Having managed to buy a 26.8mm seat post instead of a 27.2 this week I’ve got to thinking. How is it that we’ve ended up with so many different diameters?

Was it done as a ‘buy our posts, not theirs’ type deal?

Different tube sizes innit.

I suspect it was dictated by the inner diameter of standard sizes of steel (and later) aluminium tubes. Older narrower diameters being from steel tubes. As tube sizes increased so did seat post diameters.

But .4 of a mm though!

At this stage I can’t rule out that it’s an intricate plot designed purely to wind me up.

Somebody post the XKCD…

Because the bike industry hates you.

Somebody post the XKCD…

This one?

http://xkcd.com/927/

I sometimes wonder how humans can’t manage to measure twice, buy once….

I sometimes wonder how some humans manage to never make a single mistake.

Seat posts are easy to figure out. Headsets are utter nonsense.

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.

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 are

Seat posts are easy to figure out. Headsets are utter nonsense.

Especially when some headset manufacturers cannot make them the stated size….

Seatpost standards are one of the few things in cycling that are becoming easier . There now are 4 main sizes used 27.2 , 30.9 , 31.6 , and the big size that Scott use . There used to be all sorts of different sizes used .

The thing that bugs me is brake pads.

I mean does there really need to be nearly 40 different shapes of brake pads? Really?

james – Member

Giant (used to?) do a 30.8mm, where there are loads of 30.9mm out there

And Ragley invented the 27.3…

In theory, direct mount front mechs could free desiners up to do lots of silly things with the seat tube size but I imagine dropper post manufacturers will have the last word on seatpost sizes.

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.

And Ragley invented the 27.3…

Because they bought a dodgy Chinese reamer?

I build all my frames to be 27.2. Because I bought one good Silva reamer and I’m not buying more.

bencooper – Member

Because they bought a dodgy Chinese reamer?

No, it was definitely done for performance reasons, after all the factory they chose is the BEST. 27.3 is one more. Well, .1 more.

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.
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.

thats because there is no “Standard” even HS are messed up now, 1inch used to be std, then it went 1,1/4 then we std out on 1 1/8 now its all cocked up again.