- Seat post standards, why so many?
I think it came about because most tubing in engineering terms is spec’d by the od rather than id. You couldn’t get a seat tube 0.4 mm bigger in od because you still had to fit a front mech. Therefore, to make this tube lighter, you make the walls thinner and hence need a slightly larger seat post for it.Posted 6 years agojamesMember
as above afaik there are 3 (common?) front mech clamp sizes (and 4 seat clamp sizes?). So differing wall/strength seat tube specs/what not in different materials/different alloys of result in so many seatpost sizes
“But .4 of a mm though!”
There is a 27.0mm standard too in between your 26.8mm and 27.2mm
Giant (used to?) do a 30.8mm, where there are loads of 30.9mm out there
Just look down the options of a thomson elite or a brand x seatpost on crc for how many similar sizes there arePosted 6 years agomick_rSubscriber
For a long time, a common steel bike frame seat tube size has been 1 1/8″ = 28.6mm. The approximate standard (reamed) bore for this size on a finished frame is 27.2mm.
However the actual bore depends on lots of things:-
The exact tube used (thinner / thicker walls, butting etc).
Amount of heat distortion when frame was made.
Amount of reaming after welding / brazing.
Hence there are posts 26.8 / 27 / 27.2 etc
This tube size is sometimes used as-is for mtbs (hence you can get 28.6mm seat clamp collars), but is a bit beefier used with a lugged frame or an extra outer sleeve (so you also get 30mm clamp collars etc).
Bikes like Inbreds have a nominal 28.6mm seat tube which is externally butted at the top rather than being sleeved – so it takes a standard 27.2mm post and a 29.8mm clamp collar.
There is a book “Touring Bikes” by Tony Oliver that goes into intimate detail about various steel tubes if you really want to read more……
Not a clue about the history in aluminium frames.Posted 6 years agomattsccmMember
Tony Oliver argues, with some conviction, that 531 for example should be built with 27.4 mm pins in mind as this is the correct ID of the tube. 27.2 exists because that size reamer fits best in the slightly distorted tubing.Posted 6 years ago
As 531 was for amny years an industry standard it is to a great extent the reason why 27.2 was common.
Columbus steel ranged from 27.2 to 27.6 ID Tange from 26.8 to 27.4 and Vitus from 27.0 to 27.4
Hence the variety of sizes even in the days of pretty well everyone needing the same OD.
With manufactures able to do what they please it amazes me that we have only 2 “standard” plus lots of odballs.
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