One up dropper - Short Stack- Mtbr.com
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  1. #1
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    One up dropper - Short Stack

    I'm a little confused about the one up dropper posts.

    One of the key selling points seems to be the shortest insertion and stack height of competing posts.

    I'm looking at moving from a 125 to a 150mm post on my spec enduro.

    Been running the numbers and something does not add up with these posts.

    I'm comparing a 125mm xfusion post to a 150mm one up V2.

    The stack heights are

    xfusion @ 125mm 178mm
    one up @ 150mm 183mm

    In reality you're only really getting 5mm of additional clearance despite the 25mm increase in drop. The shorter stack at the saddle clamp means most of the benefit of the additional drop is offset by needing to have your post set higher to achieve the same extended height.

    Am I missing something?

    If I compare this to the KS Lev SI @ 150mm it has a stack of 200mm which would give me 22mm more clearance than the xfusion and 17mm more clearance than the one up at the same travel.

  2. #2
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    What do you mean by clearance?

    Stack height is drop plus the distance of the bottom of the stanchion collar to the rails when fully dropped. The smaller the collar-rails number the more drop you can fit between your butt and seat post clamp at optimal pedaling height. By your numbers One up is 33mm and xfusion is 53 mm. One-up has the potential to fit more drop.

    The other limitation in insertion depth of the seat tube. The post bottoming in the seat tube can also limit the drop you can fit.
    Do the math.

  3. #3
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    Quote Originally Posted by crembz View Post
    Am I missing something?
    Yes. Primary being the amount of extended seatpost required for you on your particular frame. That number doesn't change from one seatpost to another, but it is a huge determinant of which post offers the best fit if you're following the plan of fitting as much drop into your frame as you can. That's going to determine how high above the seat clamp the collar must be to achieve a given seat height with any particular post.

    It almost sounds to me like you're assuming that all posts would be slammed collar of the dropper to the seat clamp. That's not going to happen if you set everything up right. If your xfusion is currently slammed, you will not be able to run a 150mm oneup at full drop.

    Taking Lone Rager's measurements (33 vs. 53mm) to the next level, you have to consider that there's 25mm of extra drop on the oneup PLUS 20mm of difference in the compressed collar-rails dimension. That means that if the max saddle height is the same, the oneup post is going to be MORE THAN 25mm lower than the xfusion.

    How much is a little muddy, since the 33/53mm dimensions are split between the post head and the collar height. The difference between post head heights will make for a greater difference in actual seat height in the dropped position. Differences in collar height won't affect difference in dropped saddle position, but it may affect post insertion.

    Assuming both posts are slammed to the collar in the frame, this means that a 150mm oneup post will both be taller at full extension (by 5mm) AND lower at full drop (by 20mm) than the xfusion.

  4. #4
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    Yes so let's go with the 33 vs 53mm measurement. That would mean that @33mm (all other things equal) the saddle height is going to be lower than at 53mm by 20mm. To compensate for that, you'd need to raise the collar by 20mm to achieve the same extended seat height. Its that not the case?

    So in reality, your 25mm of added drop yields 5mm of additional saddle clearance once the saddle is dropped.

  5. #5
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    The significance of the 33 vs 53 is that with 33 you might be able to fit a post with more drop. At max extension you want the saddle to be at optimal pedaling height. At that height, you need to fit the drop and stack between the rails and post clamp on the seat tube. For example, with a 53 stack, you might only be able to fit a drop of 125mm. With a 33 stack, you might be able to fit a 150mm drop. So less stack can, but not always, buy you the space for more drop.
    Do the math.

  6. #6
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    Yeah so my point is what's the point of having additional drop if you need to compensate the lower stack by raising the collar to achieve the correct extended saddle height? I suppose this would be a benefit if your post is already jammed into the seat tube as far as it can go.

    If not you may be better off with a post that has more stack, especially on the seat rail end.

  7. #7
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    Quote Originally Posted by crembz View Post
    ...If not you may be better off with a post that has more stack, especially on the seat rail end.
    While more stack might be acceptable in some situations, I can't think of a situation where more stack would ever be desirable or useful.
    Do the math.

  8. #8
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    Ok here's an example.

    I have 2 posts. One is the 125mm xfusion with 53mm stack. At full extension it is 178mm from the collar to the rails

    The other is a 170mm one up with 33mm stack. 203mm at full extension.

    In order to achieve the correct riding height of 240mm gives me a difference of the collar height from the top of the seat tube of roughly 25mm. Yes the one up drops lower by 25mm. However it has 45mm more travel, so half of the additional travel is lost. I've measured this up on my bike to confirm.

    Now my other bike would only fit a 150mm one up due to an interrupted seat tube. Despite 25mm more travel than the xfusion, the actual dropped seat height would be almost the same as the 125mm xfusion.

    I believe this is because the stack is shorter at the rail end rather than the collar end. If the stack was shorter below the drop rather than above I think the result would be different.

  9. #9
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    Quote Originally Posted by crembz View Post
    Ok here's an example.

    I have 2 posts. One is the 125mm xfusion with 53mm stack. At full extension it is 178mm from the collar to the rails

    The other is a 170mm one up with 33mm stack. 203mm at full extension.

    In order to achieve the correct riding height of 240mm gives me a difference of the collar height from the top of the seat tube of roughly 25mm. Yes the one up drops lower by 25mm. However it has 45mm more travel, so half of the additional travel is lost. I've measured this up on my bike to confirm.
    In this case, the collar height doesn't play in. If the ride height is 240mm the OneUp will be at 70mm when lowered and the X-fusion 115mm when lowered. That's a 45mm difference.


    Quote Originally Posted by crembz View Post
    Now my other bike would only fit a 150mm one up due to an interrupted seat tube. Despite 25mm more travel than the xfusion, the actual dropped seat height would be almost the same as the 125mm xfusion.

    I believe this is because the stack is shorter at the rail end rather than the collar end. If the stack was shorter below the drop rather than above I think the result would be different.
    Here the dropped OneUp would be 25mm lower than the X-fusion. The datum is always the raise saddle height because that is fixed by your optimal saddle position.

  10. #10
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    Thanks mate, Yes you're right, I've just modeled it all in a spreadsheet. I think I've been over thinking this one.

    For the record I've had a one up 170mm V1 dropper for a while and have just ordered a V2 180mm

    I do have a question though. The V1 had 50mm adjustment range via the plastic shims. The V2 has a much shorter adjustment range. Any particular reason this was changed in the V2?

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