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  1. #1
    Senior Member Lexi01's Avatar
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    Setback or Straight Seatpost

    Hi Guys,

    I'm a tall rider and have found myself needing an extra little bit of height since moving from platform pedals to clipless.

    I'm 6'5" and have a XXL bike with a 873 standover and 620mm top tube and I have a 400mm seatpost extended to the limit (in fact about 5mm past the limit).

    I noticed that thomson make a 410mm post and was going to get one of those...then I saw they come in a straight and a setback. I'd like to go for the setback but had one question:

    When they're measured flat (top to bottom with a straight ruler or something) would they both be exactly the same height or would the setback be a little shorter due to the slight angle at the top?

  2. #2
    Senior Member Retro Grouch's Avatar
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    My GUESS would be they would measure the same. Why don't you email Thompson to ask them and be sure?

  3. #3
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    You do lose a bit of height, but only a tiny bit, in the range of a few millimeters.

    Thomson uses the same post, and makes a bend of 10-15 degrees or so (never measured it) 4" or so from the top, so there will be a drop, but it will be less than 0.10" (cosine of the angle x 3").

    Hopefully, you're not at a height where that would make any practical difference.

    BTW- if the math is an issue, ypu can sketch it out. Use a ruler and make a mark 4" up the edge (height of straight post to bend). Draw a reference line inward & make a mark 16mm (3/4") for the offset. Draw a line from there back to the bottom corner to make a triangle. Measure 4" up the new line, and see how far short of the reference line it comes. That's the drop.
    Last edited by FBinNY; 07-20-11 at 08:30 AM.
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  4. #4
    biked well well biked's Avatar
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    One other note: Thomson setback seatposts only have 18mm of setback, even though the bend in the post looks pretty dramatic. Most conventional setback seatposts have 20-25mm of setback.

  5. #5
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    Setback in regards to the bike has a plumb line thru the BB axis, as a zero point .
    so a tall post, long way from the crank axis , first off will be quite a ways apart
    at the top of the angle .

    the Knee over the pedal measurement is, to some extent, dependent on dimensions,
    length, of thigh and foot of the rider, plus setback .

    Get on the Bike and see how it all sorts out..

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    ^ We have a winner,no numbers/math involved....
    Everything should be as simple as possible...But not more so.---Albert Einstein

  7. #7
    Senior Member Lexi01's Avatar
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    Quote Originally Posted by FBinNY View Post
    Hopefully, you're not at a height where that would make any practical difference.
    Unfortunately I am. I have a 400mm post now that's sitting 10mm past the limit line. This Thomson post is 410mm...so an 1/8 of an inch may make a difference to me...hence this question.

    I actually found a BBB post called the Longscraper that's 450mm. I've ordered one from my LBS. That should be plenty. It's not a setback, but at 6'5" with an exceptionally long inseam I don't think I can afford to be choosy - my knees are killing me!

    Thanks for your replies everyone.

  8. #8
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    Will your position change if you have a set back or not? I've found that not a whole lot of people really need a setback unless (and this varies from case to case) they are mostly "all leg" and need to set themselves further back in their positioning. If that isn't the case... then a straight is fine

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