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I've been wondering where the tradition for "weird" sized standards comes from. For example, 27.2, 30.9 or 31.6mm seat post standards. Are they a metric conversion of something imperial, or are they a throwback to the inner diameter of some standard pipe size that BITD was used to build frames? Or is someone just messing with us?
See also : bar diameters (not 35mm which is a nice round number, but even that still has 22.2 mm at the ends).
They are metric conversions of imperial I believe.
22.22 = 7/8" for example
27.2 - 1-1/16" or there abouts allowing for limits and fits
Yup, mostly imperial conversions. Not sure how 30.9 came about.
What Jef said, they're all old imperial sizes which would have been the internal diameter of the available tubes BITD.
Road handlebars were 'metric' 26mm for ages not 25.4 like MTB's, then went to 31.8 around the same time as MTB's, but labeled it as 31.9 (the actual diameter is 31.8something).
Not sure how 30.9 came about.
As it's a seatpost it's probably something to do with an imperial size and then not reaming out as much material, something like 1.1/5 tubing, reamed out a small fraction to make it uniform?
Nearest I could estimate was 1 - 3/16" as a standard bore for a seat tube? 30.9mm seat post would be a nice sliding fit.
35mm bar standard isn't 35mm - it's 34.9, or 1&3/8"
1-7/32" is roughly 30.96 mm.
https://mdmetric.com/tech/cvtchtfdm.htm
I'd always assumed that the internal seattube diameter was 31 mm and the posts were 30.8 or 30.9 mm to give enough clearance.
Historically it mostly came from the ID's (reamed?) of available standard tubes, so 27.2 typically equated to a 28.6 (1-1/8") OD steel seat tube, interestingly enough I've got a 90's kona at home with a 27.0 ID seat tube...
There were some frames that took 26.8 posts and of course 25.4 (1") and 22.2 (7/8") going further back in history...
As Aluminium tubes tend to come in larger diameters and became more common throughout the 80s-90s so the posts increased in diameter (as well as front mech clamps), but they tended to stick mostly to more or less imperial sizes (but still labelled in their metric dimension to 1DP typically). Until you get to today where (presumably) for the purposes of parts interchangability with metal bikes, and possibly providing a bit of comfort where smaller diameter posts are used, carbon bikes (which have nothing really constraining the diameter of their seat posts) frames still mostly use the same sizes...
Considering how "StaNdardmaggedon" is in full swing I wouldn't be shocked if MTBs suddenly settled on a nice big, metric (say 40mm) diameter seat post to accomodate more robust, chunkier dropper post designs for carbon and aluminium bikes and steel fabricators be damned, sooner or later something similar would be done to road bikes as another simple way to force the backwards compatability issue...
35mm bar standard isn’t 35mm – it’s 34.9, or 1&3/8″
You've ruined my day :O 😀
Thanks for all the info, everyone. I knew someone here would know 🙂
30.9 seatpost Alu frames use 34.9 seatpost collar/fmech clamp
34.9od - 30.9id = 4mm
2mm thick alu tubing init?
no real need to overthink it any further unless you're building/designing frames.
I'm assuming 30.9 came about because some **** figured out what was the closest to 31.6 they could make it and still be incompatible.
I’m assuming 30.9 came about because some **** figured out what was the closest to 31.6 they could make it and still be incompatible.
So much this.
My Indy Fab has a 29.4mm seatpost.