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Lengthening a seat post?

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Lengthening a seat post?

Old 06-15-24, 08:04 PM
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Lengthening a seat post?

I wasn’t sure whether to post this here, or with the mechanics. Hoping for some creative solution help!
I need to add 5 inches to a seatpost.

This is for a folding bike. I have a telescoping seatpost and it it’s fine height wise. The problem is, the lower section is not long enough to push through the bottom of the frame and act as a support.

The pics below tell the story. Stock post is too short, but the 580mm 34.9 dis post is the right length to act as a support when folded.

the new telescoping post gives me the height I need when riding, but the lower 33.9 dia portion is about 5 inches too short to function as the bike support.

Any suggestions as what I could use to scab on another 5” of non structural post? Wedge a 3/4” piece of alum tubing in the rubber plug? Find someone to weld on a section of tube and the grind it smooth? Other? Thanks for any suggestions!!!



This is the lower section of the new telescoping tube that I need to add 5” to

This is how it should sit. This is with the stock post (but this post doesn’t extend high enough for me, thus the issue)

This is my new post. The rubber plug at the base has a 3/4” aperture and extends about an inch or so into the tube.
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Old 06-15-24, 09:29 PM
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By "support when folded" do you mean long enough to act as a foot for the folded bike to rest on? The term "telescoping" to me suggests two post shafts, one larger that is tightened by the frame's seat post binder and the other (being of smaller diameter) is slid in the first and secured by a second binder at the top of the larger diameter lower shaft.

Seatposts generally want their lower end to be inserted down in the frame's seat tube an amount that has the post end overlapping the frame's joint. For the usual double diamond frame this is just below the top tube/seat tube joint and often referenced by a 2.5" post insertion marking (minimum inserting). For your bike's frame this insertion amount is more likely around 4" or so, by the looks.

When your seat is at the riding height how much of the post is still in the frame? How much is in the frame when the bike is folded shouldn't be a structural issue.

Seatposts want a pretty specific fit to the frame (or that lower telescoping section) and even a couple of tenths of a MM make the difference between a good fit and potential damage to the frame's binder (or another stuck post). Any adding length to your post will need to duplicate the specific diameter as well as be coaxial to the post. Far easier said than done. Not a simple "toss a piece of pipe on the end" type of job. Andy (trying to understand the details so he can offer specific help)
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Old 06-16-24, 06:26 AM
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Thank you Andy, what you have said is spot on. When the post is fully lowered I need it to act as a “foot” for the bike to rest/balance against. Also correct in that there is an upper/narrower section of post, and a lower larger post (33.9mm). When I have things set up to ride, I have plenty of post in the frame, over 6”, so more than twice what would be required by the min insertion mark.

The issue is basically this: The stock post is 580mm long, which is the exact length needed for it to act as a “foot” when the seat post is lowered completely into the frame. The new telescoping post, while offering much greater overall length due to the two sections, only has a 450mm lower section, so that’s where the 5” shortfall comes from when the seatpost is fully lowered, as it can’t be pushed into the frame any deeper than the binder at the top of the 450mm section of 33.9mm post.

That was why I was thinking anything added to the bottom of the post could be a smaller diameter. It would never contribute to minimum insertion, I just need it to come into play when the post is fully inserted into the frame and this portion sticks out the bottom to act as a foot. I even thought of trying to find a carbon tube that I could epoxy inside the lower alu tube (with something like a 3 1/2” overlap).



Measuring how much post I have in the frame when riding. Ignore the upper section, it has a min insert too and I safe in that as well (I just don’t have if fully extended for the height I require in this picture).
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Old 06-16-24, 06:29 AM
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I’ve also looked into simply replacing the lower section of the telescoping tube with a longer section but have not been able to find anyone who makes a 33.9mm tube of sufficient length (580mm) which has a binder at the top - seems to be a pretty specialty piece of work. Bummer because I bought this bike which fits and rides well, but which I can’t fold and store as intended.
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Old 06-16-24, 10:57 AM
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Sorry Andy, forgot to tag you in my responses, thanks for the thoughtful reply! - Tom
Originally Posted by Andrew R Stewart
Andy (trying to understand the details so he can offer specific help)
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Old 06-16-24, 11:17 AM
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Originally Posted by Plainsman
I’ve also looked into simply replacing the lower section of the telescoping tube with a longer section but have not been able to find anyone who makes a 33.9mm tube of sufficient length (580mm) which has a binder at the top - seems to be a pretty specialty piece of work. Bummer because I bought this bike which fits and rides well, but which I can’t fold and store as intended.
1.375" tubing (with various wall thicknesses and almost any length) is commonly available and is ~1 mm thicker than your desired 33.9mm. I would think any machine shop (or even an amateur with a lathe in their garage/basement) could turn it down to size. I don't know how the binder attaches, but if it's just a reduced diameter section at the top of the tube with a longitudinal slit cut in it, that too is easy for a shop to do. Of course, even though a machinist could do this in a few minutes, it would probably cost a pretty penny from a professional machine shop.
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Old 06-16-24, 03:53 PM
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You are correct, the binder at the uppermost part is the 33.9mm tube slides over the top of the tube, which is reduced to receive standard 27.2mm seatpost.

Originally Posted by wayold
1.375" tubing (with various wall thicknesses and almost any length) is commonly available and is ~1 mm thicker than your desired 33.9mm. I would think any machine shop (or even an amateur with a lathe in their garage/basement) could turn it down to size. I don't know how the binder attaches, but if it's just a reduced diameter section at the top of the tube with a longitudinal slit cut in it, that too is easy for a shop to do. Of course, even though a machinist could do this in a few minutes, it would probably cost a pretty penny from a professional machine shop.
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Old 06-16-24, 03:59 PM
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If I could find a 27.2 x 600mm post that might also work, as I could use that for my upper, and when the telescoping post was fully collapsed the 27.2 dia portion would extend out the bottom of the 33.9mm section the required distance to act as a support foot.
Originally Posted by Plainsman
You are correct, the binder at the uppermost part is the 33.9mm tube slides over the top of the tube, which is reduced to receive standard 27.2mm seatpost.
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Old 06-16-24, 04:33 PM
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I also found this lightweight portion of 3/4” light weight tubing on Amazon. I thought about taking a rubber mallet at tapping it into the bottom of the stopper that’s in the larger lower diameter of post, then tapping a simple rubber chair foot onto it. It’s a snug fit in that apperture in the plug at the bottom of the 33.9mm tube, but it might work.

3/4” craft tubing I bought next to original telescoping post.


Plug in bottom of 33.9mm lower tube with 3/4”


Thinking tap this into the aperture for a snug fit?
Originally Posted by wayold
1.375" tubing (with various wall thicknesses and almost any length) is commonly available and is ~1 mm thicker than your desired 33.9mm. I would think any machine shop (or even an amateur with a lathe in their garage/basement) could turn it down to size. I don't know how the binder attaches, but if it's just a reduced diameter section at the top of the tube with a longitudinal slit cut in it, that too is easy for a shop to do. Of course, even though a machinist could do this in a few minutes, it would probably cost a pretty penny from a professional machine shop.
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Old 06-16-24, 07:49 PM
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I also have considered just buying a new inner post from Amazon (about $25 worth of post combined here) so the inner would be long enough to act as the support foot. Would just need to figure out how to bind these two sections together.
Originally Posted by Andrew R Stewart
Andy (trying to understand the details so he can offer specific help)

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Old 06-17-24, 10:14 AM
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A plug bonded in place could join two pipes (or hollow seat posts). Seems to me that for the stresses acting on a "kickstand" functioning foot likely being small, the form of attachment is of less concern and nearly zero safety concerns. Also I believe that the foot could well be of a smaller diameter than the rest of the lower seat post, as that portion won't need to be clamped tight (like with a seat post binder collar). This lessening of the diameter size might open up the extension foot or joining options.

Like said, for someone with access to a lathe this project would be fairly easy. I have such lathe but before I got it I developed relationships with some local machine shops for the project steps I can't do. Retired machinists and Maker Spaces are two other possible sources for this kind of work.

A wood dowl hand filed to fit and just jammed up in the post would be a quick trial and perhaps long term solution. Andy
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Old 06-17-24, 10:34 AM
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Andy, thanks for taking the time to respond! I think I might try this “dowel-seque” solution. While I like the idea of the post doing double duty as a kickstand, I think this 3/4” dia 1.2mm wall thickness aluminum craft tube I just bought could be the answer. Looks like if I tap it into the hole in the bottom of the seatpost plug it and stick a rubber foot on it, it might do the trick and also have a decent finished look to it. Unless I’m misunderstanding your suggestion.

$8 .75” OD/1.2mm ID alloy tube beside the post I’m trying to modify.

Looks like it would have a good snug fit in the bottom plug of the seatpost if I were to tap it in with a rubber mallet.
Originally Posted by Andrew R Stewart
A plug bonded in place could join two pipes (or hollow seat posts). Seems to me that for the stresses acting on a "kickstand" functioning foot likely being small, the form of attachment is of less concern and nearly zero safety concerns. Also I believe that the foot could well be of a smaller diameter than the rest of the lower seat post, as that portion won't need to be clamped tight (like with a seat post binder collar). This lessening of the diameter size might open up the extension foot or joining options.

Like said, for someone with access to a lathe this project would be fairly easy. I have such lathe but before I got it I developed relationships with some local machine shops for the project steps I can't do. Retired machinists and Maker Spaces are two other possible sources for this kind of work.

A wood dowl hand filed to fit and just jammed up in the post would be a quick trial and perhaps long term solution. Andy
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Old 06-17-24, 11:26 AM
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Originally Posted by Andrew R Stewart
A plug bonded in place could join two pipes (or hollow seat posts). Seems to me that for the stresses acting on a "kickstand" functioning foot likely being small, the form of attachment is of less concern and nearly zero safety concerns. Also I believe that the foot could well be of a smaller diameter than the rest of the lower seat post, as that portion won't need to be clamped tight (like with a seat post binder collar). This lessening of the diameter size might open up the extension foot or joining options.

Like said, for someone with access to a lathe this project would be fairly easy. I have such lathe but before I got it I developed relationships with some local machine shops for the project steps I can't do. Retired machinists and Maker Spaces are two other possible sources for this kind of work.

A wood dowl hand filed to fit and just jammed up in the post would be a quick trial and perhaps long term solution. Andy
And, a wood dowel extension would be kind to any nice floors the folded bike is set on. Perhaps making it a very welcome long term solution.

Another small point - an extension that is undersized would allow for a small amount of error in machining/shaping/jamming together and still allow the assembled post to slide as easily as it currently does. Also wood swelling. (Summer humidity? A post stuck 'till January? Sorry, grew up in New England where windows worked in winter when you kept them closed but not in summer when you need them to open. )
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Old 06-17-24, 02:23 PM
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Now I get you, thanks. I’ve been running down the road of option one (I have everything but the internal rubber bushing which I can easily make from a stock rubber plug), but I think you are talking about option three substituting a wood dowel in place of what I’m showing as a coupling tube to slide into each half of 27.2 post.
Originally Posted by 79pmooney
And, a wood dowel extension would be kind to any nice floors the folded bike is set on. Perhaps making it a very welcome long term solution.

Another small point - an extension that is undersized would allow for a small amount of error in machining/shaping/jamming together and still allow the assembled post to slide as easily as it currently does. Also wood swelling. (Summer humidity? A post stuck 'till January? Sorry, grew up in New England where windows worked in winter when you kept them closed but not in summer when you need them to open. )

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Old 06-18-24, 08:39 AM
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Thanks all. Someone in the mechanics forum has this bike, so moving the discussion over there. I appreciate all of the Feedback!
Originally Posted by Plainsman
Now I get you, thanks. I’ve been running down the road of option one (I have everything but the internal rubber bushing which I can easily make from a stock rubber plug), but I think you are talking about option three substituting a wood dowel in place of what I’m showing as a coupling tube to slide into each half of 27.2 post.

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