Cutting seatpost

Discuss light weight issues concerning mountain bikes & parts.

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jacobHansen
Posts: 34
Joined: Fri Apr 26, 2013 10:58 am

by jacobHansen

Hello guys,

Im currently making my bike lighter, so... How much Can i Cut my seatpost if i want to save those few grams?

Friend of mine Said that i need to have 8cm of seatpost inside the grams When it has been Cut to my leg length, what is your opinion?


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Attermann
Posts: 237
Joined: Sun Jan 05, 2014 11:04 pm
Location: Denmark

by Attermann

why don't you just measure how much your seat posts need? from the bottom to the minimum line, and then cut after that, my carbon rose post needs 10 cm's :)

by Weenie


jacobHansen
Posts: 34
Joined: Fri Apr 26, 2013 10:58 am

by jacobHansen

Cause I wanted to be sure Thanks! Will do


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DanW
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Location: Here, there and everywhere

by DanW

Also check the frame's documents about how much seatpost needs to be in the frame. Some frame's need more than others although 10cm seems quite normal.

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Juanmoretime
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Location: Urbana, Illinois

by Juanmoretime

What Danw said X 2. You want the seat post to extend below the junction of the top tube to the seat tube.
RESIDENT GRUMPY OLD MAN.

xc71
Posts: 102
Joined: Fri Oct 14, 2011 4:36 am

by xc71

There is a formula alot of the bike manufactures use: 2.50mm x seatpost diameter.
e.g. 2.50mm x 31.6mm = 79mm of insertion.

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dwaharvey
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Location: USA

by dwaharvey

xc71: that seems a bit odd. Why would a 27.2mm post need less insertion than a 31.6mm post (and taken to the extreme it clearly seems nonsensical)?

As others have said the min-insertion should be the larger of the seatpost's min-insertion, and what the frame requires (normally bottom of post passing the toptube-seattube junction, but in some cases like Ti frames where there's a seatpost insert, basically just reaching to the bottom of that insert since anything below that is just hanging in space).

xc71
Posts: 102
Joined: Fri Oct 14, 2011 4:36 am

by xc71

dwaharvey wrote:xc71: that seems a bit odd. Why would a 27.2mm post need less insertion than a 31.6mm post (and taken to the extreme it clearly seems nonsensical)?

As others have said the min-insertion should be the larger of the seatpost's min-insertion, and what the frame requires (normally bottom of post passing the toptube-seattube junction, but in some cases like Ti frames where there's a seatpost insert, basically just reaching to the bottom of that insert since anything below that is just hanging in space).

Got this info straight from one of the owners of Ibis bikes. It is the exact formula they use for their frames/seatposts.

DanW
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by DanW

Such a formula might have little relevance to other frames though which the OP should be aware of. Frames which have a brace between the top tube and seattube often require longer insertion for example. As before, the best thing to do is check with the frame manufacturer.

mattr
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by mattr

xc71 wrote:Got this info straight from one of the owners of Ibis bikes. It is the exact formula they use for their frames/seatposts.
so that would be one bike manufacturer, rather than lots.

Personally, as long as it extends below the top tube cluster, or to the end of the insert/reamed section of seat tube, you should be Ok. For most frames this is going to be about 10-12 cm.

xc71
Posts: 102
Joined: Fri Oct 14, 2011 4:36 am

by xc71

mattr wrote:
xc71 wrote:Got this info straight from one of the owners of Ibis bikes. It is the exact formula they use for their frames/seatposts.
so that would be one bike manufacturer, rather than lots.

Personally, as long as it extends below the top tube cluster, or to the end of the insert/reamed section of seat tube, you should be Ok. For most frames this is going to be about 10-12 cm.

No, he mentioned several other manufactures use the same formula and they had adopted the same formula.

mattr
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by mattr

I suspect he was either a) feeding you a line to shut you up or b) dressing a vague rule of thumb up as a calculation based on mechanical principles.

Cos 2.5 times diameter would only apply to one specific design of frame, in one particular material. Unless he was just talking about the seatpin (not the frame at all) which is a bit daft, as the insertion requirements for the frame take precedent.

You may well find it is just a rule of thumb minimum, i.e. 2.5 times diameter rounded up to the nearest whole inch. A lot of "design rules" eventually come down to this once you ask the right questions.

xc71
Posts: 102
Joined: Fri Oct 14, 2011 4:36 am

by xc71

mattr wrote:I suspect he was either a) feeding you a line to shut you up or b) dressing a vague rule of thumb up as a calculation based on mechanical principles.

Cos 2.5 times diameter would only apply to one specific design of frame, in one particular material. Unless he was just talking about the seatpin (not the frame at all) which is a bit daft, as the insertion requirements for the frame take precedent.

You may well find it is just a rule of thumb minimum, i.e. 2.5 times diameter rounded up to the nearest whole inch. A lot of "design rules" eventually come down to this once you ask the right questions.

Too funny, Ibis had also posted the same info on their Forum for other customers inquiring about seatpost insertion.
At any rate Jacob, your frame manucturer will give you the min. insertion number. I'm 1mm difference than your buddy's suggestion if you have a 31.6mm seatpost.

by Weenie


jacobHansen
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Joined: Fri Apr 26, 2013 10:58 am

by jacobHansen

Thanks guys, good Info :)


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