fit 30.8 mm seat post in 30.9 mm frame?

Discussion in 'Bicycle Mechanics and Repairs' started by 02GF74, 14 Feb 2008.

  1. 02GF74

    02GF74 Über Member

    Ok, it is a 0.1 mm difference - the width of a human hair - but will there be any problems using a 30.8 mm seat post in a frame designed to take 30.9 mm?

    (just curious - I have 30.9 on order so won't be trying it out).

    And why is it necessary to have so many differet seat post diameters? Surely 2 or 3 should cover all possible frames.:smile:
  2. walker

    walker New Member

    Bromley, Kent
    I thought there was only two diametres? 27.2 and 30.9
  3. simonali

    simonali Guru

    No there are loads of them. I think my old MTB uses a 26.8.
  4. simon_adams_uk

    simon_adams_uk Über Member

    SW London
    Should be fine - if there's a small amount of play and it does slip you can always shim it using a piece of coke can...

  5. Not quite, Coke can is 0.2mm.
  6. Smokin Joe

    Smokin Joe Legendary Member

    They are the same size, I believe. Bike dimensions were traditionally in imperial measurements and the 0.1mm variation depends on whether the manufaturer rounds up or down when converting.
  7. andygates

    andygates New Member

    Diet Coke then.
  8. softpeadals

    softpeadals New Member

  9. 30.9mm is a Specialized only post size.

    In the olden days the external diameter of a seat tube was 1 1/8''. The introduction of oversized aloonyman frames required the use of tubes with greater outside diameters, hence; 1 1/4'' and 1 3/8''. By amazing coincidence these sizes correspond perfectly to the modern metric sizes of 28.6mm, 31.4mm and 34.9mm. There are only three, three is the number of seat tube external diameters.

    Back to the olden days, given that there is only one size of front mech clamp to fit only one size of seat tube why were there so many different seat post diameters? Well the answer is straightforward. High end tubing such as 531 (manganese/molebdinum alloy) had an internal diameter (and hence seatpost outside diameter) of 27.2mm. High quality, thin walled tubing. Less expensive and therefore heavier tubes have the same external dimension and a thicker wall section and therefore a smaller internal diameter, 26.8mm in the case of Reynolds 501 (Chrome/molebdinum alloy)for example.

    I dont know why Specialized felt the need to invent their own size, I expect there is a very valid technological reason for it. Until they started plating silly buggers everyone graded tubing and hence seatposts in 0.2mm increments. A fifth of a mm. Which is the exact thickness of the material used to manufacture Coke cans.

    (Wrong, it's 0.1mm)
  10. simonali

    simonali Guru

    Surely to shim a seatpost by 0.2mm you'd need 0.1mm material, so all this talk of Coke cans is irrelevant?
  11. simon_adams_uk

    simon_adams_uk Über Member

    SW London
    Sorry for the slight confusion - I had in mind (but not put down in word!) that if there was some slippage (due the theoretical size difference AND any manufacturing tolerance difference) then a coke can shim could work.

  12. shoot, arse, toss, ****, bugger. :wacko:

    Absolutely right, a coke can is 0.1mm.
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