Seatpost diameter sizes (standards) - article comments

[BikeGremlin]

To prevent article pages from being miles long, but preserve all the useful questions and answers provided over time, I've decided to copy/paste the website comments to the forum - and "move" further discussions here.

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Seatpost diameter sizes (standards)

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If you can't find the answer to your question in this thread, please open a separate thread with your question/problem, in an appropriate forum section (this is the Frames, forks & seat-posts section).

Relja

 

[BikeGremlin]

1. bruce prator
   31/08/2019 at 16:56
   I want to use a 31.8 or 30,9 dropper post on my 34.9 seatpost. what shims would work for this?
   • 
     
     Relja
     01/09/2019 at 08:57
     I’d look for an adapter that notes those very dimensions: 34.9 to 31.8, or 34.9 to 30.9 (depending on the choice).
2. 
   
   bruce prator
   02/09/2019 at 00:31
   where can I find that
   • 
     
     Relja
     02/09/2019 at 08:48
     Amazon search (affiliate link):
     Seatpost reducer (adapter, shim)
     Bike24
     https://www.bike24.com/p2315002.html?q=seatpost+shim+adapter
     Bike components de
     https://www.bike-components.de/en/s/?keywords=seatpost reducer
     It might take time to source the particular needed combination, but Google and local bicycle shops are a way to go.
3. 
   
   Brian
   23/01/2020 at 04:25
   What size diameter seatpost do I need for my 1960’s 3 speed Tote Cycle and where can I get original old stock chrome one. Thank you.
   • 
     
     Relja
     23/01/2020 at 06:05
     Don’t know – best way to find out is to measure the bike’s seat tube inner diameter.
4. 
   
   brian
   23/01/2020 at 17:06
   Thank you, Will do.
5. 
   
   Fred
   18/04/2020 at 15:15
   Another way to measure the circumference, albeit not super exact, is to use a measure tape and tie it around the seatpost.
   • 
     
     Relja
     18/04/2020 at 18:16
     If memory serves me well, dividing that with the number Pi should give the diameter.
     Haven’t tried the method, but I suspect it to be too imprecise for this use.
   • 
     
     Bezeelel
     09/09/2020 at 00:53
     I’m upgrading my bike and my post I’m cangi g from a 26.8 to a 31.6. I’m buying a vernier because I cannot get a accurate measurements for a setpost adapter. The spacer I’m hoping that will work is a 26.8-31.8. My question is will I have to purchase to seat post clamps for this to work and be safe?
   • 
     
     Relja
     11/09/2020 at 14:07
     Not sure I understand the setup, but, if a narrower seatpost is to be inserted, using an adapter to fit the difference between the seat tube, and the seat post – then no need to change the clamp, if it’s in good condition.
     However, if seatpost is to be extended by more than some 20 centimetres, I would look for a well matching seatpost, not use an adapter – because adapters are usually not very deep (long), so won’t offer much of a support for the seatpost. Hope I’ve explained well what I mean.
   • 
     
     Greg
     10/01/2021 at 16:11
     Measuring the circumference of the post and dividing by Pi is a good method. There are even circumference tape measures to do it.
     If you don’t have calipers, you’re hard-pressed to get the diameter. An adjustable wrench can be used as calipers, then measure the gap on the wrench. However, it’s almost impossible to distinguish 0.2 mm increments with a tape measure.
     But measuring the circumference means you only have to distinguish a 0.6 mm difference (because everything is multiplied by 3.14…), which is doable.
     Measuring the size of the hole in the seat post—well, that requires ID calipers, gauges, or sticking something into it then marking it so you can measure it like the seat post. That will introduce more error. For example, take some poster board (stiff paper), roll it up, insert it, and let it expand to fill the hole. Making sure it’s fully expanded & secure, mark the edge of the paper with a marker. Pull out the paper, unroll it, and measure between the edge and the mark (and divide by Pi).
     I’ve done it before, and it does work well enough.
   • 
     
     Relja
     10/01/2021 at 16:42
     Thanks, Greg. All good ideas that would work, better, or worse, depending on one’s preference, and availability of tools.
     This reminded me of the barometer question/dilemma.

 

[BikeGremlin]

1. SnarkerBob
   20/01/2021 at 01:37
   Your article is confusing, because of some obvious errors. First, “Picture 1” is not labeled, and nowhere do your explain which blue arrow is pointing at which part.
   Also, at several parts you refer to Vernier calipers, but none of your images have Vernier calipers. You show just plain old digital calipers that don’t have a Vernier scale. Look up what a Vernier caliper actually is.
   Finally, the suggestion that a less than 0.2mm difference in seat post diameter would make the post too small to fit snugly in the seat tube is laughable. Any halfway decent clamp should easily account for 1mm error or more.
   • 
     
     Relja
     20/01/2021 at 11:35
     Although it seems a bit snarky, your comment has been very useful.
     One of the WordPress updates seems to have removed labels on picture galleries (where more than one picture is placed side by side). Hadn’t noticed it. It’s fixed now, to the best way I could fix it with this “version” (doesn’t allow labeling each picture separately, without the label being put over a picture, instead of beneath it).
     “Vernier calipers” – English is not my native. The name of those calipers, whether analogue (with a Vernier scale), or digital, is the same in my native (“Šubler” – an “imported” German term). I will correct the term once I double check the proper English technical terminology.
     As for the 0.2 mm difference, it is probably counter intuitive for many people, but in practice it can cause one, or several of the following:
     – Seatpost slipping under rider weight and road bumps (either down, or turning sideways – or both).
     – Seat tube cracking near the seatpost clamp area (steel frames are the least susceptible to this, while carbon fiber ones are the most “sensitive” to such problems).
     I would normally say: “don’t take my word for it, try it, with a smaller diameter seatpost (0.2 mm is enough, but you can go 0.5, or 1 mm, as you said, to get “faster” results)”. However, this does pose a risk of damaging a frame, so do it only on a frame you are willing/prepared to damage.
     Thank you for what is in effect a very helpful, constructive feedback.
     If you find any other mistakes, feel free to note them. I try to keep this as correct, and up-to-date as possible.
   • 
     
     Peter
     30/09/2022 at 15:20
     yes old thread
     1 mm take up is incorrect.
     seat post will slip down as it will rock from side to side, it happened to me
     no more than 0.1 clearance
2. 
   
   Bradley Stenstrom
   06/03/2021 at 16:26
   Most of your information is geared towards the later 20th Century bicycles. I’m trying to find information about the size of a 1948 Columbia Custom Deluxe Ladies bicycle seat post. I measured it with a digital caliper and it reads 15.75 mm. I checked Sheldon Browns site and his list did not show Columbia’s. This bicycle was my mothers and I’m trying to put it back together for her 85th birthday in April or at least by Mothers Day.
   So far I have not found any seat posts that are that small (no clue what became of the original) Any advise would be greatly appreciated.
   • 
     
     Relja
     06/03/2021 at 18:49
     Reading this with a smile on my face – a lovely idea for a birthday surprise.
     15.75 mm is very narrow – I’ve never seen any seatpost with an under 20 mm outer diameter.
     Of course, where there’s a will, there’s a way. I have a few ideas:
     1)
     Does the frame have some shims that reduce the seat-tube’s inner diameter, allowing such a narrow seatpost to be held in place?
     If yes – maybe those could be taken out to fit a “more standard” diameter seatpost?
     2)
     Most seatposts are a lot wider than 15.75 mm. Machining the sidewalls down would weaken them tremendously in my opinion.
     But, a narrower pipe could probably be sourced, or even a steel rod – then it’s up to finding a turner to process it into the appropriate diameter size.
     Painting, or chrome plating could also be done – just take it into account when machining to not have the new seatpost too narrow, or too wide.
     Having said that: going a bit too narrow can be fixed with some DIY shims from Coca-Cola cans (that’s one thing that drink is good for ),
     while going a bit too wide would call for some more grinding – but it’s probably a safer way to go (“measure three times, cut once” is a local saying).
     If making a custom seatpost, it’s probably a good idea to keep the upper part with about 23 mm outer diameter, to fit most saddle adapters (Amazon affiliate link – though I prefer buying locally and supporting the local small shops).
     My schedule is hectic and I can’t offer to make that myself and mail it, while this is a rather time-sensitive operation (I also don’t have a very good experience with our mail system’s “swiftness” and reliability) – but if you have any other questions, let me know. More than happy to help however I can. Good luck!
     Relja
3. 
   
   charlie
   26/03/2021 at 15:01
   hi i have a bought a used carbon bike the seatpost 31.6mm is stuck solid could i cut it and insert an 30.2mm or 27.5mm seatpost inside?
   • 
     
     Relja
     26/03/2021 at 15:23
     If all else fails, and taking a frame to a certified carbon repair shop is not an option, that could be attempted.
     Will it work well? I’m not sure.
     What I would try is to create leverage to remove the seatpost.
     John Allen’s method sounds reasonably:
     https://www.sheldonbrown.com/stuck-seatposts.html#carbon
   • 
     
     Fred
     26/03/2021 at 15:47
     Fred here again. I would install a (expendable) saddle on the seatpost and “screw” the seatpost out by turning the saddle, if it’s too stuck i would hang up the bike by the saddle so the saddle sits in fixed position and then “screw” the bike frame so that gravity helps the seatpost go out.
   • 
     
     Timothy Takemoto timtak
     12/06/2022 at 09:38
     I am thinking of doing a similar thing and think it could be possible. I have a carbon frame with an aluminum shim and a stuck 27.2mm seat post. I cut the top of the 27.2mm seat post off (because it was offset forward and I like rearward offset these days) and am thinking of inserting the top of a 25mm Look Ergopost. Unfortunately, the inner diameter of the cut 27.2mm seat post is a bit too small – less than 25mm. I may sand down the Look Ergopost to fit and perhaps put something inside it to strengthen it. You may find likewise that the inner diameter of your 30.2mm seat post is smaller than 27.5. However, one can purchase carbon tube in a variety of sizes and use that to strengthen anything you have had to reduce in diameter.
     I tried using a slide hammer to get the seatpost out of the frame but cracked the frame instead.
     I had also thought to use a impact wrench on the seatpost to knock it into a circle and I had created a socket that would go over the seatpost with a hole in it (through which I would have posted something through the socket and seatpost) as shown on YouTube but I don’t think it would have worked.
     Thanks to Bike Gremlin, the new Sheldon Brown (?)
4. 
   
   Edward
   17/04/2021 at 15:23
   HI,
   I read your article and it prove most useful.
   I measured my seatpost diameter and it turn out to be 40mm.
   I know it is large by modern standard do you have any idea where i can find such a seatpost clamp.
5. 
   
   Erik
   13/07/2021 at 22:30
   I would love to know why every 25.4mm seatpost i have been trying to get for my vintage bike (need something ~300mm long) is not actually 25.4mm. Two “25.4” posts so far have been measured at .988 inches. Infuriating . The one that was on the bike is exactly 1″/25.4mm so everything else will not clamp/fall in.
   • 
     
     Relja
     14/07/2021 at 10:25
     Hi Erik,
     “Give me the metric system, or give me death!”
     Joking a bit.
     Anyway, when I turn .988″ into mm, it is below 20.1 mm (20.0952 to be exact).
     As far as seatpost tolerances go, that’s significantly more than 25.0 mm (enough to not fit into a 25.0 mm seat tube), and a lot less than 25.4 – practically “two sizes” smaller, enough to make the tube crack even if the clamp manages to keep such a seatpost in place.
     Can’t understand that much of a mismatch. I value quality and precision over quantity, but it seems that many manufacturers have a different view on that. No other explanation comes to mind.
     Ritchey company used to be quite good, but I haven’t bought anything from them for a while – and they don’t seem to be making any 25.4 mm sized ones.
     The standard seems to be considered obsolete.
     Locally (in Novi Sad), I can source 25.4 mm seatposts, but of poor quality – the clamp has very coarse saddle angle adjustment, so depending on the seat tube angle, the saddle can end up being either too much upwards, or downwards tilted (if you know what I mean).
     2nd hand market (and local bike shops) is where I’d look. With a wallet in one hand, and accurate & precise callipers in the other.
     Relja

 

[BikeGremlin]

1. bob
   20/07/2021 at 05:10
   Is it possible to sand down a 25.8mm aluminum seatpost to 25.4mm?
   • 
     
     Relja
     20/07/2021 at 20:24
     Hi Bob,
     I would rather not risk making the seatpost walls thinner (and thus weaker).
     But it is possible.
     Relja
2. 
   
   drona
   03/08/2021 at 12:00
   Hi, My bike frame seat post is 30.9mm in diameter, what size dropper post do I need for it to seamlessly fit in the seat tube?
   I have a Merida Big 9 15 by the way
   • 
     
     Relja
     03/08/2021 at 12:13
     Hi Drona,
     If the current seatpost is a well-fitting one, and it is 30.9 mm, I’d look for a 30.9 mm dropper seatpost.
     Seamless sliding is not always possible – but it shouldn’t fit too loosely (you shouldn’t be able to rock it left-right when it’s inserted), nor take too much force to slide it in (no punching, or using full strength to get it in – that means it’s too wide).
     Relja
   • 
     
     drona
     03/08/2021 at 12:47
     Hi Relja,
     Sorry, I meant that my seat tube is 30.9mm. What seat post would best fit in that seat tube, I was thinking 27.2 but wouldn’t that be too narrow?
     Also I am having trouble looking for 27.2mm dropper posts with at least 100mm travel under $170 AUD.
     Do you know any dropper post which suits what I mentioned above?
   • 
     
     Relja
     03/08/2021 at 13:15
     Hi Drona,
     The seatpost depends on the inner seat tube diameter. The outer diameter does not reliably tell us the exact inner diameter (that depends on the wall thickness).
     The safest way to determine this is to measure (I like to say that one good measurement is better than a thousand expert opinions ).
     It is easiest to measure the existing seatpost outer diameter – if there is a well-fitting seatpost on the bike. In case there’s no seatpost, we would need to measure the seat-tube’s inner diameter (using calipers, and minding to measure the tube itself, not the seat-clamp’s inner diameter).
     From what I can Google, Merida Big Nine 15 has a 27.2 mm seatpost (Merida website link). Still, I would rather measure before buying anything – just to be on the safe side.
     As for the dropper seatpost recommendations, I would ask on bikeforums.net MTB section (link to bikeforums website). There are many experienced cyclists and mechanics on that forum. I’m not an expert on dropper seatpost models. With a note that it’s a good idea to state one’s country, since otherwise many shopping advice are, how to put it… “US-centric.”
     Relja
3. 
   
   mike
   18/08/2021 at 08:20
   alot of bikes are made with missmatched seatpost and you will also find some bikes that have a build up of paint around the seatpost hole,on most older steel bikes you can remove that old paint build up with a smooth round file and chase up the correct size seatpost,even some weld splatter stops seat post from fitting correctly,very common on older bikes,you seatpost should fit in without too much force and should not be floppy when you slide it down,you can buy seat post in just about every size,the older 25.4mm ones can still be found on new steel bikes,discarded old bikes are a very good place to look for hard to find seatposts.
4. 
   
   Gary Harthill
   11/09/2021 at 08:46
   Hi
   I want to purchase a Pinarello f10 seatpost, I have seen one on eBay which states
   23mm x 60mm cross section length 350mm, would this hit my f10.
   I am sceptical about things on eBay the only reason I looked was because of price as a new one is £300. The ad states genuine Pinarello seatpost but the price is £130 so you can understand my reluctance.
   Many thanks
   Gary
   • 
     
     Relja
     11/09/2021 at 11:23
     Hi Gary,
     I’m not sure. It’s worth measuring the inner dimensions of your seat-tube just to make sure (callipers are a good tool for that).
     It’s even easier to measure a fitting seatpost if you have one.
     As far as I know, 150-sh pounds is what these go for 2nd hand/used (ebay and the likes) – but take my info with a grain of salt.
     Relja
5. 
   
   Brian Piersol
   25/02/2022 at 04:37
   not a mention here of size 26.0 post on my old TREK antelope. That is the number stamped on the current post and it fits snug but slides nice so it must be correct. And the caliper confirms too. I am trying to find a seat thudbuster for that size. May be impossible as it looks to be a very rare size seatpost. suggestions? thanks, Brian maybe I should get a newer bike!!!!! It’s an antique.
   • 
     
     Relja
     25/02/2022 at 06:56
     Hi Brian,
     Yes, 26 mm is not the most commonly used size (though it is listed in Table 1 above), but I could find it online.
     Here’s one example (Amazon affiliate link).
     Here’s a zero setback one from bike24.com.
     Another one, with setback, from bike24.com

 

[BikeGremlin]

1. kevin
   30/08/2022 at 11:17
   Thanks again for your information. I have 2 comments. When trying to purchase a seatpost for an oversized seat-tube, I eventually noticed a comment that the seat post outer diameter should be 1mm less than the measured seat tube inner diameter, (on one of UK based online cycle shops – I forget which). That did give a snug fit, but I also note your measurements show seat post stated diameters are not exact.
   Secondly in an old comment above you state.. “Anyway, when I turn .988″ into mm, it is below 20.1 mm (20.0952 to be exact).” I’m old enough to have been brought up with imperial measures in the UK, and that didn’t look right. I use 25.4mm to an inch as a rule of thumb, which gives 25.095mm for 0.988″ when I calculate. These days I also exclusively use the metric system. Except when using sites from the USA.
2. 
   
   Ian Brown
   12/11/2022 at 13:14
   Regarding, measuring the internal diameter of the frame seat tube, I wouldn’t just measure at the top, as it could be splayed out. I would measure further down, as well, using a pair of internal callipers, or an inside micrometer.

 

[BikeGremlin]

This topic is closed for further replies.

If you can't find the answer to your question in this thread, please open a separate thread with your question/problem, in an appropriate forum section (this is the Frames, forks & seat-posts section).

Relja