Seatpost confusion
 

In another thread today I was reading about a bike with a tight fitting seatpost. The opening measured 27.29mm at it's widest and 26.9mm front to back. I guess this is common?

I've only recently entered the magical world of C&V bikes, having cut my teeth on fairly new bikes where I buy the correct size seatpost and it just fits. Now I find myself in a place where I'm not even sure that a seatpost "fitting" is a solid indication that it is the correct size. Round pegs, oval holes...what are you supposed to do?

Let me offer a few data points from my personal experience:


Case 1: Pinarello Gran Turismo

When I bought this bike it had a 27.2 seatpost. The tubing is Columbus SL so 27.2 sounded right. However, the seatpost was in there really tight, to the point of leaving deep scratches from the extraction process. I moved that post to another bike where it fit just fine. A different 27.2 seatpost was similarly stubborn about going into or out of the Gran Turismo. After a little research I decided that maybe this bike needed a 27.0 seatpost after all. A 27.0 seatpost slides in relatively easily and without any wiggle. Ultimately (because it was what I could find at the price and quality I wanted), I ended up getting a 27.2 seatpost and sanding it down until it would go in without a fight. It's probably around 27.05.


Case 2: Austro Daimler Team

When I bought this bike it had a 27.0 seatpost. The bike has Reynolds 531 butted tubing, so I was surprised by that, but a 27.2 post wouldn't fit and other 27.0 posts did. When I wanted a nicer post I bought a 27.0 and it fit.


Case 3: Pinarello Montello

I bought this as a bare frame so it had no seatpost. It's Columbus SLX, so I expected it would need a 27.2. A modern 27.2 seatpost that I tried didn't want to go in. I could probably have forced it in, but it didn't want to go. A vintage but NOS SR 27.2 seatpost went right in and fit correctly. Breaking out the calipers I found the modern post measured slightly over 27.2 while the vintage post measured slightly under. This bike is off being painted right now, but I think I'll use a 27.2 seatpost with it one way or another.


Case 4: Motobecane Grand Record

I bought this as a bare frame but got a discount because there was a seat post stuck in it. It turned out that the post wasn't even seized, it was just (I think) the wrong size. I think it was 26.6. Research suggests the Grand Record will need a 26.4? This one is still in the early project stage, so I'm not sure what it will end up with.


Anyway.... The point of all that data is just to provide some context for my confusion.

My question is, how do you decide what size seat post to use with a bike, particularly a bike where the seat tube is at least partially ovalized? Do you try to correct the shape? Do you just use whatever seems to fit best? Do you use what the frame "should" need?

You should only use the correct size seatpost that coincides with you seat tube ID.
Trying to make too small a seatpost work by overtightening your seatpost clamp is a big no-no. You just end up damaging your seatpost clamp area and sometimes trashing it all together when clamp ears crack, and from the overtightening, and the seat tube stretching and distorting.
Each brand/model tubset has a corresponding correct seatpost size. Tables for such must exist in the net. Try looking in Sheldon Brown's site.....

Right. I guess what I'm asking is how I figure out what is the "correct" size.

Clearly using a post that's too big is a problem. Using a post that's too small risks other troubles. What do you do when the opening measures 27.2 in one direction and 27.0 in another?

I read somewhere that based on diameter and thickness calculations you'd expect Columbus SL tubes to need a 27.4 seatpost but that because of distortion introduced during frame building it actually requires 27.2. But obviously the amount of distortion introduced during the frame building isn't constant from frame to frame.

Usually Columbus SL seat tubes are 27.2.

It's the "usually" part that has me concerned.

After brazing, the seat tube isn't round anymore. Go measure your frame at different points, you'll find some variation. It's common to use a reamer on a new frame. It's better than sanding, as it actually removes material in just the high points, and regains it's "round".

Your Motobecane is proably 26.4.

I have a reamer if you ever need to borrow it.

Not sure if this will help.
On my gas pipe Raleighs have had similar problems with ovalising and burrs which revealed themselves only when fitting a better seat post. I hammer an old steel post in now before fitting. That seems to even things out.

You're not alone with this, Andy. Here's my take. Through years of use, that post is wallowing under stress, even minimal as may be its also a softer metal than the frame and gouges the post. You can apply anti-seize much as you like but if that seat tube slit is gouging the post, one will fight it and further mar the post. Next thing you know, it has to be cleaned up, usually by sanding, including doing the seat post itself. The same old situation is repeated through that bikes life of usage.

These thin wall steel bikes do take a beating, especially with heavier riders. Common as they age, will have some out of round. Thick wall gas pipe jobbies seem less of an issue. At least in my experience.

My solution is to correct it best as possible and trial fit the best fitting post, disregarding the size stamped in it.

Good to know. You've got a more ultimate set of tools than a television repairman.

So the last time I took the trouble to worry about this I dug up some old thread where Sheldon was advocating reaming and a bunch of people said it was a bad idea because the tubes were so thin already. I think that was in a case where the "correct" size was 27.0. The argument against increasing the seat tube diameter was convincing enough that I will never attach my cylinder hone to a power drill again, but if the tube is "supposed to be" 27.2 (and presumably has that inner diameter somewhere below the lug) then reaming to correct the size seems to make sense.

But am I to believe that these frame have just gone through 30+ years of never having been fixed? It seemed more likely that the tube got bent out of shape over time.

Reaming was my first thought when I saw this thread. A scenario that is quite likely would be that the bike came with a Campy post. Those things last forever. So, even if the bike changed hands a few times, the post stayed in (or was removed and replaced until it came time to sell it (to you). So the now quite scored post was pulled and tossed.

You might look to see if the seat pin recesses on the seat lug look like they have been distorted inward by over-tightening. I could imagine that over-tightening would tend to curl the edges of the slot in or forward, lessening the fore and aft inside dimension of the seat tube as you observed. If that is the case, tend to it before you ream. (I have a '73 Raleigh Competition with that issue. Seat pin sat poorly in the recesses and the 531 tube required a 27.0 post. My frame builder removed the old ears, brazed on new ones, then reamed. 27.2 on the nose and a joy to work with (and adjust seat height).

Ben

You know, I bought a Moto GJ frame and a 26.6 slides in perfectly. So that is what I use. I have found on French bikes I cannot make an assumption, 26.4 , 26.6... Then there are exceptions, like a team champion of Columbus SL... 27.2

As stated earlier, the heat of fabrication distorts seat tubes often, undersized seat posts can deform seat lug ears to mess up the roundness. I have often found opening up the slot is required to start the assessment.
A reamer is your friend. Adjustable readers are around, not cheap but useful. A builders quality 27.2 reamer and handle can set one back a dear penny but they work so well.

It's not unusual. The top tube is constrained at both ends, and when it is heated for brazing, it expands. When the joint is at brazing temperature (particularly with brass, as the temperature is higher than for silver), the metal is quite soft and the expanding top tube squashes it into an oval shape. This can happen at both the head lug and the seat lug; it's just more obvious at the seat lug. Best practice, as noted elsewhere above, is to ream the opening after brazing.