Seat post size
12-03-18 | 05:41 PM
#1
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Seat post size
I am going to use a Reynolds 631 seat tube 31.75mm diameter double butted 0.8/0.5/0.8 The nominal internal diameter of the seat tube is 30.15mm
It will be quite a large frame so there will be plenty of thicker butted section at the top to support the seatpost.
Build will be silver brazed lugs.
I am trying to source a seatpost and am not sure what range of sizes I could use. None of the sizes seem common so the more choice the better.
Would a 30.0 mm post fit? I know they have to be slightly smaller than the seat tube hole but would this work. It would be handy for me as I already have an adjustable reamer that will reach about 30.1
What would be the maximum size? Could I ream out nearly all of the butting to fit a 30.6 seatpost? Would this compromise the strength? I am aware people use single butted seat tubes 0.5 thick which makes me think it is fine, but then I also read you should use an externally butted tube.
Would 30.2 or 30.4 be a better compromise.
Not keen on using shims!
Thanks
Mike
It will be quite a large frame so there will be plenty of thicker butted section at the top to support the seatpost.
Build will be silver brazed lugs.
I am trying to source a seatpost and am not sure what range of sizes I could use. None of the sizes seem common so the more choice the better.
Would a 30.0 mm post fit? I know they have to be slightly smaller than the seat tube hole but would this work. It would be handy for me as I already have an adjustable reamer that will reach about 30.1
What would be the maximum size? Could I ream out nearly all of the butting to fit a 30.6 seatpost? Would this compromise the strength? I am aware people use single butted seat tubes 0.5 thick which makes me think it is fine, but then I also read you should use an externally butted tube.
Would 30.2 or 30.4 be a better compromise.
Not keen on using shims!
Thanks
Mike
12-03-18 | 05:58 PM
#2
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Joined: Jun 2004
Posts: 3,146
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From: Torrance, CA
Bikes: Homebuilt steel
Typically you want the seat tube ID to be about .2mm larger than the post size. You can ream to various sizes, even removing all the .8mm butt. Whatever you decide just be sure to check for available seat posts. There aren't many 30.0mm or 30.6mm seat posts around other than a Thomson and some elchepo's, but anything from 30.0 to 30.6 should be viable.
Last edited by Nessism; 12-03-18 at 06:02 PM.
12-03-18 | 09:12 PM
#3
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Joined: Feb 2012
Posts: 19,344
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From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Not sure i'd want to shave a ST top wall down to .6mm if the stays were attached to the ST directly. But we don't know about this aspect yet, or whether the joining will be brazed or welded. Andy
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AndrewRStewart
AndrewRStewart
12-04-18 | 03:33 PM
#5
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From: NW,Oregon Coast
Bikes: 8
Relax, Machined seat post sizing Shims work very well ,
I now have 3 bikes , over sized seat tubes, with sizing shims
to use common 27.2 seatposts , 2 are cane creek Thudbuster seat posts..
in fact one of them, 3rd example; a Bike Friday Tikit,
I took out the 27.2 Kalloy Uno
seat post, head slipped angle too easily..
and reused my 25.0 <C> SR* with a 25>27.2 USE seat post shim,
so its a Sleeve inside a sleeve, inside the seat tube.
* came out of my screwed & Glued AlAn..
I now have 3 bikes , over sized seat tubes, with sizing shims
to use common 27.2 seatposts , 2 are cane creek Thudbuster seat posts..
in fact one of them, 3rd example; a Bike Friday Tikit,
I took out the 27.2 Kalloy Uno
seat post, head slipped angle too easily..
and reused my 25.0 <C> SR* with a 25>27.2 USE seat post shim,
so its a Sleeve inside a sleeve, inside the seat tube.
* came out of my screwed & Glued AlAn..
