Seat post standard 27.2 or 31.6
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  1. #1
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    Seat post standard 27.2 or 31.6

    I'm getting ready to have a Ti frame built and just found out that I have a choice of a 27.2mm seatpost or a 31.6mm seatpost.

    Is there a standard emerging here? similar to the handlebar standard emerging at 31.8mm, sometimes called oversize. We are seeing more and more availability of handlebars and stems at 31.8mm rather than 25.8mm.

    Would the 31.6mm be stronger but heavier?

  2. #2
    Potatoes
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    31.6

    Currently, I'd say newer frames are utilising 31.6mm. Mainly because a thin large tube is stiffer, stronger and lighter than the equivalent small tube (which would be achieved by making the tube thicker). I'd say go with what the builder recommends for your style of riding etc.

    Also keep in mind that you can always use shims to fit a smaller post into your frame- not the other way round.

  3. #3
    wim
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    Don't know if an oversize seat tube is becoming the standard, but keep in mind that it could also affect tread, aka q-factor. On some frames with a fat seat tube, you need to move the crank out with a longer-than-normal bottom bracket spindle to accomodate front derailleur cage travel. If you're used to a very narrow tread, you may not care for that. Check with your builder.

  4. #4
    duh...
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    Quote Originally Posted by wim
    Don't know if an oversize seat tube is becoming the standard, but keep in mind that it could also affect tread, aka q-factor. On some frames with a fat seat tube, you need to move the crank out with a longer-than-normal bottom bracket spindle to accomodate front derailleur cage travel. If you're used to a very narrow tread, you may not care for that. Check with your builder.

    are you sure? that's seems like it would F alot of things up... if the der & crank gotta move outwards, chainline moves out too, but ony in the front since wheel/cassette standard has not changed... lots of bikes have 31.6 seat tubes (down near the BB), it's the 34+ that you might be more concerned with

  5. #5
    wim
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    are you sure?
    No, I'm not. I can't know if the oversize tubing offered by lawrence's builder will affect anything other than seatpost size, but that's why I wrote " . . . could also affect tread." It's just something to think about when given a choice of seat tube inside diameters. A change of dimension can ripple all across the bike.

    FWIW, 31.6 mm is an inside diameter according to the OP, so my guess is that the outside diameter of such a seat tube is more than that.
    Last edited by wim; 08-23-2007 at 10:11 AM.

  6. #6
    duh...
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    Quote Originally Posted by wim
    No, I'm not. I can't know if the oversize tubing offered by lawrence's builder will affect anything other than seatpost size, but that's why I wrote " . . . could affect tread." It's just something to think about when given a choice of seat tube inside diameters. A change of dimension can ripple all across the bike.

    FWIW, 31.6 mm is an inside diameter according to the OP, so my guess is that the outside diameter of such a seat tube is more than that.


    if it's a competent builder it shouldn't be an issue

  7. #7
    i like whiskey
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    In all instances where you are not sure what to spec on a custom frame, defer to the builders expertise and opinion.

    FWIW, 27.2 seatposts are readily available at virtually every bike shop if you ever need to replace. I don't recall seeing 31.6 posts, but I don't really look at post sizes on anyone elses bikes.

  8. #8
    Steaming piles of opinion
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    Quote Originally Posted by innergel
    In all instances where you are not sure what to spec on a custom frame, defer to the builders expertise and opinion.

    FWIW, 27.2 seatposts are readily available at virtually every bike shop if you ever need to replace. I don't recall seeing 31.6 posts, but I don't really look at post sizes on anyone elses bikes.
    Agreed. Not that you need to find any more than one, but 27.2 is 'standard', 31.6 has a much narrower set of offerings. If there is some particular design or style of seatpost you like, you may want to check it's availability in the larger size. Yeah, they can be shimmed, but that's a less-than solution.

    That said, a Thomson in 31.6 is the 'right' answer, IMO.
    A good habit is as hard to break as a bad one..

  9. #9
    eminence grease
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    I just completed a bike with a 31.6 post and it was a pain in the butt trying to find a post that met my wishes for the build. I can't use a straight and the Thomson setback is ugly in my estimation.

    There are not many choices out there in that size. Of course, you can shim it down to 27.2 and open an entire universe of choices. Or, you can ask your builder to incorporate a him in the seat tube and end up with the same result.

    My opinion - if you don't need it for whatever magic reason, don't do it.
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  10. #10
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    Thumbs up 31.6 is the way to go

    I find it interesting that virtually ever tube size on modern bikes has increased greatly over the years except seatpost size. Cannondale, the mother of all things oversize, has for some reason always kept thier seatposts at the standard 27.2 size. Klein, on the other hand has always used 31.6. Finally, Cdale seems to be going with 31.6 with the new system six frames. The increased size undoubtably helps with the whole weight/ stiffness/ strength equation, so lets dump the decades old 27.2 standard and go with the 31.6.

  11. #11
    Juanmoretime
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    Quote Originally Posted by terry b
    I just completed a bike with a 31.6 post and it was a pain in the butt trying to find a post that met my wishes for the build. I can't use a straight and the Thomson setback is ugly in my estimation.

    There are not many choices out there in that size. Of course, you can shim it down to 27.2 and open an entire universe of choices. Or, you can ask your builder to incorporate a him in the seat tube and end up with the same result.

    My opinion - if you don't need it for whatever magic reason, don't do it.
    I have a 31.6 too and found quire a few that work well with setback. FSA makes the Lite post with 25mm or 35mm, Ritchey WCS carbon with 25mm or the Ferrari of posts the Ax Lightness Daedalus is my favorite.
    For my next trick I will now set myself on fire!

  12. #12
    eminence grease
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    Quote Originally Posted by Juanmoretime
    I have a 31.6 too and found quire a few that work well with setback. FSA makes the Lite post with 25mm or 35mm, Ritchey WCS carbon with 25mm or the Ferrari of posts the Ax Lightness Daedalus is my favorite.
    Yep, but this time I was looking for a silver one. And that was a bit harder.
    You'd be better off with a netbook, they do everything better.

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