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Input? On a Frame Design I Have Been Playing With.

Discussion in 'Downhill & Freeride' started by FCLinder, Dec 29, 2005.

  1. FCLinder

    FCLinder Turbo Monkey

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    Ok, I know there has been a lot of Monkeys throwing frame designs up for show lately. I just want some input on what I have been working with.

    The frame is designed much like BCD,s design and works off a 2 shock platform. The difference is I am not using a gear box and the frame is setup for a lower center of gravity feel.

    The concept suppose to work like this. Two 2" stroke air shocks combined to give 4" of shock stroke at lighter weight. The two shocks are setup on a 45 degree access off the main pivot. When the rear triangle sets in to motion the top shock will compress to give the first 5"s of travel. When that shock is fully compressed it will push on the lower shock and get another 5"s of travel. Reasons behind the two shock setup are one, you can fully adjust your travel at higher and lower compressions. Two, the rear triangle will have two movement patterns. This will give a much better fill when hitting a bump and track better too.

    Like I said I am just playing around with the idea right now. Any input will be nice.
     

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  2. Superdeft

    Superdeft Monkey

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    This resembles a bike from another thread, using 2 shocks and 2 axle paths, can anyone help me remember what the name is?
     
  3. berkshire_rider

    berkshire_rider Growler

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  4. JAB

    JAB Chimp

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    I would think you'd need to put anonther brace between the rear end and main frame, because right now the bushings on the upper shock looks like they will wear out super fast (probably loose air preasure too) due to flex.

    If you haven't designed a suspension frame before I would strongly recommend you start with a single pivot. Not dissin' but that is in my opinion the best place to start...
     
  5. OGRipper

    OGRipper Turbo Monkey

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    Well he said it's designed much like BCD's so you are probably thinking of the BCD 2X4. :)

    The difference (in concept) anyway is that as I understand the BCD, both shocks are able to move at once, creating a zone of axle travel rather than a specified wheel path. Based on your description that is not what you are going for, but looking at your design it might actually work that way depending on the shock set up.

    It seems the second stage shock would need to be set up with a very high threshold to avoid actuating before the first stage shock is fully compressed...hmmm, cool design but if you don't intend a "zone" of axle path this seems like an overly complicated way of getting two-stage shock characteristics.
     
  6. JAB

    JAB Chimp

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    By the way is this a lurker thread??? Me: Feb 2004 with 51 posts, thread starter: Mar 2002 with 106 posts...
     
  7. FCLinder

    FCLinder Turbo Monkey

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    I have not put it in a CAD design yet. Like I said, I am playing around with the idea. When I put it in CAD, then I may learn more about the path of the rear axle.
     
  8. FCLinder

    FCLinder Turbo Monkey

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    Dude, what the F are you talking about?
     
  9. allsk8sno

    allsk8sno Turbo Monkey

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    looks to me like you will have major interference issues if the vertically mounted shock cycles anytime before the horizontal shock is fully compressed...and yeah a double link off where the horizontal shock mounts might add some stiffeness/bearing life
     
  10. FCLinder

    FCLinder Turbo Monkey

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    Thanks for the input.
     
  11. Jeremy R

    Jeremy R <b>x</b>

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    Johnny Number 5 needs MORE INPUT.
     
  12. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    If you look at the BCD 2x4 you will notice that the two pivots are a long way apart (one being around the BB shell, the other one being quite high above it and slightly behind it). This means that the wheel has a considerable rearwards-movement capability. Both your pivots are relatively low and close together (as well as being almost in line with each other and the axle), and as such I am inclined to think that you would not gain as much in terms of bump absorption as the BCD design (the vertical range of axle position may be similar but the horizontal range would not be anywhere near as big). Due to the closer pivots though, the handling and pedalling would probably be more predictable than the BCD.

    Also you will note on the BCD that the "secondary" swingarm (the one attached to the wheel, with the higher pivot) pushes the shock back against the other swingarm to which it is mounted, rather than the front triangle as yours does. This again allows for more independent movement... but once again the tradeoff would probably be the handling characteristics.
     
  13. ÆX

    ÆX Turbo Monkey

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    it looks good.

    if what you are going for is a 4'' stroke shock to get low leverage,
    weight, and tun-ability.

    if you are looking for max ability for your tire to stay in contact
    with the ground pivot separation like thaflyinfatman said is key.

    cool to see other iterations come along.

    looks stiff enough to me.

    chainstay length might be hard to get under 17.

    you going to weld one up?
     
  14. FCLinder

    FCLinder Turbo Monkey

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    Thanks man, glad to get some input from you. I do have a friend that is a good at welding and fab work. He told me to come up with the CAD designs and he will build a Prototype for me. I am still working on all the angels and where to put the pivots. The drawing is some thing I put together on the Drive to NJ over Christmas. I do want to make sure the bike tracks well and has no pedal feed back. Any feedback you may have would be greatly appreciated.

    Cecil
     
  15. Jeremy R

    Jeremy R <b>x</b>

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    .........

     
  16. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Again the only way to have *no* pedal feedback (chain extension) is to do it the way BCD has, with one BB-centric pivot, and a chain pulley that is concentric to the other pivot. Not to say you couldn't create a design that has unnoticeable amounts of pedal feedback without doing that however.
     
  17. SCABRIDER

    SCABRIDER Monkey

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    it would never work imo...
     
  18. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Then your opinion is wrong.
     
  19. zedro

    zedro Turbo Monkey

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    i just cant help feeling that a simpler high pivot design will achieve the same basic results, although being able to 'tune in' big-hit appeal vs. short chainstay handling is kinda interesting, just not sure if it goes beyond the theoretical benefits.

    But then again just making something different kicks way more ass than the toe-the-line-i-only-buy-from-the-store lame-o nay'sayer crowd :D
     
  20. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    I tend to agree here. Not only that, but you can tune the pedalling and braking characteristics far more easily/accurately with a fixed axle path.

    Personally I am inclined to think that traction (the only real advantage of having such good bump absorption IMO) is not THAT critical in this regard. I think the gains in handling/pedalling characteristics given by a fixed axle path would be more beneficial than the increase in traction on really rough terrain.
     
  21. dw

    dw Wiffle Ball ninja

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    If you want to have zero pedal feedback, then you have to use either zero chanistay length change and a singlespeed with the same sprockets front and rear.

    In order to have zer pedal feedback with more conventional gearing, you have to have some amount of chainstay lengthening effect.

    With conventional gearing (bigger sprocket in front) and zero chainstay length change, your pedals will rotate forward with suspension compression, and kick back under suspension extension.

    Dave
     
  22. thaflyinfatman

    thaflyinfatman Turbo Monkey

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    Hmm good point, I hadn't actually thought of that. So the axle path needs to be perpendicular to the chainline in order to maintain zero pedal feeback under suspension compression/extension, correct?
     
  23. dw

    dw Wiffle Ball ninja

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    I will start off with saying again that I will buy a 2X4 one day, not for a performance reason, but because I think it is a cool bike, hand made by a friend.

    The one challenge with this type of design is that there is no way to plan any prescribed amount of anti squat through the travel (to a point where it would be helpful to try). you can plan a range, but because the wheel can move in almost any direction, that range will vary wildly from massive pro-squat to massive anti-squat.

    Additionally, it is difficult to control the rear wheel axle path during cornering, and this forces the rider to either move their center of mass with trail imperfections to maintain traction, or go slowler to maintain traction. Human reaction times dont really allow the first scenario, so we are stuck with #2. For most of us reading this its no big deal, we aren't riding at the level that it would matter, but for Sam Hill, Fabien, Rennie, it's a big deal becuase they are almost always over the edge of traction.

    Additionally, unless you have your sprockets pivoting at both pivot axis like the 2X4, you can never have the same amount of pedal feedback for the same bump twice. The bike will always have a different "feel" at the pedals. Personally, I think the widely held opininon that pedal feedback is unsettling is overrated anyways, but thats my opinion. As long as you can't feel it, how much does it matter? The issue with varying pedal feedback is that your body can feel it. So to clarify, I'm saying that the existence of pedal feedback is not as much of an issue as the PERCEPTION of pedal feedabck. A minor point for so much writing. :) heh

    So you trade those factors off with the ability of the wheel to move rearward with most bumps up to a point. If you start looking at transmission angles of forces through your wheel and reaction rates of suspension components, and then couple that with what types of travel you need at different points in the rear wheel travel, and how you are making up time on a racecourse or comport or fun on a trail, then you can maybe start to decide whether the tradeoff is worth it or not.

    Bike will look cool, and different, thats another plus for some I think too.

    Good luck Cecil, have fun!

    Dave
     
  24. dw

    dw Wiffle Ball ninja

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    Well, not really, pedal feedback is a function of chainstay length change plus or minus the amount of chain length change you have for a given angular change of travel. If you search in the archives I have documented this with pictures pretty well a few years ago.

    Dave