# Cutting seat post



## boostmiser (Sep 10, 2008)

Has anyone cut their seat post for the sake of saving some weight? Is there a certain length the tube has to be seated into the frame?


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## QuattroCreep (Nov 30, 2009)

Yes there is a minimum length of seat post that needs to be in the frame. There should be a line on the post marked as minimum.

Not worth the risk of messing up the jewels for a couple of grams.


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## I'm not that guy (Aug 15, 2010)

Have to agree with QuattroCreep. There's safer ways to drop 10 grams, like spitting a few times before going for a ride.


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## frdfandc (Nov 27, 2007)

As long as the seat post extends past the seat tube/top tube junction, you will be fine shortening the seat post. Measure 3-4 times and don't cut too much off. 

However, if its a nice seat post, you will be limited in resale if you choose to do this.


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## I'm not that guy (Aug 15, 2010)

If you want to do it and be mathematical about it, here's how it's done:

1) With your post clamped in the frame adjusted to your fit, mark the post where the clamp is.
2) Find the "minimum insertion" mark on your post
3) Measure the distance between the "minimum insertion" mark and the mark you made where you clamp down on your post.
4) Cut that exact length off the bottom for your post. 

I still say don't do it because if the post ever failed for any reason, you've got no recourse with the manufacturer because you cut a product not designed/intended to be cut.


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## Gearhead65 (Jan 23, 2010)

I have a KCNC zero setback that I cut approx 8 cm off. Used a tubing cutter. Just didn't need that much and I still had plenty in the frame....and I'm a weight weenie.


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## QuattroCreep (Nov 30, 2009)

How much post are we talking about? How much does it weigh?

If you want to save grams for free try only wrapping your bars up to your hoods and leaving the drops bare, or going with only one bottle cage, or leave your flat kit at home.

If you must cut fallow *That Guy's* advice and use a miter box and a fine tooth hack saw to make sure the cut is square/matches the current angle at the end of the post.


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## cxwrench (Nov 9, 2004)

Gearhead65 said:


> I have a KCNC zero setback that I cut approx 8 cm off. Used a tubing cutter. Just didn't need that much and I still had plenty in the frame....and I'm a weight weenie.


tubing cutter? was that a carbon post?


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## Peter P. (Dec 30, 2006)

The seatpost remaining after you cut MUST extend BELOW the bottom of the top tube, a distance at least equal the diameter of the seat tube.

For instance, if your seat tube is 30mm in diameter, then the seatpost should be long enough to extend 30mm below the bottom edge of where the top tube and seat tube intersect.

The reason is, you don't want the leverage of your seatpost as you sit on it exerting maximum force on the seatpost clamp or the joint of the seat tube/top tube. Maximum support is reached when the length of insertion equals the diameter of the seat tube.

Make sense?

Do not cut your post with a tubing cutter. It will flare the end and you'll have to file it afterwards so it will insert into the frame, and it won't look as professional. Use a couple of hose clamps cinched around the post at the point of cut, and a hacksaw. Clean up the joint with some sandpaper, which should be minimal.


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## boostmiser (Sep 10, 2008)

Thanks for the reply's....I'm not going to do it, more informational.
When I pulled my seatpost out and saw how long it was, it crossed my mind that people must do it to save weight.


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## Blue CheeseHead (Jul 14, 2008)

Peter P. said:


> The seatpost remaining after you cut MUST extend BELOW the bottom of the top tube, a distance at least equal the diameter of the seat tube.
> 
> For instance, if your seat tube is 30mm in diameter, then the seatpost should be long enough to extend 30mm below the bottom edge of where the top tube and seat tube intersect.
> 
> ...


1.) Sorry, as an engineer I do not see your logic of the maximum support being reached when the length of insertion = diameter past bottom of the TT. It just is not true. The seat post insertion needs to resist the moment created by the load on the end of a cantilever. the longer the cantilever, the more moment resistance you need. More insertion = more resistance. I would expect a longer seatpost to have a longer minimum insertion, thus if you cut one, in theory you would not need as much minimum insertion. To be safe, I would maintain the minimum insertion length as marked by the manufacturer.

2.) When cutting with a pipe cutter the flare is created on the inside of the post, not the outside. Even without filing, it will not impact the ability to slide into the seat tube smoothly.


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## Gearhead65 (Jan 23, 2010)

Blue CheeseHead said:


> 1.) Sorry, as an engineer I do not see your logic of the maximum support being reached when the length of insertion = diameter past bottom of the TT. It just is not true. The seat post insertion needs to resist the moment created by the load on the end of a cantilever. the longer the cantilever, the more moment resistance you need. More insertion = more resistance. I would expect a longer seatpost to have a longer minimum insertion, thus if you cut one, in theory you would not need as much minimum insertion. To be safe, I would maintain the minimum insertion length as marked by the manufacturer.
> 
> 2.) When cutting with a pipe cutter the flare is created on the inside of the post, not the outside. Even without filing, it will not impact the ability to slide into the seat tube smoothly.


+1 with Maytag Blue.
What he said. Tubing cutters work great. It's the same (non) issue on copper tubing...if the flare ended up on the outside, you couldn't get it shoved down in the copper elbow before you sweated the fitting.


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## medimond (Apr 26, 2009)

I cut mine off leaving the min insertion length and then weighted the aluminum piece ... drum roll, it weighed 50 grams.


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## Argentius (Aug 26, 2004)

I cut mine off with a dremel.

When you have a 400 mm seatpost so that it can fit both road, and mountain, and you have a non-compact frame, no reason at all one can't bring it back down to 250 or 300.

It's "not much," but, hey. Every second counts, right?


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