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Old 07-19-11, 07:09 PM   #1
Lexi01
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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?
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Old 07-20-11, 07:55 AM   #2
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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?
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Old 07-20-11, 08:17 AM   #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.
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Old 07-20-11, 08:27 AM   #4
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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.
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Old 07-20-11, 08:45 AM   #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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Old 07-20-11, 09:35 AM   #6
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^ We have a winner,no numbers/math involved....
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Old 07-20-11, 08:29 PM   #7
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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.
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Old 07-21-11, 12:20 AM   #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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