Seatpost sizing question

[rhyscycles]

Hi guys,

I have a rather nice Red Ringle Moby 26.8 seatpost which I would like to use on a build but the frame to be used requires a 27.0 post!

Now here we are talking .2 of a mil' - it seems nobody provides a shim that thin but then I can clamp it tight but I'm worried about stressing the seat tube - should I be!?

What should I do?

Cheers.

 

[Corduroyboy]

What's your favourite fizzy drink?

 

[tintin40]

Certainly don't tighten it up. :shock: There are good reason why seat tubes/seat posts come such small increases.
You do have bit of a problem :?

 

[rhyscycles]

What an idiot - I'm sat here looking at my shim!! :roll:

And the donor is.....drum roll please..........A Diet Coke can!

Nice one fella!

 

[rhyscycles]

Certainly don't tighten it up. :shock: There are good reason why seat tubes/seat posts come such small increases.
You do have bit of a problem :?

Yeah, in an ideal world I would be able to find the Red Moby in a 27.0 :cry:

Ah well, maybe I will just have to use something else instead for now if the Diet Coke shim doesn't work.

I'm curious now, the obvious good reason for such small increases would be a perfect fit but why do bike manufactures vary so much in the use of sizes? .2mm difference just seems a bit daft surely they could use either 26.8 or 27.0 or 27.2 as a standard.

Is a manufacturers choice of size driven by the choice of material or maybe the chosen manufacturing process? How would they justify using 26.8 instead of 27.0? :?

 

[Anthony]

I thought a coke can was more than 0.1mm

If it's of any interest, I have two Kilaueas and an Explosif, all of which have 27.2 posts because that's what fits. But 95+% of owners will have 27.0 posts fitted, because that's what Kona says you should use. If there was any reason to worry about clamping hard enough to squeeze the tube for that 0.2mm, then those 95+% of Kilauea/Explosif owners would be suffering seat tube breakages. But I've not heard of any reputation for weakness there, so my guess is it isn't too much of a problem.

Just to be clear, a 27.0 post in any of those three frames will just about slip down the tube under gravity without you hardly touching it, and yet Kona say that's ok.

 

[Anthony]

I'm curious now, the obvious good reason for such small increases would be a perfect fit but why do bike manufactures vary so much in the use of sizes? .2mm difference just seems a bit daft surely they could use either 26.8 or 27.0 or 27.2.
Is a manufacturers choice of size driven by the choice of material or maybe the chosen manufacturing process? How would they justify using 26.8 instead of 27.0? :?

Almost all steel seat tubes used to be 28.6mm tubes with external butting at the top - the seatpost size then depends how thick the tube walls are - the thinner the walls, the more room there is left for the seat post. That's why late 80s/early 90s bikes had as little as 26.4 posts - the wall thicknesses in those days used to be that much greater.

 

[rhyscycles]

I'm curious now, the obvious good reason for such small increases would be a perfect fit but why do bike manufactures vary so much in the use of sizes? .2mm difference just seems a bit daft surely they could use either 26.8 or 27.0 or 27.2.
Is a manufacturers choice of size driven by the choice of material or maybe the chosen manufacturing process? How would they justify using 26.8 instead of 27.0? :?

Almost all steel seat tubes used to be 28.6mm tubes with external butting at the top - the seatpost size then depends how thick the tube walls are - the thinner the walls, the more room there is left for the seat post. That's why late 80s/early 90s bikes had as little as 26.4 posts - the wall thicknesses in those days used to be that much greater.

Cheers Anthony, so one would assume as time has moved on and stronger, lighter materials are used the tube walls mentionned have become thinner and/or tubing diameter has increased resulting in the need for larger diameter posts?

Oh and just if you're interested I found two measurements on Google for the thickness of the wall of a Coke Can, 0.0762mm and 0.10922. So Anthony if we were to go with the later result you would be right!

Cheers.

 

[Geoff]

I thought a coke can was more than 0.1mm

If it's of any interest, I have two Kilaueas and an Explosif, all of which have 27.2 posts because that's what fits. But 95+% of owners will have 27.0 posts fitted, because that's what Kona says you should use. If there was any reason to worry about clamping hard enough to squeeze the tube for that 0.2mm, then those 95+% of Kilauea/Explosif owners would be suffering seat tube breakages. But I've not heard of any reputation for weakness there, so my guess is it isn't too much of a problem.

Just to be clear, a 27.0 post in any of those three frames will just about slip down the tube under gravity without you hardly touching it, and yet Kona say that's ok.

and yet for some reason, my Kil has a 26.8 Thomson which does not slip down under gravity! :?

I always just worked on the principle that if the bolt had to be done up so tight the slot looked really pinched together at the top, post was too small.

 

[Anthony]

Cheers Anthony, so one would assume as time has moved on and stronger, lighter materials are used the tube walls mentionned have become thinner and/or tubing diameter has increased resulting in the need for larger diameter posts?
Oh and just if you're interested I found two measurements on Google for the thickness of the wall of a Coke Can, 0.0762mm and 0.10922. So Anthony if we were to go with the later result you would be right!
Cheers.

Well I think a 27.2 is about the biggest post you'll see in an mtb with a 28.6 tube. The 97 Kilauea has a 28.6 x 1.3/0.6/0.9 seat tube and the 97 Lava Dome has a 28.6 x 1.3/0.7/1.0 (where the 1.3 is 0.6 and 0.7 respectively plus external butting). So it isn't really any surprise that the Kilauea takes a 0.2 bigger post than the Lava Dome. And to go back to your original question, my cynical guess is that Kona spec'd the same size post for both because there were economies of scale in it for them and they thought clamping a bit harder for the bikes that could really do with a 27.2 wasn't a big issue.

I think 0.6 has been about the thinnest steel has gone for seat tubes, although e.g., Columbus make a 0.75/0.4/0.6 tube in the Life series, but they don't recommend it for mtb - and it's only 31g lighter than the 1.0/0.6/0.8 externally-butted tube that they do recommend for mtb, so why take a risk with your ass?

I found the difficulty with coke can was to get it to fit without a sharp/f'ugly bit poking out of the top - bit of a shame to spend a lot of money on a cool seatpost and then have a f'ugly bit of coke can making an eyesore of itself to make it fit!