Go Back  Bike Forums > Bike Forums > Classic & Vintage
Reload this Page >

Seat Post to Tube Clearance?

Notices
Classic & Vintage This forum is to discuss the many aspects of classic and vintage bicycles, including musclebikes, lightweights, middleweights, hi-wheelers, bone-shakers, safety bikes and much more.

Seat Post to Tube Clearance?

Old 03-23-17, 05:45 AM
  #1  
Hudson308 
Mr. Anachronism
Thread Starter
 
Hudson308's Avatar
 
Join Date: Jan 2013
Location: City of Lakes
Posts: 1,930

Bikes: fillet-brazed Chicago Schwinns, and some other stuff

Mentioned: 25 Post(s)
Tagged: 0 Thread(s)
Quoted: 478 Post(s)
Liked 155 Times in 106 Posts
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)?
__________________
"If we don't change direction soon, we'll end up where we're going." -Irwin Corey
Hudson308 is offline  
Old 03-23-17, 06:07 AM
  #2  
FBinNY 
Senior Member
 
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 35,964

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

Mentioned: 124 Post(s)
Tagged: 1 Thread(s)
Quoted: 4360 Post(s)
Likes: 0
Liked 15 Times in 15 Posts
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.
__________________
FB
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FBinNY is offline  
Old 03-23-17, 06:13 AM
  #3  
John E
feros ferio
 
John E's Avatar
 
Join Date: Jul 2000
Location: www.ci.encinitas.ca.us
Posts: 19,780

Bikes: 1959 & 1960 Capo; 1982 Bianchi; 1988 Schwinn KOM-10;

Mentioned: 33 Post(s)
Tagged: 0 Thread(s)
Quoted: 802 Post(s)
Liked 150 Times in 119 Posts
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.
__________________
"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324
Capo: 1960 Sieger (2), S/N 42624, 42597
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069
John E is offline  
Old 03-23-17, 06:42 AM
  #4  
Hudson308 
Mr. Anachronism
Thread Starter
 
Hudson308's Avatar
 
Join Date: Jan 2013
Location: City of Lakes
Posts: 1,930

Bikes: fillet-brazed Chicago Schwinns, and some other stuff

Mentioned: 25 Post(s)
Tagged: 0 Thread(s)
Quoted: 478 Post(s)
Liked 155 Times in 106 Posts
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!
__________________
"If we don't change direction soon, we'll end up where we're going." -Irwin Corey
Hudson308 is offline  
Old 03-23-17, 06:49 AM
  #5  
T-Mar
Senior Member
 
Join Date: Nov 2004
Posts: 19,684
Mentioned: 511 Post(s)
Tagged: 1 Thread(s)
Quoted: 3232 Post(s)
Likes: 0
Liked 639 Times in 501 Posts
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.
T-Mar is offline  
Old 03-23-17, 07:54 AM
  #6  
Hudson308 
Mr. Anachronism
Thread Starter
 
Hudson308's Avatar
 
Join Date: Jan 2013
Location: City of Lakes
Posts: 1,930

Bikes: fillet-brazed Chicago Schwinns, and some other stuff

Mentioned: 25 Post(s)
Tagged: 0 Thread(s)
Quoted: 478 Post(s)
Liked 155 Times in 106 Posts
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.
__________________
"If we don't change direction soon, we'll end up where we're going." -Irwin Corey
Hudson308 is offline  
Old 03-23-17, 10:45 AM
  #7  
FBinNY 
Senior Member
 
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 35,964

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

Mentioned: 124 Post(s)
Tagged: 1 Thread(s)
Quoted: 4360 Post(s)
Likes: 0
Liked 15 Times in 15 Posts
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.
__________________
FB
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.

Last edited by FBinNY; 03-23-17 at 11:47 AM.
FBinNY is offline  
Old 03-23-17, 12:49 PM
  #8  
fietsbob 
coprolite
 
fietsbob's Avatar
 
Join Date: Jun 2010
Location: NW,Oregon Coast
Posts: 41,886

Bikes: 8

Mentioned: 190 Post(s)
Tagged: 0 Thread(s)
Quoted: 7138 Post(s)
Liked 650 Times in 431 Posts
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.




fietsbob is offline  
Old 03-23-17, 02:54 PM
  #9  
Scooper
Decrepit Member
 
Scooper's Avatar
 
Join Date: Aug 2005
Location: Santa Rosa, California
Posts: 10,472

Bikes: Waterford 953 RS-22, several Paramounts

Mentioned: 66 Post(s)
Tagged: 0 Thread(s)
Quoted: 617 Post(s)
Likes: 0
Liked 24 Times in 22 Posts
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.
__________________
- Stan

my bikes

Science doesn't care what you believe.
Scooper is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
cqlink
Classic and Vintage Bicycles: Whats it Worth? Appraisals.
3
01-16-19 01:21 PM
crandress
Classic & Vintage
4
01-06-18 05:56 PM
Heyspike
Bicycle Mechanics
12
01-23-17 08:48 PM
cpsqlrwn
Classic & Vintage
14
04-26-16 11:12 AM
Just Riding
Bicycle Mechanics
2
04-23-12 07:46 PM

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.