Short seatpost shim - dangerous?

Discussion in 'Cycling Equipment' started by Jean, Jun 7, 2003.

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  1. Jean

    Jean Guest

    I just bought a new seatpost for my road bike, but I need to use a shim for it to fit the seat tube
    (25.4mm post => 26.0mm seat tube inner diameter).

    The local bike shop has a shim available, but it is only about 48mm long. I vaguely remember that
    the minimum seat post insertion is supposed to be at least 2.5 times the seat post diameter
    ...which, in my case, yields a minimum insertion of 65mm.

    Also, the short 48mm shim would not extend below the welds on the seat tube -it would have to be at
    least 52mm long to extend below all the welds.

    Will using the "too short" shim unduly stress the seat tube (steel) and/or the seat post (aluminum)?
    Would the post wobble in the tube if subjected to heavy load?

    Thanks for any guidance,

    Jean
     
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  2. The reason for the amount of insertion into the frame is so that enough of the tube is supported.
    The shim is used to make up for a slight difference in tupe/post sizes, usually only a few
    thousanths of an inch. It is simply for the purpose of tightening the frame tube around the post.
    Rarely more than the top inch or so of the frame tightens around the seat post even without a shim.

    As long as you have at least the minimum amount of seat tube inside the frame, you should be fine.

    May you have the wind at your back. And a really low gear for the hills! Chris

    Chris'Z Corner "The Website for the Common Bicyclist": http://www.geocities.com/czcorner
     
  3. A Muzi

    A Muzi Guest

    "Jean" <[email protected]> wrote in message news:[email protected]...
    > I just bought a new seatpost for my road bike, but I need to use a shim
    for
    > it to fit the seat tube (25.4mm post => 26.0mm seat tube inner diameter).
    >
    > The local bike shop has a shim available, but it is only about 48mm long.
    I
    > vaguely remember that the minimum seat post insertion is supposed to be at least 2.5 times the
    > seat post diameter ...which, in my case, yields a minimum insertion of 65mm.
    >
    > Also, the short 48mm shim would not extend below the welds on the seat tube -it would have to be
    > at least 52mm long to extend below all the
    welds.
    >
    > Will using the "too short" shim unduly stress the seat tube (steel) and/or the seat post
    > (aluminum)? Would the post wobble in the tube if subjected
    to
    > heavy load?

    I agree that sounds marginally unsafe. Since 26.0 posts are very available in 350 and 400 mm ( they
    only cost twice what the liner did!) why not get the right part??

    Selling you a 25.4 post for a 26.0 frame sounds like a case of ignorance at the bike shop in the
    most magnanimous reading. . .

    --
    Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971
     
  4. On Sun, 8 Jun 2003, A Muzi wrote:
    > "Jean" <[email protected]> wrote in message news:[email protected]...
    > > I just bought a new seatpost for my road bike, but I need to use a shim
    > for
    > > it to fit the seat tube (25.4mm post => 26.0mm seat tube inner diameter). The local bike shop
    > > has a shim available, but it is only about 48mm long.

    I successfully make shims out of mylar (some 'plastic') film. Very cheap, if you only find a source,
    and easy to trim. I even mailed some out to a friend in Malaysia, to his full satisfaction.

    It comes in sheets. So, any size is available.

    Sergio Pisa
     
  5. Jean

    Jean Guest

    >A Muzi wrote:

    |"Jean" <[email protected]> wrote in message
    |
    | I just bought a new seatpost for my road bike, but I need to use a shim
    for
    | it to fit the seat tube (25.4mm post => 26.0mm seat tube inner diameter).
    |
    | The local bike shop has a shim available, but it is only about 48mm long.
    I
    | vaguely remember that the minimum seat post insertion is supposed to be at least 2.5 times the
    | seat post diameter ...which, in my case, yields a minimum insertion of 65mm.
    |
    | Also, the short 48mm shim would not extend below the welds on the seat tube -it would have to be
    | at least 52mm long to extend below all the
    welds.
    |
    | Will using the "too short" shim unduly stress the seat tube (steel) and/or the seat post
    | (aluminum)? Would the post wobble in the tube if subjected
    to
    | heavy load?

    >I agree that sounds marginally unsafe. Since 26.0 posts are very available in 350 and 400 mm ( they
    >only cost twice what the liner did!) why not get the right part??

    >Selling you a 25.4 post for a 26.0 frame sounds like a case of ignorance at the bike shop in the
    >most magnanimous reading. . .

    >Andrew Muzi

    I bought the seatpost (via mail-order) because it has zero set-back --which I need for proper fit.
    The post is only made in two diameters - either 25.4 or 27.2, and it comes with a shim of your
    choice. Unfortunately, the supplier did not have any 26.0 shims. Hence my search for a shim at my
    LBS. Sooo, neither the seller nor I have made any mistakes ... yet ...

    Jean
     
  6. On 10 Jun 2003, Ken wrote:

    > Needed to replace my seatpost on my roadbike. It has a strange diameter of 28mm.
    .............
    > Sorry got carried away with my reply,

    This is the last time I post about shims. Promise!

    Why don't you guys give up depending on fancy-looking and apparently high-tech solutions?

    First of all any aluminum seatpost can be somewhat reduced in diameter by polishing. Next, get
    yourself some mylar film to make all possible shims. Here is a reprint of private correspondence
    with a friend in Malaysia, to which I mailed mylar shims in fact ... .

    Sergio Pisa
    _____________________________________________________________________________
    Hi Sergio, Still using the shim... on second bike. Excellent!

    One thing good about the shim is that it's much easier to handle than metal that have sharp edges
    that cuts. Regards, KW
    _____________________________________________________________________________
     
  7. > Any suggestions as to what kind of store I should call for mylar sheets? Thanks,

    1) A good hardware store;
    2) a shop that overhaules, and rewinds, electric motors;
    3) any high-tech (University, perhaps) experimental lab.

    If you can't locate it send me your address and postage refund so that I can mail it to you. I got
    about one square meter of it ... .

    Sergio Pisa
     
  8. Jean

    Jean Guest

    "Sergio SERVADIO" <[email protected]> wrote in message
    news:p[email protected]...
    | > Any suggestions as to what kind of store I should call for mylar sheets? Thanks,
    |
    | 1) A good hardware store;
    | 2) a shop that overhaules, and rewinds, electric motors;
    | 3) any high-tech (University, perhaps) experimental lab.
    |
    | If you can't locate it send me your address and postage refund so that I can mail it to you. I got
    | about one square meter of it ... .
    |
    | Sergio Pisa
    |

    Thanks for the advice. I'll try to find some locally, and if that fails I'll contact you.

    Jean
     
  9. Kw Mui

    Kw Mui Guest

    You may want to try a stationary shop.

    The mylar that Sergio sent me looked like the thickest transparency sheet used for projection. Trust
    that 3M ones will be robust enough.

    No seat post slippage nor worries about metal fusing together. I've used the mylar set up in rain
    and no water seeped into the seat tube. The mylar has some compression, which is good.

    Regards, KW Mui

    Sergio SERVADIO <[email protected]> wrote in message
    news:<[email protected]>...
    > > Any suggestions as to what kind of store I should call for mylar sheets? Thanks,
    >
    > 1) A good hardware store;
    > 2) a shop that overhaules, and rewinds, electric motors;
    > 3) any high-tech (University, perhaps) experimental lab.
    >
    > If you can't locate it send me your address and postage refund so that I can mail it to you. I got
    > about one square meter of it ... .
    >
    > Sergio Pisa
     
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