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Seat tube sizing.

Old 09-15-20, 01:36 PM
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Jgarcia2020
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Seat tube sizing.

I am looking to swap out my seat post on my 90's trek mtb,the manufacture states that the diameter of the seat tube is 27.5. As we all may know, bike parts are very hard to come by these days. I am wondering if I could go down some mm's and fit a 26mm seat post to my bike without damaging my frame?
What is the narrowest diameter seat post would you recommend I not exceed without having to resort to using a shim?

Thank you.
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Old 09-15-20, 02:18 PM
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Originally Posted by Jgarcia2020 View Post
I am looking to swap out my seat post on my 90's trek mtb,the manufacture states that the diameter of the seat tube is 27.5. As we all may know, bike parts are very hard to come by these days. I am wondering if I could go down some mm's and fit a 26mm seat post to my bike without damaging my frame?
What is the narrowest diameter seat post would you recommend I not exceed without having to resort to using a shim?

Thank you.
That would be possible, but in my opinion I would rather find a 27.2 (a very common size), and you could shim that easily.
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Old 09-15-20, 02:25 PM
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As above, much more likely to be 27.2. I have never seen a 27.5mm post, but can't say for sure that it does not exist.

But here is an article with a decent size list of old frames and their seatpost sizes, and 27.5mm does not exist under the Trek listings. I didn't check if it was anywhere else.

https://www.sheldonbrown.com/seatpost-sizes-m-z.html#t
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Old 09-15-20, 02:39 PM
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Short answer - No.

Considering seat post sizes differ by 0.2mm That is a way big jump without a shim.

https://www.modernbike.com/seatpost-shims
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Old 09-15-20, 03:00 PM
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I did get one, but its too big. From my understanding, shims are meant to fill the void of too much space. Can they be used the other way around?
In other words, I can make the tube small (by shimming) but I cant make it larger?
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Old 09-15-20, 06:53 PM
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You did get one what? A seat post or a shim?
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Old 09-15-20, 07:08 PM
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Originally Posted by Jgarcia2020 View Post
I did get one, but its too big. From my understanding, shims are meant to fill the void of too much space. Can they be used the other way around?
In other words, I can make the tube small (by shimming) but I cant make it larger?
C'mon...think about it for 2 or 3 seconds...how would that work? I'm betting the post size that fits your frame is either 26.8mm or 27.2mm. It would be exceptionally strange if it were anything else.
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Old 09-15-20, 08:40 PM
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Originally Posted by Iride01 View Post
You did get one what? A seat post or a shim?
Seat post 27.5
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Old 09-15-20, 08:46 PM
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Originally Posted by cxwrench View Post
C'mon...think about it for 2 or 3 seconds...how would that work? I'm betting the post size that fits your frame is either 26.8mm or 27.2mm. It would be exceptionally strange if it were anything else.
I know, but I momentarily thought that could have been a thing.... I'm learning. Thanks
Its not 27.2 because the one I purchased is not a proper fit, so it must be the former.
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Old 09-15-20, 08:49 PM
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Originally Posted by Jgarcia2020 View Post
I did get one, but its too big. From my understanding, shims are meant to fill the void of too much space. Can they be used the other way around?
In other words, I can make the tube small (by shimming) but I cant make it larger?
The way I read your post is you want to install a 26mm seatpost in a 27.something seat tube. Meaning the frame tube is larger than the seatpost you want to install.
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Old 09-15-20, 09:03 PM
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Originally Posted by dedhed View Post
The way I read your post is you want to install a 26mm seatpost in a 27.something seat tube. Meaning the frame tube is larger than the seatpost you want to install.
I initially thought I had a 27something seat tube so I purchased a 27.2mm and it was too big. That was a inherent technically issue that proved to be abundantly apparent. My question was, what would be the absolute smallest diameter would I be able to use without damaging my frame. This is taking into consideration that the parts are hard to come by these days, however, another user alluded to me that I most likely had a 26.8mm or 27.2mm seat tube. I've already figured out its not a 27.2, so my next move is to try a 26.8, if I cant find one that size I'll opt for a smaller size diameter and use a shim, something that I did not want to have to resort to but will do if necessary.
Thank you for your suggestions, they are greatly appreciated.
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Old 09-15-20, 09:15 PM
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There is only ONE correct size. It might be 27.0mm, but 26.8mm was more popular. You can go smaller ONLY if you have to correct shim. If you determine your seat tube is 26.8mm you can use whatever diameter post you want as long as you also use a shim that brings the O.D. to 26.8mm. There are lots of shims available, you must have the correct one.
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Old 09-16-20, 03:36 AM
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Originally Posted by Jgarcia2020 View Post
I am looking to swap out my seat post on my 90's trek mtb,the manufacture states that the diameter of the seat tube is 27.5.
Where does the manufacturer state that the (inner) diameter of the seat tube is 27.5? If on line, please provide a link. If in printed material, please check it again. A measurement of 27.5 is highly unlikely.

One common problem that can prevent a seatpost of the correct diameter from fitting is damage from previous insertion of an undersized seatpost. If the slot at the back of the seat tube is narrower at the top than at the bottom, that's evidence that an undersized seatpost had been installed at some point.

If that's the case with the frame in question, you can widen the slot by (carefully) inserting a large flat-blade screwdriver from above and lightly tapping with a hammer. Only widen the slot enough to admit the seatpost; you don't want to end up splitting the seat tube at the bottom of the slot.
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Old 09-16-20, 05:38 AM
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Does the frame have a tubing type decal on it? Aluminium or steel?
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Old 09-16-20, 08:29 AM
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digital calipers would prevent a lot of aggravation
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Old 09-16-20, 08:42 AM
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Originally Posted by cyclist2000 View Post
digital calipers would prevent a lot of aggravation
measuring the area around the clamp is useless. You need to know what the tube size is down below the clamp area. Most common caliper whether digital or not are not going to get inside far enough to get a real measurement of that.

Especially on bikes that have gone through a lot of tightening and untightening the clamp area will not be true to the tube internal size.
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Old 09-16-20, 12:58 PM
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Originally Posted by cyclist2000 View Post
digital calipers would prevent a lot of aggravation
This is true.
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Old 09-16-20, 12:59 PM
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Originally Posted by dedhed View Post
Does the frame have a tubing type decal on it? Aluminium or steel?
Chromoly.
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Old 09-16-20, 01:02 PM
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Originally Posted by Trakhak View Post
Where does the manufacturer state that the (inner) diameter of the seat tube is 27.5? If on line, please provide a link. If in printed material, please check it again. A measurement of 27.5 is highly unlikely.

One common problem that can prevent a seatpost of the correct diameter from fitting is damage from previous insertion of an undersized seatpost. If the slot at the back of the seat tube is narrower at the top than at the bottom, that's evidence that an undersized seatpost had been installed at some point.

If that's the case with the frame in question, you can widen the slot by (carefully) inserting a large flat-blade screwdriver from above and lightly tapping with a hammer. Only widen the slot enough to admit the seatpost; you don't want to end up splitting the seat tube at the bottom of the slot.
I found the info regarding the tube size on bikepedia.
Yeah, what you're speaking of, is not the case. I did see RJ the bike guy address that issue on youtube though.
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Old 09-16-20, 01:04 PM
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Originally Posted by cxwrench View Post
There is only ONE correct size. It might be 27.0mm, but 26.8mm was more popular. You can go smaller ONLY if you have to correct shim. If you determine your seat tube is 26.8mm you can use whatever diameter post you want as long as you also use a shim that brings the O.D. to 26.8mm. There are lots of shims available, you must have the correct one.
For sure. When I first show the shims, the distinguishing sizes kinda confused me but after some study, I was able to understand how they work.

Thanks again.
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Old 09-16-20, 01:56 PM
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Originally Posted by Jgarcia2020 View Post
Chromoly.
But does it say something like Tange, Ishiwata, Tru Temper with a designation of tube name? With that information you can find out what the ID of the tube would be.
The Bicycle Info Project :: True Temper Bicycle Tubing Catalog circa 1987

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Old 09-16-20, 04:58 PM
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Originally Posted by Iride01 View Post
measuring the area around the clamp is useless. You need to know what the tube size is down below the clamp area. Most common caliper whether digital or not are not going to get inside far enough to get a real measurement of that.

Especially on bikes that have gone through a lot of tightening and untightening the clamp area will not be true to the tube internal size.
but he could measure the old seatpost
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Old 09-16-20, 08:01 PM
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My early Trek 970 made of Tru Temper AT seatpost measures 26.6mm
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