Cutting seatpost question

Discuss light weight issues concerning road bikes & parts.
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dgasmd
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by dgasmd

Is there a minimum safe distance/amount that must be left below the clamp when one cuts a seatpost? Considering cutting a post, but don’t want to compromise safety for the sake of a few grams.

Advice appreciated!


by Weenie


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ms6073
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by ms6073

Yes, and the minimum insertion depth is usually represented on the seatpost by a graphic.

Image
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Marin
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by Marin

dgasmd wrote:
Tue Dec 19, 2017 4:29 pm
Is there a minimum safe distance/amount that must be left below the clamp when one cuts a seatpost? Considering cutting a post, but don’t want to compromise safety for the sake of a few grams.

Advice appreciated!

It depends on the frame, the seatpost should extend below the top tube joint at least.

The min insert mark is of course irrelevant after cutting.

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ms6073
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by ms6073

Marin wrote:
Tue Dec 19, 2017 4:44 pm
The min insert mark is of course irrelevant after cutting.
:smartass: It was kind of implied that the OP was to take a measurement from the end of the post to the minimum insertion mark, and then use that length as the minimum insertion length after the post is cut.
Michael - The Anaerobic Threshold is neither...

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Calnago
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by Calnago

Ha! I have to say, I thought of adding the exact same cautionary note that @Marin did when I saw the pic. After some things I’ve seen people do, I could totally see where someone may cut a big chunk off but still not get that the min insertion line has to be moved accordingly. And we’re all assuming that the wall thickness is the same throughout the entire length of the seatpost. I actually think that’s a safe assumption but with all the fancy layups these days I could imagine a scenario where a seatpost is purposely beefed up in the clamping range area but somewhat “tuned” above that. I would think a post like that would be covered in warnings about not cutting it however.
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Mr.Gib
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by Mr.Gib

Calnago wrote: After some things I’ve seen people do, I could totally see where someone may cut a big chunk off but still not get that the min insertion line has to be moved accordingly.
It is rather disconcerting to know that I might be out on the road with such people. Should they even be allowed to have a bicycle?
wheelsONfire wrote: When we ride disc brakes the whole deal of braking is just like a leaving a fart. It happens and then it's over. Nothing planned and nothing to get nervous for.

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Lelandjt
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by Lelandjt

Most posts have 100mm below the min insertion line but that's to provide an extra margin for use with any frame. 1cm below the bottom of the top tube is safe. Weight weenie it up!

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Calnago
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by Calnago

:). One man’s “safe” is another man’s “hey, why is my seatpost creaking all the time?”. Or worse. 1cm below the bottom edge of the top tube on my C60 wouldn’t even extend past the seat cluster lug. I’d personally like a little more margin than that. Colnago actually supplies different length seatposts depending on whether it’s a sloping or traditional frame. But it’s hit or miss I think whether you receive the shorter one if you order a traditional frame. I was going to cut one of the longer ones down to match the shorter length version (since all my frames are traditional) but in the end didn’t bother. The weight savings is really minimal.
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
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topflightpro
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by topflightpro

Calnago wrote:
Tue Dec 19, 2017 5:57 pm
And we’re all assuming that the wall thickness is the same throughout the entire length of the seatpost. I actually think that’s a safe assumption but with all the fancy layups these days I could imagine a scenario where a seatpost is purposely beefed up in the clamping range area but somewhat “tuned” above that. I would think a post like that would be covered in warnings about not cutting it however.
All posts should be designed to be clamped at any point above the minimum insertion line, otherwise they would need a maximum insertion point as well. I could easily see someone on too large a frame inserting some posts almost all the way into the frame.

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Calnago
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by Calnago

Excellent point. I wasn’t even thinking of that but you’re absolutely right.
Whew! See Mr. Gib... that idiot you’re afraid to be riding behind could easily have been me.
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
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wheelsONfire
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by wheelsONfire

10cm is ordinary for safety reasons.
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bremerradkurier
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by bremerradkurier

149102cut or right angle?

by Weenie


Johnny Rad
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by Johnny Rad

Calnago wrote:
Tue Dec 19, 2017 5:57 pm
...And we’re all assuming that the wall thickness is the same throughout the entire length of the seatpost. I actually think that’s a safe assumption but with all the fancy layups these days I could imagine a scenario where a seatpost is purposely beefed up in the clamping range area but somewhat “tuned” above that. I would think a post like that would be covered in warnings about not cutting it however.
This is exactly what Martin @ MCFK told me with their new "2018" seat post - don't cut it!

http://www.mcfk.de/en/products/sattelst ... tpost.html

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