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I want to shorten my seatpost it fouls the non-OEM kickstand on my folding bike when lowered.

  1. What is the best way to cut a (33.9mm alu) seatpost? Can I just use a rotary plumbing pipe cutter? The edges of a seatpost appear to be deburred or smoothed, what's the best way to achieve this?

  2. There's no minimum insertion markings. Is there a rule of thumb in their absence?

tubing cutter

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    I'd cut off nothing, and look for a better kickstand that clamps on the rear axle instead. Just an option. – Criggie Jan 15 '17 at 19:17
  • I wanted a dual leg kickstand as I have a trailer. And my bike has oversized hub axles and 20" wheels, and there's a dearth of rear wheel dual kick stands that attach to the rear axle. – RoboKaren Jan 15 '17 at 19:39
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    I've generally used a hacksaw for cutting off seat posts. Follow up with a file to debur the end. As to insertion length, if the post is sticking out at the bottom (when set to riding height), any of that bit that's visible can be cut off, with no effect on structural integrity. And one would generally assume that so long as the replacement post is as long as the OEM post, there would be no structural problem. (I like to see about 5"/12cm of insertion.) – Daniel R Hicks Jan 16 '17 at 1:55
  • Your scenario is a bit confusing. Close as I can determine, a Brompton has a seat post that is more or less "normal", and then a long seat tube that slides in and out of the frame. The seat tube needs to be nearly fully inserted into the frame (for the 10 inches or so that they overlap) in order to provide structural integrity. The seat post needs to be inserted into the seat tube perhaps 5 inches. I would have grave reservations about cutting the seat tube. – Daniel R Hicks Jan 16 '17 at 2:23
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    Understand that the length of the exposed post is as critical as the length of the post as it fits into the seat tube. The longer the exposed portion, the greater the bending forces on the junction between post and tube. Thus, if a long portion is exposed then a relatively long portion (perhaps 1/3 as much) needs to be pushed into the seat tube, to resist bending at the joint. – Daniel R Hicks Jan 16 '17 at 3:32
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I agree with @ojs answer on point (1), you can just cut it with a hacksaw but with the exception of point (2) "the common consensus seems to be at least 5 cm of insertion" - the standard minimum insertion mark on a seatpost is 100mm, double this. You may get away with a bit less, like ~90mm or maybe even 80mm but 50mm would seem very much on the short side to me.

The general idea is that the seatpost should extend a decent amount beyond the bottom of the top tube. 5cm is unlikely to accomplish this on most bikes. He may have been thinking inches.

I'd leave this as a comment on his answer rather than an answer on its own but I don't have the reputation and it is an important issue for safety.

  • Do you have any sources for this important issue? The factory limit is less than 10 cm on most seat posts and includes a safety margin. In most of quality frames the seat tube is butted and does not even touch the seatpost below a few centimeters at top. – ojs Jan 15 '17 at 17:00
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    Many seat tubes are single butted and do make continuous contact with the post. And in terms of sources, go measure some posts. 5cm is not a number you will see. – Nathan Knutson Jan 15 '17 at 17:02
  • @ojs- do you have any sources for your 5cm? I have been riding bikes for decades and I have never seen a minimum insertion that short. The 100mm I got from a quick Google, but it gels with my approximate experience. I happen to have a spare seatpost I just swapped yesterday and measuring it the minimum insertion is just over 9cm (I know it looks like 10 in the photo, but it is closer to 9). From Googling "minimum insertion seatpost 5cm" I can see a few people suggesting 5cm below the bottom of the top tube, which might be where you got the idea. – Ivan McA Jan 16 '17 at 7:26
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    Note that saying "bottom of the top tube" only applies to conventional diamond frames -- doesn't apply to "girl frames", mixtes, folders, etc. More generally, the seatpost must extend below the top of the seat tube (ie, into the tube) by a "decent amount". – Daniel R Hicks Jan 16 '17 at 19:01
  • @ojs: Actually, I have a broken seat post right here that had a min insertion rating of 75mm, but broke when it was inserted to a depth of 110mm. That's 35mm more insertion that the rating required, and it still broke! You don't want to ride a seat post that's inserted less than its rated min. That would be inviting desaster. – cmaster Jun 6 '18 at 20:49
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  1. Pipe cutter or hacksaw cut the post well but leave sharp edges. These are easily rounded with sand paper or files (rat tail for inside, flat for outside). To get a factory-like finish you'd need a metal lathe, but a rougher finish isn't going to cause structural problems and will be hidden inside the frame.

  2. The common consensus seems to be at least 5 cm of insertion. [EDIT] outside bicycles.stackexchange. See comments, downvotes and other answers for other opinion.

  • I'd go with all of @ojs answer above. A rotary tool with cut off wheel would also work. If you don't have easy access to tools, you can still smoothen it by rubbing flat and angled on a somewhat smooth concrete, rock, or a sharpening stone with water or oil. Go a few round on the inside at an angle with the rod of a hardened steel object such as a screwdriver will help smoothen the inside edge. – người Sàigòn Jan 15 '17 at 13:29
  • @ngườiSàigòn Rocks make poor tools for any job - fine in an emergency, but there's always a better tool. – Criggie Jan 15 '17 at 23:38
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    5cm (2") does not sound right to me. – Daniel R Hicks Jan 16 '17 at 1:58
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For insertion length, 72-80mm is the shorter end of what you see marked on seatposts, up to around 100mm. Seatpost length (leverage) and material are relevant factors there. In addition to what's safe for the post though, what's safe for the frame is also a part of this. The rule of thumb in most cases there is for at least the entire seat cluster to be well supported, including for example seatstays that are fastbacked into the seattube at points somewhat lower than the more normal area by the toptube intersect, although on some frames they go so low that one has to start assuming the designer knows there won't be post support down there. I would never go less than about 70mm, even if a need arose to cheat a little bit, and 72mm is a fairly common place I think for the mark to be on short (traditional length for non-sloping frames), high-end aluminum posts.

For a folding bike though, with their very long, thick-walled posts and more arbitrary frame designs, I would say all bets are off in terms of rules of thumb and you want to just play it very safe with how much of the post is being supported by the frame and vice versa. I'm also surprised that there are really no markings, since there's always a safe limit.

Rotary pipe cutters are capable of inadvertently flaring out the end of the tube slightly. It's not the end of the world because you can just file it down, but it is a nuisance. For this reason, hacksaws are generally a better choice for everything on bikes, where the exact outside diameter of the tube almost always matters somehow. The go-to approach usually is to use a normal threadless steerer cutting guide in a vise, but a 33.9mm post won't fit in one, so in a shop that would mean using an oversize guide. Barring access to a clamp-on guide you could improvise something with zip ties, hose clamps, etc to at least make it easier to keep the cut flush. The fast way to deburr and neatly taper the cut end is to use an internal/external tube deburring tool. I always gently sand the end face with a circular motion to erase the hacksaw marks, but I probably should stop. If you don't have any kind of specific deburring tool, you can make it acceptable with just sandpaper or a file. The common Park threadless steerer cutting guide, which along with a hacksaw the go-to tool in many shops for cutting seatposts and bars. Only works with tubes up to 33mm though. Several other companies have very similar tools.

The common Park threadless steerer cutting guide, which along with a hacksaw is the go-to tool in many shops for cutting seatposts and bars. Only works with tubes up to 33mm though. Several other companies have very similar tools.The Park oversize tube cutting guide, which mainly exists for cutting aero seatposts and seatmasts, but would also do your post just fine. It's maximum size is 45mm.

The Park oversize tube cutting guide, which mainly exists for cutting aero seatposts and seatmasts, but would also do your post just fine. It's maximum size is 45mm.An internal/external tubing deburrer

An internal/external tubing deburrer.

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    Nathan, can you label your images. The last image is obviously a deburring tool but the first two are unclear if they are tubing cutters- and if so, what type. – RoboKaren Jan 15 '17 at 18:50

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