fit 30.8 mm seat post in 30.9 mm frame?


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:


New Member
Bromley, Kent
I thought there was only two diametres? 27.2 and 30.9
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)


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.

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