understand how there could be any travel at the shock without ripping the frame apart in short order.

    Please enlighten me.

  2. #2
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    They use flex in the lower stays to approximate a pivot. It does place a lot of stress on the BB-chainstay welds but if engineered properly it can last. It looks like about an inch of travel total.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
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  3. #3
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    it just flexes a bit. like less than half an inch so well designed frames will last.

  4. #4
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    called a "soft tail"

  5. #5
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    Quote Originally Posted by Dougal
    They use flex in the lower stays to approximate a pivot. It does place a lot of stress on the BB-chainstay welds but if engineered properly it can last. It looks like about an inch of travel total.
    Okay, that sorta makes sense. The engineer in me (not that I am one), says it's a bad design that will fatigue-fail in time. Do they do this design with AL frames at all, or just Ti? I wouldn't buy one in AL for sure - not thinking I feel comfortable buying a Ti frame of this design even.

  6. #6
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    Quote Originally Posted by DPDISXR4Ti
    Okay, that sorta makes sense. The engineer in me (not that I am one), says it's a bad design that will fatigue-fail in time. Do they do this design with AL frames at all, or just Ti? I wouldn't buy one in AL for sure - not thinking I feel comfortable buying a Ti frame of this design even.
    i think they use steel most commonly here.

  7. #7
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    Quote Originally Posted by b-kul
    i think they use steel most commonly here.
    That would make sense, as steel certainly has better fatigue resistance than AL - not sure how it compares to Ti, generally speaking.

  8. #8
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    If you want see a long travel (4+") pivotless travel frame look for a picture of an Ibis Bowtie . The frame was made in 97. You can most likely find a pic on Castello Desings site.

  9. #9
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    Quote Originally Posted by DPDISXR4Ti
    That would make sense, as steel certainly has better fatigue resistance than AL - not sure how it compares to Ti, generally speaking.
    Ti is even more flexible than steel, so ideal for this application. By some accounts the Ibis Bow-Ti pushed that theory a bit too far.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
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  10. #10
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    Quote Originally Posted by Dougal
    Ti is even more flexible than steel, so ideal for this application. By some accounts the Ibis Bow-Ti pushed that theory a bit too far.

    I thought I read somewhere that Ti is super hard and not very flexible and that is the reason you dont see bends in any Ti frames.

  11. #11
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    Titanium is roughly twice as flexible as steel (elastic modulus about half). If you've got a steel and titanium tube of the same dimensions then the ti tube will deflect twice as much for the same load.

    But because titanium has a high yield and a low elastic modulus it flexes a long way before it will stay bent and has a huge spring back. This makes it difficult to bend accurately.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
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  12. #12
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    Quote Originally Posted by Dougal
    Titanium is roughly twice as flexible as steel (elastic modulus about half). If you've got a steel and titanium tube of the same dimensions then the ti tube will deflect twice as much for the same load.

    But because titanium has a high yield and a low elastic modulus it flexes a long way before it will stay bent and has a huge spring back. This makes it difficult to bend accurately.

    I guess flexing and bending are different things. Makes sense.

  13. #13
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    Quote Originally Posted by mullen119
    I thought I read somewhere that Ti is super hard and not very flexible and that is the reason you dont see bends in any Ti frames.
    Don't tell the guys at Litespeed that!

    http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT

    I hate providing eBay links, but I don't find a picture of this frame anywhere else on-line. Here's the listed info...
    Litespeed Unicoi Titanium Frame in Small. Top tube ctc is 22". Top Tube effective is 21.75". Seat tube ctc is 17.5" Seat tube ctt is 19.5.

    Interesting how the seat tube dims would typically indicate a frame in the Medium to Large range. That bent top tube makes things a little weird.

  14. #14
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    All you have to do is consider the aluminium is already at its fatigued strength (which has a limit) and go from there. It would change performance until it got there though.
    Die to ride, ride to die

  15. #15
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    Quote Originally Posted by Bikesair
    All you have to do is consider the aluminium is already at its fatigued strength (which has a limit) and go from there. It would change performance until it got there though.
    Sorry, what?
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
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  16. #16
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  17. #17
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    Quote Originally Posted by DPDISXR4Ti
    Okay, that sorta makes sense. The engineer in me (not that I am one), says it's a bad design that will fatigue-fail in time. Do they do this design with AL frames at all, or just Ti? I wouldn't buy one in AL for sure - not thinking I feel comfortable buying a Ti frame of this design even.
    These have been doing well since the old Ibis days

    http://www.castellanodesigns.com/fango.html

  18. #18
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    Engineered flex.
    Works in coil spring shocks, in both steel and ti flavors. Ti springs use fewer coils to achieve the same spring load as steel.
    **** censorship

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