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Seat post sizing

Old 01-24-18, 11:49 PM
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Seat post sizing

Hi all
My MTB has a 30.8 seat post. Not a common size. I was wondering if a 30.9 (more common) would fit.
It seems a small difference of only 0.1, but before I order a seat post, I just want to make sure
It would be great to hear from anyone has any direct or similar experience with this.

Thanks
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Old 01-25-18, 12:15 AM
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It probably will not. But you could find out if anyone has a seat tube reamer and open your seat tube to 30.9.
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Old 01-25-18, 12:20 AM
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I have to admit I don't remember a need for a 30.8 post before, not saying it hasn't happened. Just not enough to remember. Do you still have your 30.8 post? If so how snug does it fit (and this is some of the answer)? Do you need a longer post then currently? Was it stolen? Did you measure the frame instead of reading off a known post?


On top of all this is that post "sizes" are not actual measured sizes. Typically post are slightly undersized but can be over and even tapered a tad. It's all in the tolerances and production control. Think of the labeled size as clothing sizes, worth understanding but not always a measured truth. So what to do? Like buying clothes a post is really only judged by it's fit. Better shops have a range of old posts that they can use to judge a fit and then order (if not on hand) the one their experience and policies say is best. Andy
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Old 01-25-18, 01:09 AM
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One thing about posts is that the change in circumference is about 3.14 x the length of a change in diameter.

So, say a 0.2mm difference in post size would mean the clamp would have to either open or close 0.63 mm more.

A 0.1mm difference, however, is pretty close, and may well just squeze in with a little gentle persuasion.

If it is an aluminum post with a moderately thick wall, you may also be able to simply sand about 0.05mm off of the diameter of the post all the way around. Perhaps just a little polishing of the post with sandpaper.
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Old 01-25-18, 01:35 AM
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Just checked Google shopping. There are tons of new 30.8 seat posts available. Buy the right size.
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Old 01-25-18, 10:01 AM
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Originally Posted by Kontact
Just checked Google shopping. There are tons of new 30.8 seat posts available. Buy the right size.
I suppose if my OP was worded: "Can anyone find a 30.8mm seat post for me?" This would be an appropriate reply.
But it would have been worded:

"Can anyone find me a 30.8 mm seatpost that is:
Light weight Al alloy
Dual bolt
Europe based vendor
Reliable vendor (no Ebay, Aliexpress,etc)
No Chinese knock-offs (Ritchey, Race Face, Cinelli, etc)?"

As this was not the post, I appreciate your reply the same

Cheers
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Old 01-25-18, 10:04 AM
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Originally Posted by Andrew R Stewart
I have to admit I don't remember a need for a 30.8 post before, not saying it hasn't happened. Just not enough to remember. Do you still have your 30.8 post? If so how snug does it fit (and this is some of the answer)? Do you need a longer post then currently? Was it stolen? Did you measure the frame instead of reading off a known post?


On top of all this is that post "sizes" are not actual measured sizes. Typically post are slightly undersized but can be over and even tapered a tad. It's all in the tolerances and production control. Think of the labeled size as clothing sizes, worth understanding but not always a measured truth. So what to do? Like buying clothes a post is really only judged by it's fit. Better shops have a range of old posts that they can use to judge a fit and then order (if not on hand) the one their experience and policies say is best. Andy
I will measure the post and the seat tube to see if there is any hope. I don't want to use a shim. I'll keep on looking
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Old 01-25-18, 10:28 AM
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Originally Posted by MarcusT
Hi all
My MTB has a 30.8 seat post. Not a common size.

Thomson makes 30.8 seat posts. Two bolt, straight or set back, silver or black, in four different lengths.


Originally Posted by MarcusT
I don't want to use a shim.


A shim with a larger post?

Last edited by SquidPuppet; 01-25-18 at 10:47 AM.
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Old 01-25-18, 10:36 AM
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Originally Posted by MarcusT

"Can anyone find me a 30.8 mm seatpost that is:
Light weight Al alloy
Dual bolt
Europe based vendor
Reliable vendor (no Ebay, Aliexpress,etc)
No Chinese knock-offs (Ritchey, Race Face, Cinelli, etc)?"
Thomson Elite InLine Seatpost | Chain Reaction Cycles

Don't screw around with trying to use the wrong size.
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Old 01-25-18, 03:37 PM
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Originally Posted by fietsbob
Here is a 31.9
Why do you keep referring to 31.9 posts when the OP needs a 30.8?
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Old 01-25-18, 11:15 PM
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Thank you for the suggestion of the Thompson. I'll will take a look and see if it is what I am looking for.
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Old 01-26-18, 12:49 PM
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Originally Posted by fietsbob
they really have a 1.25" seat post the metric system and math in general is showing failures in education, as this forum repeatedly exposes.
What are you talking about?

standard seat posts go in 0.2mm steps you have to machine down a 32, on your own to get a 31.9
Except we're discussing 30.8 and 30.9, so the 0.2mm step up would be 31.0 or 31.1, not 31.9. 31.9 is 1.0mm increase.

You aren't talking about relevant, useful numbers or have explained what inch sizes have to do with a seat post that is a full mm smaller than any common inch size.
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Old 01-26-18, 01:26 PM
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Originally Posted by fietsbob
1.25" = 31.75 mm are you going to go over to his guys house and measure his stuff for him.. ?

Smart Phones = dumb people who argue useless drivel , over the internet when they should be doing the job at work they are paid to do..

get over yourself.

...
Again, what does 31.75, 31.8 or 31.9 have to do with the OP's 30.8 seat tube?

Why did you jump up a full 1mm in size? What is the OP supposed to do with what you're talking about?
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Old 01-26-18, 01:29 PM
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Full ignore mode.. engaged
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Old 01-26-18, 02:11 PM
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Originally Posted by fietsbob
Full ignore mode.. engaged
Well that's not very nice. I was just trying to understand your posts and if you had just mistaken one number for another.

I guess we'll never know...
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Old 01-26-18, 02:21 PM
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Originally Posted by MarcusT
Hi all
My MTB has a 30.8 seat post. Not a common size. I was wondering if a 30.9 (more common) would fit.
It seems a small difference of only 0.1, but before I order a seat post, I just want to make sure
It would be great to hear from anyone has any direct or similar experience with this.

Thanks
not a mechanic?

Get a Common size, like 27.2 seat post, then buy a machined seat post sizing ship sleeve with a 27.2 ID ,
and what ever OD your frame's ID happens to be.

getting it 0.1 mm oversize and sanding off a little from the shim sleeve can be done...
and Rough ID, burrs and so forth, of the frame wont contact your new seat post, just the shim sleeve's outside.


[ID, inside diameter, OD, outside diameter]


.....

Last edited by fietsbob; 01-26-18 at 03:11 PM.
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Old 01-26-18, 02:25 PM
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I have 2 Bike Fridays both used seat post sizing shims and a smaller common 27.2 seat post..

sources (wholesale to shops) 'Wheels' and QBP, in the states..



I kept the seat post out of my AlAn, and used it in a steel frame, with a similar shim.

in that case the shim sleeves came from the British USE company.. they made few sizes of seat posts ,
and offered shim sleeves with ID of 25.0 and 27.2..

.....

Last edited by fietsbob; 01-28-18 at 11:20 AM.
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Old 01-28-18, 02:35 AM
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Originally Posted by MarcusT
Hi all
My MTB has a 30.8 seat post. Not a common size. I was wondering if a 30.9 (more common) would fit.
It seems a small difference of only 0.1, but before I order a seat post, I just want to make sure
It would be great to hear from anyone has any direct or similar experience with this.

Thanks
Short answer:

Try a 30.9, if you can't find a 30.8 seat post. If it slides in without using the slightest hand pressure (not forcing it), it's too big and you'll need a 30.8.
Another way would be to measure the frame width, or check manufacturer's specs - what seat post diameter was it designed for.


Long answer:
Everything is machined to some tolerance. So there's always a difference between a nominal size and the actual size (there's also a measurement error, just to make things more interesting). So your post might be of 30.9 nominal width, and the frame might have been manufactured for 30.9 - which is a lot more common size.

For even more details - how to measure, what the standards are, how to "round up" the measured diameters, I put it on a web page, with a few pictures and table charts for help, the link:

Seatpost diameter sizes (standards) - Bike Gremlin - Bicycles

Last edited by Bike Gremlin; 08-24-20 at 05:12 AM.
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Old 01-28-18, 03:07 AM
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Originally Posted by Slaninar
Short answer:

Try a 30.9, if you can't find a 30.8 seat post. If it slides in without using the slightest hand pressure (not forcing it), it's too big and you'll need a 30.8.
Another way would be to measure the frame width, or check manufacturer's specs - what seat post diameter was it designed for.


Long answer:
Everything is machined to some tolerance. So there's always a difference between a nominal size and the actual size (there's also a measurement error, just to make things more interesting). So your post might be of 30.9 nominal width, and the frame might have been manufactured for 30.9 - which is a lot more common size.

For even more details - how to measure, what the standards are, how to "round up" the measured diameters, I put it on a web page, with a few pictures and table charts for help, the link:

Seatpost diameter sizes (standards) - Bike Gremlin - Bicycles
Thanks. After doing a fair bit of reading. I found an old post from 2010 with the same question and they went 30.9 without a problem. I measured the post with calipers (analog) and it looked like 30.9. In the seatpost tube it fits loosely and there is a small amount of play.
I looked at the Thompson from CRC and they only have one inline post at 30.8. I wanted with layback.
I found a 30.9 Truvativ T40 for a great price. Ordered that and waiting for it to come in.

Cross your fingers
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Old 01-28-18, 03:53 AM
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Originally Posted by MarcusT
Thanks. After doing a fair bit of reading. I found an old post from 2010 with the same question and they went 30.9 without a problem. I measured the post with calipers (analog) and it looked like 30.9. In the seatpost tube it fits loosely and there is a small amount of play.
I looked at the Thompson from CRC and they only have one inline post at 30.8. I wanted with layback.
I found a 30.9 Truvativ T40 for a great price. Ordered that and waiting for it to come in.

Cross your fingers
How did you know it takes a 30.8 in the first place? It seems odd that either will work.

Originally Posted by Slaninar
Try a 30.9, if you can't find a 30.8 seat post. If it slides in without using the slightest hand pressure (not forcing it), it's too big and you'll need a 30.8.
Why would a larger post fitting in a smaller hole easily mean that it is too big?

Last edited by Kontact; 01-28-18 at 03:57 AM.
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Old 01-28-18, 04:07 AM
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Originally Posted by Kontact
How did you know it takes a 30.8 in the first place? It seems odd that either will work.



Why would a larger post fitting in a smaller hole easily mean that it is too big?
Yup, thanks for the correction. If it doesn't slide easily in, it's TOO big, of course.
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Old 01-28-18, 04:18 AM
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Originally Posted by Kontact
How did you know it takes a 30.8 in the first place? It seems odd that either will work.
Factory seat post is stamped "30.8"
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Old 01-28-18, 11:16 AM
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whats so bad about that one?
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Old 01-28-18, 12:33 PM
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Originally Posted by fietsbob
whats so bad about that one?
It has a smart phone built in so it = useless drivel.
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Old 01-28-18, 01:24 PM
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..
...I to browse the mechanics forum in the wintertime.

Why are there no active chain lube threads right now ? This is the time of year for them, people.
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