xccx

A dumb question. If a seatpost says 27.0 on it, is it actually 27.2? I told you it was dumb.

gruppo

"Yes" it is a dumb question and one which refutes the claim, there are no dumb questions, and "No" to the seatpost size question.

damian_

Its actually a bit smaller.

TallRider

It's smaller by 0.2mm. Which does matter.
27.2mm is the most common seatpost size for which bicycle frames are designed, but 27.0mm seatposts and frames designed for them do exist.
I have seen one case in which a frame that should have had a 27.2mm post had a 27.0mm, and the seatclamp was crimped at a weird angle to get it tight. I uncrimped it and put a 27.2mm post it. But I have no idea if this is common.

Grand Bois

If you wrapped a piece of notebook paper around the post, it would increase the diameter by 0.2. In my experience, manufacturer's tolerences are not that close. It took a rubber mallet to get my 26.1 Zeus post into my Raleigh's 26.4 seat tube, and that was after sanding and polishing the post and using a brake hone in the tube.

TallRider

At least on better-quality bikes, they are that close, and then some. In the case I mentioned above, I was able to realize that the 27.0mm post was too small because there was excess play upon sliding the post into the frame, and when the seatpost bolt was tightened enough to hold the post, the seat-collar lug was clearly crimped too far shut. Putting in a 27.2mm post in solved both of these issues.
A piece of notebook paper would not meaningfully increase the width once it was put under compressive force.

FlatFender

A cut up beer can makes a good shim.

Grand Bois

Did you really think I was suggesting that he shim the post with a piece of paper? A post that is .2 too small will not caise the pinched look that you describe because the difference only equals the thickness of a piece of notebook paper.

The statement about better quality bikes is about as appropriate as your avatar.

cs1

I had a similar problem on my Waterford 1200. I came with a 27.2 seatpost where a 27.4 should have been. The collar lug was almost touching. It took a little work to fix it. Now it has a 27.4 and it works very well. So, yes .2mm does make a difference.

If you need a special size just go to ebay. I found a 29.4 seatpost for my son's old Trek MTB bike there. Just about any size you need can be found on ebay. Good luck

Tim

operator

Take it easy gentlemen...

TallRider

Nah, I didn't think that at all; sorry if my response was unclear. My point was that 0.2mm does actually make an easily noticeable difference, which is surprising on the face of it but pans out in most experience. In the case I'd mentioned above, I'd put two 27.0mm posts and two 27.2mm posts into the frame, and the difference was indeed caused by that 0.2mm.
Maybe it doesn't relate to the quality of the bikes. I mentioned quality of finishing only because lots of old Schwinns came stock with shims, I (and others) guess so that Schwinn didn't have to finish the interior of the seat tube carefully.

phoebeisis

With a lot of posts and frames, a .2mm too small post will fit the frame ok without crimping.Make sure you eyeball the cutout in the frame while you are tightening up the post.If it closes up too much, back off.You cam make a shim easily enough with aluminum flashing.This flashing measures about .007"(probably is supposed to be .010").You can double or triple it as needed.Make sure you pedal and roll the top edge, so it doesn't puch down into the frame. Like someone said above-beer cans etc should work fine also.You form the shim by rolling it around your seatpost.Luck,Charlie