Picking the Correct Size Seatpost, 26, 26.2, 26.4?
 

It's an old Japanese frame, had a non-stock 26.8 seatpost installed. Way too tight, I needed to twist it out with a pipe wrench. However, the seat tube and compression cut wasn't deformed, just over-tight at 26.8.

Would it be safe to replace with a 26.4, or perhaps down to a 26.2? Not clear on how much difference .4 actually makes in a seatpost fit.

Thanks.

Buy some vernier calipers and measure, guesswork seems to have failed the previous owner, better not make the same mistake twice,

Try 26.6. I came into a 2000 KHS Flite with a 26.8 post wedged in. I looked up the bike online and the spec'd post was 26.6. I thought, "huh, I've never heard of that..." but duly went down to my LBS and there was a small bin of used seatposts of various odd sizes among larger bins of the more popular. Long story short, using the spec'd seatpost size worked like a charm. FWIW, the frame was Reynolds 520, of which I also previously hadn't heard.

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...one of the things you realize after you've encountered enough of them is that seat posts are not always standardized at the diameter marked on them, and frames are not always reamed to the standards you expect. So if it's an alloy post, there's no reason not to carefully sand down the portion that goes into the seat tube a skosh to get .the fit you need.

If 26.8 went in there without deforming anything, and was just a little tight, going to something like 26.4 or less is a bad idea.

I cam measure ten 27.2 seatposts out in the collection in the garage, and only 5 of them will come in at what I need on a particular frame. Which is why I have a milk cratefull of seat posts. :(

You might want to look into getting the seat tube reamed to accept a 27.2mm post as you'll have many more posts to choose from. BUT...…., have a good mechanic inspect it first to determine if the frames wall thickness is enough to do this. The .2mm that would need to be shaved off is very, very minor but still needs someone who knows what they're doing to make a judgement.

The time tested way to insure the best seat post fit it to try various posts and go with the best fit.

One question about the old/tight post. When removed were there any gouges from a burr inside the seat tube? If so then at least hone smooth the seat tube before testing for size. Andy

I had to do that and it worked out well. It was a 27.2 post to replace a 27.2 post but the new one wouldn't slide in. I ended up sanding, carefully, for about two hours because I didn't want to take off too much. I started with medium-coarse emory cloth in a shoe-shine motion but really didn't make much progress and switched to a power sander. Just a little at a time, stopping and checking often, until it fit. Steel, not alloy.

The previous seatpost was pretty smooth, obviously some twist marks. OK, so I ordered a 26.6 replacement and keeping fingers crossed.

These sizes of seatposts aren't in my junkbox, circa 1980 frame.

0.2mm can make a huge difference. If the bike takes a 26.6mm post, a 26.4 will be too sloppy and the 26.8mm will be far too tight. Start with a 26.6mm post but be aware that 26.6mm posts are rather rare. See if someone around you has a Stein seatpost gauge. That will help.

I replace a lot of seatposts at my local co-op and if a frame takes a 27.2mm post, for example, I can pull out any post stamped with 27.2mm at random and it will fit. That includes posts made by Thomson to generic steel posts taken out of 1980s practically throwaway bicycles. There’s lots of seatposts out there and there’s really no need to sand one down.

On that we can agree.

I don’t know where you get your posts or what is wrong with them but I’ve never had a problem swapping posts on a bike. My Moots 27.2 post will fit in my Dean frame (although they are dissimilar frames and might explode due to the incompatibility:rolleyes:). My wife’s mountain bike uses a 27.2mm post and I put a generic post, a Race Face, and a Tito titanium post in it. My Cannondale takes a 27.2mm post and I’ve had the OEM Cannondale post, a couple of Race Face posts and a Thomson post in it without any issues of size. The same holds for the other 10 bikes in my garage. I’ve never had to measure a seatpost of any size for any bike...ever. I’ve been through a lot of posts in 39 bikes over my life while suffering from a severe case of upgradeitis.

Both my 80s Panasonic frames, take 26.6 seat posts. Both from Japan obviously, ditto on the suggestion to try a 26.6 one.
Tim

It is difficult to measure seatposts with calipers and get an accurate reading. I keep meaning to buy a seat tube sizing gauge like this,


https://www.universalcycles.com/shop...BoC0MoQAvD_BwE

I've measured a few frames for seatposts. Calipers have worked -- either poorly or not at all. The reason is that, if I'm resorting to measuring the frame, it's often because I also have a reason not to trust the diameter at the top of the tube. I've had good results with an inside spring caliper for going a couple inches down into the post and see if the diameter is uniform before making the final call. Getting within 0.2 mm takes some practice, and there's a good reason why machinists don't use those tools any more if they can avoid it.

An error of 0.2 mm in diameter would be 0.6 mm in circumference -- enough to notice in deformation of the slot.

This thread reminded me yet again of my seatpost woes. My Kuota takes a 30.0 post, a size only available in alloy from Thompson as a replacement. I tried a cheap Chinese carbon post in 27.2 (nothing wrong with it at all) and a shim, but I can't get it to stay put. It is always slipping down on a ride until it bottoms out on the bottle boss. Yes, I tried carbon paste too. The original post from Kuota had the ridges stripped that held the saddle and the saddle wouldn't stay with that one. So I went on ebay and lo and behold an original Kuota was up for cheap. I bought it but my concern is it will have the same problem as my original. We'll see.

I'll use the Chinese carbon post on my commuter as an upgrade. It won't make any difference in the ride but what the hell, it's carbon.

Most of the low-medium-level Japanese bike-boom frames use 26.4. Higher-tier frames have thicker seatposts.

I have a TANGE #5 frameset here that takes a 26.8mm seatpost. I asked a professional framebuilder about reaming it out to 27.2mm and he said no way he'd do that as the seat tube walls would be too thin after.

Cheers

i have a schwinn super le tour spec'd for a 26.4 post. i wanted to try a 26.8 post before i knew what it was spec'd for and it simply wouldn't fit. sounds like you made the right choice

Seatpost size is a frequent topic here. Almost a year ago I took the time to measure a few posts I have. here's a link to that thread.

https://www.bikeforums.net/bicycle-m...post-size.html

BTW the sizing tool (Stein's or other copies) is only the start of proper sizing. It will gage the top of the seat tube, about .5" depth (this for my Odyssey brand size gage). Great to have a sort of first size to trial fit. But seat tube distortion (from either heat and fit up at the time of the frame's being made or too small a post being used previously) is often focused at the bottom of the lug (or where the top tube or seat stays attach). Way too deep for the sizing gage to reach. I've fitted test posts fine at the seat tube's top only to have that post jam when inserted a few inches more. Andy

...yeah. Stewart, I think maybe I'm your new hobby. :) Not sure what your problem is with me, but it's pretty obvious that you are once again filled with more opinion than fact. Good luck with your opinions, Bubba. Keep on fighting the good fight.

Considering that you are the one who claimed that seatposts vary all over the place, I find it rather interesting that you claim my experience is “opinion”. I don’t measure posts on a regular basis but I don’t find that there is enough variance that once a post size is known that any other post of the same size won’t fit. I certainly have never found only half of all posts of a given size will fit in the same frame.

I have 11 bikes and 2 frames in my garage. One of the bikes and one frame use 30.8mm posts. The rest use 27.2mm posts. That’s 9 bikes with that post size. I have swapped posts around on all of them. Not a single post has every not fit in any of the frames. Not too tight or too loose. Each has fit exactly as I would expect it to fit...i.e. just right. I have exactly the same experience with the 30.8mm posts. I have the same experience at my co-op where I replace a lot of posts. That’s not opinion. It’s fact.

I replace so many posts at my local co-op that I’ve gotten pretty good at guessing the size and being close in most cases. And, again, if the bike takes a certain size, all of the post marked with that size fit like they were made to fit.

And, since I know you’ll never believe me on anything, check Andrew R Stewart’s link. He saw some variance but , honestly, it would be so small as to not make any difference in the way the posts fit in a frame. He certainly didn’t seem to find that 50% of the posts were out of the standard.

...seat posts and steel frames (my particular interest...I honestly don't know much about aluminum) are made to standard. The standard varies. In seatposts and steel frames it's not uncommon to see variations as much as .1 mm. That's enough to make for a good fit, a loose fit, or a tight fit. Which is the OP's issue. If you don't measure posts on a regular basis, why are you an expert on this topic ? (As you are on so many, many others, Stewart.)

...the fact that you replace a lot of posts in a co-op environment is unimpressive. I've worked in a co-op environment, and I know the QC standards for what rolls out the door. Your idea of a good solid slip fit between post and seat tube is not the same as mine. Some of us have high standards, Stewart.

Not sure why you continue to tell me about all the bikes you have now, have had in the past, and your continuing work as an unpaid volunteer. Keep your day job, though.

I only have about four project bikes going right now out in the garage, but I think at one time I had as many as 20 of them, all lined up and ranging in age from the 1950's to the 1980's (maybe one or two from the 90's). Which is why I have the milk crate full of various sized seat posts. Right now I'm working on a '58 Lenton.

I tell you this not to impress you, only to point out that once I quit managing the co-op here on Saturdays, I had a lot more time to work on them. :) The Lenton and a Raleigh Record Ace from 1973 might be the last of them,

...maybe this will help:


....again, and for the last time I hope, your idea of a good fit and mine may or may not be the same. I want the post I ride on to have a solid contact with the seat tube it rides within, so I don't need to worry about over tightening the seat clamp if and when something that is a little loose in how it slides slips. I'm glad that this has worked out so simply and so well for you, because you seem to have a penchant for the absolute. In this case, over the years that frames and posts have been manufactured, it saddens me that we, the rest of the world, have let you down. :( "Out of standard" is a pretty poor describe what's going on here. "Standard" varies with your particular purpose and use.

It's wrong to say I don't ever believe you. You're my go to guy on chemistry.

...did you go to that thread and actually read through it ? :wtf:

Yes. And I saw nothing that suggested that half of all seatposts are the wrong size. Did you see something there to suggest that? Please provide a link.

I don’t measure seatpost on a regular basis because there is no need to in my experience. I don’t waste time on doing trivial measurements that are unnecessary. If a bike takes a 27.2mm post, every 27.2 mm post in the bin will fit in that frame. I’ve not run across a single marked seatpost that wouldn’t fit in a frame designed for it. That’s steel, aluminum, titanium and carbon.

Perhaps you worked in a bad co-op but we wouldn’t send someone out of the shop with an improperly sized post unless they absolutely insisted and even then I’d try to persuade them not to do it because they are only going to be back when it becomes a problem.


I continue to bring up my work at my local co-op because it’s the data set I have to work with. I see 30 to 40 bikes per day that I work and I’ve been doing it fairly consistently for 10 years. I see 1500 bikes per year and I replace about 150 posts per year. Do the math, that’s a lot of seatposts. If I were going to see fit problems, I would have seen them by now. I simply don’t see seatpost fit as a problem...and I know how a seat post should fit.


It’s worked out so simply for me because it’s not as complicated problem as you make it out to be. Seat posts just aren’t something that has a fit issue unless you use the wrong sized post as is the case with FenchFit bike. Someone jammed the wrong post into a frame. The correct post will fix the problem. Seatposts don’t vary as much as you made them out to vary.

The reason I know a lot about bicycles is the same reason I know a lot about chemistry...I study, read and pay attention. Paying attention, being willing to learn and remembering the lessons learn from mistakes is what builds knowledge. Putting hands on 15,000+ bikes means I learn a whole lot. If seatposts were as large a problem as you’ve made them out to be, I’d have noticed.

I agree that there is some variance in seatpost diameter. That’s normal. But it ‘s a matter of magnitude. I just don’t see variance as large as you make it out to be.

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...just to be clear, Stewart, I don't know jack about chemistry. :)