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Seat Post to Tube Clearance?

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Old 03-23-17, 05:45 AM
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Hudson308
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Seat Post to Tube Clearance?

Hey, guys; What's a typical/optimal clearance between the seat post and the seat tube? .002" total (OD to ID)?
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Old 03-23-17, 06:07 AM
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Yes,

The production reamers made for seat tunes are 0.05mm larger than the nominal, ie. 27.25mm. The posts are supposed to be true to size.
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Old 03-23-17, 06:13 AM
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Originally Posted by FBinNY View Post
Yes,

The production reamers made for seat tunes are 0.05mm larger than the nominal, ie. 27.25mm. The posts are supposed to be true to size.
Thank you for quantifying the difference -- I did not know what the fit margin is. "True to size" does match my experience w/ seat posts.
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Old 03-23-17, 06:42 AM
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Originally Posted by FBinNY View Post
Yes,

The production reamers made for seat tunes are 0.05mm larger than the nominal, ie. 27.25mm. The posts are supposed to be true to size.
Perfect. Thanks so much!
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Old 03-23-17, 06:49 AM
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Typically, posts are spec'd to be 0.2mm under the nominal inner diameter of the seat tube. For instance, an imperial Columbus SL seat tube has a nominal outer diameter of 28.6mm and a nominal wall thickness of 0.6mm at the post end, which results in a nominal inner diameter of 27.4mm. The standard post diameter for an SL frame is 27.2mm.

This would still allow for a clearance fit even if the seat tube and post were running at at heir respective minimum and maximum tolerance limits, though tubes can get distorted during the frame building process, which can cause subsequent fit issues, if not honed or reamed.
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Old 03-23-17, 07:54 AM
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Originally Posted by T-Mar View Post
Typically, posts are spec'd to be 0.2mm under the nominal inner diameter of the seat tube. For instance, an imperial Columbus SL seat tube has a nominal outer diameter of 28.6mm and a nominal wall thickness of 0.6mm at the post end, which results in a nominal inner diameter of 27.4mm. The standard post diameter for an SL frame is 27.2mm.

This would still allow for a clearance fit even if the seat tube and post were running at at heir respective minimum and maximum tolerance limits, though tubes can get distorted during the frame building process, which can cause subsequent fit issues, if not honed or reamed.
Hmm... that's four times the clearance that FB quoted. Maybe different manufacturers spec'd different clearances.
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Old 03-23-17, 10:45 AM
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Originally Posted by Hudson308 View Post
Hmm... that's four times the clearance that FB quoted. Maybe different manufacturers spec'd different clearances.
This kind of stuff is generally settled by convention.

Because an interference fit is unacceptable, the convention will stipulate that both post and frame makers will stay on their side of the line (or nominal size). This is analogous to how we drive on roads, agreeing that it's OK to be close to the center line, but not to cross it.

So for a post the tolerance will be +0.0, -0.xx. And for the frames -0.0 +0.xx.

As I said the reamers made for the job are 0.05mm over nominal (that's a hard fact, it's the business I was in). Posts are close to the nominal, but not over it. Better posts are very close to the nominal, and that's not hard to achieve on a turned post.

of course there are tolerances, and a post spot on with a frame a bit under the 0.05 allowance will make for a very snug fit. At the opposite end one where the post is at the small end of the range, with a frame that's on the high end will be a bit sloppy, but if both are within the acceptable tolerance still OK.

The 0.2mm tolerance mentioned by someone else is a mile off. Post fits with more than 0.1mm slop will exhibit noticeable rocking and creaking since they;re only constrained at the very top. You'll also get excess closing of the pinch bolt which brings the ears out of line with each other and may cause breakage of the pinch bolt.

Also keep in mind, that posts are sized on 0.2mm increments, so if the fit were that far off, we'd be spec.ing a larger post.
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Old 03-23-17, 12:49 PM
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But I have found seat post variations,, may be within that hundredths of a MM.

not exactly 27.200.

factory reamer can wear doing many thousands of frames , and done quickly..

sparse cutting oil , dragging chips around, rather than clearing them from the tool, etc.




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Old 03-23-17, 02:54 PM
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T-Mar has it right.

A bit of history on the subject:

Technical Q&A with Lennard Zinn: Component sizing; pedal overlap, wheelbuilding; aero’ lids; and crank design

Originally Posted by Lennard Zinn

Seat-post size, on the other hand, often is dictated by the tubing manufacturer. The outside diameter of the seat tube will often be an English size, say 1.375″, and the I.D. will be dictated by the wall thickness, which the frame manufacturer selects as being appropriate for that particular rider and frame size. That’s why there are so many seat-post sizes – lots of framebuilders are trying to find the best balance between stiffness, strength and weight, while they are stuck with the English-size O.D. to make sure that a front derailleur clamp will fit on, and front derailleur clamps only come in 1-3/8″ (34.9mm, a.k.a. 35mm), 1-1/4″ (31.75mm, a.k.a 32mm), and 1-1/8″ (28.6mm).

27.2mm became the standard seat-post size because most high-end road frames in the 1970s and 1980s were lugged and were almost universally made out of Columbus SL or SLX or Reynolds 531. These seat tubes were 1-1/8” in diameter, or 28.6mm. The single-butted seat tube was 0.9mm thick at the bottom and 0.6mm thick at the top. Well, 2×0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm. However, the tolerance on the wall thickness and roundness of the seat tube made it so that you rarely could fit a 27.4mm post inside, even before brazing. And then, the seat tube always got distorted during brazing, making it even less possible to fit a 27.4mm in there, but a 27.2mm fit nicely. The same goes for why a 27.0mm seat,post was often used on the bigger sizes, which were made out of Columbus SP or SPX, whose seat tubes had 1.0 X 0.7mm wall thicknesses. Because of tolerances, the predicted 27.2mm post (28.6 – 2×0.7 = 27.2mm) never fit, but a 27.0mm fit nicely.
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