carbon seatpost length? how much do you need in the seat tube?

Discuss light weight issues concerning road bikes & parts.
Catagory6
Posts: 366
Joined: Thu Feb 09, 2017 2:36 am

by Catagory6

want to cut down a carbon seatpost, but it doesn't have a minimum insertion indication

is there a a ratio, or percentage of how much needs to go in, depending on how much extends above the clamp?

by Weenie


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C36
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by C36

Catagory6 wrote:want to cut down a carbon seatpost, but it doesn't have a minimum insertion indication

is there a a ratio, or percentage of how much needs to go in, depending on how much extends above the clamp?
Use to have it marked on the posts and the frame (min insertion may be different).
On the Cannondale super six evo2 that’s 90mm.
The older rule of thumbs as 3x the diameter (in mech construction its considered as a “long-guide”).


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Lelandjt
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by Lelandjt

The bottom of the post must extend past the bottom of the top tube junction, but only by a few mm.

mattr
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by mattr

Lelandjt wrote:
Mon Sep 17, 2018 5:30 am
The bottom of the post must extend past the bottom of the top tube junction, but only by a few mm.
Thats about 4-5cm on most of my bikes..........

I usually work on about 10cm to be on the safe side. Most marked minimum insertions i've measured over the years have been around that. Most shims are also about that long as well, as is the reamed/butted depth of *most* frames (the ones i've bothered to check anyway).

Going much longer than 10cm will just leave you extra unsupported post waggling around in an open tube. It also gives you a small margin for a lower profile saddle as well (most saddles are within a ~15mm range).

sungod
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by sungod

some ww cf posts are designed to be clamped in a certain area (ax for instance), so if you took a long one and lopped the end off, the post might not be as strong in the clamping area

which post is it? if it's a 'generic' design without any variation in the cf it'd probably be ok to cut as above

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Lelandjt
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by Lelandjt

mattr wrote:
Mon Sep 17, 2018 6:49 am
Lelandjt wrote:
Mon Sep 17, 2018 5:30 am
The bottom of the post must extend past the bottom of the top tube junction, but only by a few mm.
Thats about 4-5cm on most of my bikes.
But it's longer on other bikes. That's why seatposts designed to fit any frame have longer insertion lengths. Think about it and you'll realize that the part of the post extending past the reinforced part of the frame isn't doing anything. On most frames the reinforcement is the top tube junction. Some full suspension MTBs lack this junction so their reinforcement is a thicker part of the tube, which is hard to pick out. On those you use the 90 or 100mm recommendation to be safe.

mattr
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by mattr

Lelandjt wrote:
Mon Sep 17, 2018 3:56 pm
Think about it and you'll realize that the part of the post extending past the reinforced part of the frame isn't doing anything.
I have thought about it. Which is why i usually go for about 10 cm. Which gives me either coverage to go past the top joint of any frame, or to the end of any shim/reaming/butting on any frame. Whichever is longer, priority is the shim/reaming/butt length. As this gives full support/minimum load to the seatpin/frame. It also usually gives some adjustment for pedal/shoe system changes, saddle changes, seasonal changes etc.

wallstbear
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Joined: Sun Jun 17, 2018 4:17 am

by wallstbear

I have spent a lot of time on ths question and talked to maltiple bicycle engineers.

You need to ask your self a question first: are you more worried about the seatpost breaking or the frame seat tube snapping?

This is critical because the answer affects how much seatpost you need.

Most frames are actually reinforced in the seat tube collar section and tend to be pretty solid (whether it's 34.9OD/31.6ID, or 31.8OD/27.2ID, this gives you way more wall thickness than you actually need, so the "slightly lower than the junction" theory doesn't hold water). So I suppose we need to worry about the seat post snapping a bit more.

Trial and error shows basically you are playing with fire when you go below 70mm, unless you are a tiny guy.

The diameter x 3 method simply doesn't work. If anything, a thicker seatpost actually needs less tube in the frame to be solid. On the reverse side, imagine having a very thin seatpost (say 10mm), having 3 times the length simply isnt' a good idea.

WILLIAMDENYS
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Joined: Fri Aug 24, 2018 5:06 pm

by WILLIAMDENYS

on my fsa seatpost stands 100mm length inside
- ridley fenix sl 105 - not lightweight
- sworks tarmac sl 5 r8050 di2 - vision tc 24 wheels = 7,1kg

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wheelsONfire
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by wheelsONfire

The standard has been 100mm.
Don't cut it shorter than that. Remember that the less seat post you got inside, the more you stress the frame!

Also consider some saddles are lower and some are higher.
Measure placing saddle on a table. From bottom up to top.
It can be around 15mm or so.
Bikes:

Ax Lightness Vial EVO Race (2018.12.21)
viewtopic.php?f=10&t=156137
Paduano Racing Fidia (kind of shelved)
Open *UP* (2016.04.14)


Ex bike; Vial EVO D

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dgasmd
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by dgasmd

wheelsONfire wrote:
Mon Nov 12, 2018 12:18 am
The standard has been 100mm.
Don't cut it shorter than that. Remember that the less seat post you got inside, the more you stress the frame!

Also consider some saddles are lower and some are higher.
Measure placing saddle on a table. From bottom up to top.
It can be around 15mm or so.
Totally incorrect!!!

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Lewn777
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by Lewn777

Exercise extreme caution :smartass:

I had a Fuji SL. I cracked the seat-tube. Because I had a carbon seat post that wasn't the orginal Fuji length, Fuji said that they didn't have to warranty the bike. That with the seat post I used wasn't inserted at least 10cm into the frame and therefore they are not responsible for the consequences. The post I used was shorter (350mm?) than the Fuji OEM (400mm?), but I hadn't even cut it and was nowhere near the minimal insertion line. To be fair to Fuji they have offered me a half price high-end new frameset, when the old one was low end.

In my opinion the few grams saved isn't worth the risk, unless maybe you are under 65kg. Save weight on the seat, wheels, pedals or groupset, where lightness is built in. If you are looking to save 30 grams how about changing out your cleat bolts to titanium or alloy if you haven't already. :D

But yeah, 10cm inside the frame is a good rule of thumb, so +1.

GothicCastle
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Joined: Mon Jul 04, 2016 1:52 am

by GothicCastle

Catagory6 wrote:
Mon Sep 17, 2018 12:53 am
want to cut down a carbon seatpost...
Good God, why? This is a dumb way to save almost no weight.

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Calnago
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by Calnago

^Agreed
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
Unlinked Builds (searchable): Colnago C59 - 5 Years Later; Trek Emonda SL Campagnolo SR; Special Colnago EPQ

by Weenie


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wheelsONfire
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Location: NorthEU

by wheelsONfire

Obviously Lewn777 say that minium insert lenght can be more.
I don't doubt that, i ment that usual minimum is 10cm.
But if you guys say that minimum 10cm is wrong, OK, i was wrong.
Bikes:

Ax Lightness Vial EVO Race (2018.12.21)
viewtopic.php?f=10&t=156137
Paduano Racing Fidia (kind of shelved)
Open *UP* (2016.04.14)


Ex bike; Vial EVO D

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