Risk using seatpost over stop line

Discuss light weight issues concerning road bikes & parts.
Helmutti
Posts: 19
Joined: Mon Dec 24, 2012 1:18 pm

by Helmutti

Hi all.

My first post here after lurking some time, but i thought that this must be something for carbon experts like you.

I was out looking at a lovely Look 595 ultra, but the frame have been cut to fit the former user, so if I have to fit it, I'm missing 1-1,5 cm under the stop limit at the seatpost. I have uploaded pictures below to illustrate the seatpost (that have several cuts/lose elements)

I need to tighten the seatpost right under the red spacer ~1cm under the stop.

I weight 65kg (with clothes), so is there any risk that i can damage the frame or seatpost?
Is it a go or no-go?


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Regards Lars

by Weenie


ghisallo2003
Posts: 595
Joined: Mon Jan 02, 2006 7:10 pm

by ghisallo2003

Perhaps you could find a saddle with a particularly extended rails to seat distance? I dont know how much variation is out there and whether 1 cm is feasible, but anything you could do to lower the seatpost would lower the risk you are worried about.

jano
Posts: 140
Joined: Sun Oct 21, 2012 8:15 pm

by jano

You could use lower the seat height by using shoes and pedals which give you a lower stack height.

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HammerTime2
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Location: Wherever there's a mountain beckoning to be climbed

by HammerTime2

I am not saying that this will necessarily be a good solution, but you can consider the Look "RePost"
http://www.lookcycle.com/en/us/route/accessoires/rsp.html
http://www.lookcycle.com/en/us/route/accessoires/rsp-r5.html .
I'll leave you to look into it further.

Scott!
Posts: 48
Joined: Fri Jun 30, 2006 2:55 pm
Location: Nashville, TN

by Scott!

To add to what was said by ghisallo2003, I've found variations of up to 1.75 cm from rail to top of saddle between the various road saddles I have.

Helmutti
Posts: 19
Joined: Mon Dec 24, 2012 1:18 pm

by Helmutti

Okay that's a lot everybody!

Can anyone recommend a 'tall' saddle?

I have looked, but it's hard to see which ones that is 'tall' and which is not.

This eg.g look 'tall' but i honestly don't have any idea.
Selle italia x2 flow
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RussellS
Posts: 701
Joined: Wed Feb 03, 2010 1:31 am

by RussellS

Maybe related, maybe not. I have a carbon Orbea frame with a carbon Profile Designs seatpost. The post is about 1 cm above the minimum insertion line. The minimum insertion line is about 1 cm above the seatpost clamp and visible. Using it a couple years like this.

Scott!
Posts: 48
Joined: Fri Jun 30, 2006 2:55 pm
Location: Nashville, TN

by Scott!

The lowest I've measured of the saddles I have is the Specilaized Romin. It measures a little over 4 cm from the bottom of the rail to the top of the saddle. The tallest I have is a Koobi Xenon which measures almost 6 cm. I have a couple MTB saddles which are even taller. Best bet is to use the saddle you made your initial measurement with as a baseline and head to your LBS and measure a few saddles.

Helmutti
Posts: 19
Joined: Mon Dec 24, 2012 1:18 pm

by Helmutti

I think the result must be a little over the stop and a new saddle.

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djconnel
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by djconnel

It's an interesting question: if you weigh nothing and your saddle weighs nothing you only need enough seatpost in the frame for it to clamp. The heavier you are the more seatpost you need. At some point you have enough seatpost in there that it's no longer strength-limiting. But clearly 65 kg is less than the design limit of the post so being exactly at the line isn't a problem. But 1-1.5 cm is large.

My guess is the strength is proportional to the square of the amount of seatpost in the tube but a mechanical engineer would be able to answer that better.

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Rick
Posts: 2021
Joined: Sat Aug 29, 2009 4:30 pm

by Rick

I would have thought "no problem at all".
I think most seatposts recommend about 2.5" of minimum insertion, and I have run them with only about 1" of insertion.
Going 1-1.5 cm below the manufacturer's line seems pretty safe to me because the manufacturer typically includes a very conservative safety factor.
But this is just my opinion, and like all deviations from manufacturer's specs, it is 'At your own risk."
I happen to be a mechanical engineer, but I have applied no analysis whatsoever to this. Only gut feel and experience. :)

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elviento
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by elviento

This baby will easily give you 3cm of extra height, but it's hideous (although it allows you to have children).

2.5" (6.25cm?) of grab is probably not enough. I have worked with quite a few bike engineers, and they mostly say 7-8 is required.

In fact, a manufacturer I know that uses internally butted seat tube with 2" of grab is having lots of warranty issues. Remember this is in Asia where 65kg dudes are pretty common.

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HillRPete
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by HillRPete

djconnel wrote:My guess is the strength is proportional to the square of the amount of seatpost in the tube but a mechanical engineer would be able to answer that better.

Probably closer to a log function I'd reckon, at some point the gain from more overlap diminishes.

mate
Posts: 20
Joined: Thu Sep 13, 2012 7:31 am
Location: Slovenia

by mate

I have 2 friends with 695...
You can find a guy who works with carbon and he will make the seat-tube in height you need. It will be great if you have the piece which was cutted
It should not cost more then 50 euros and here our guy make it so good nobody can find where it was putted together

by Weenie


thisisatest
Shop Owner
Posts: 1980
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by thisisatest

Typically you want the insertion to be a minimum of 2.5x the diameter. That does little for you, with a non-round post. On conventional frames, an additional rule is for the bottom of the post to pass the bottom of the top tube.

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