Seat post size, an indication of the tubes used?
 

I've been searching the forums and couldn't find a definite answer, though there are some indications that the seat post size is usually linked with specific types of tubes.

Does this thing makes any sense?
27.2mm seat post is usually related to Reynolds 531?

Are there any "rules", "guidelines" or just plain myths regarding this topic?

I can't think of any rules. In the olden days, Reynolds tubing took a lot of different seatpost sizes. At some point 27.2 kinda became a standard, but I've had 27.2, 27, 26.8 and 26.4, 26.2, 26 on French bikes. 26.8 is not uncommon on older English 531 frames.

Lennard Zinn explains seatpost sizes in a May, 2005 article that appeared in Velo News.

Oddly enough some have seen two identical bikes from the same year and manufacturer (mostly French) with different seat post sizes - Go Figure???

Then there's the whole distinction between metric and imperial tubing sizes. The seat tube on my Peugeots (Reynolds 531SL) have an outer diameter of 28.0mm, whereas the same tubeset would be 28.6 when delivered to imperial specification. One takes a 26.4 seatpost, the other a 26.2.

I believe Reynolds themselves once told a forum member here that the framebuilders reamed the seattubes themselves in a variety of sizes.

Framebuilders ream the seat tube after brazing the seat cluster as the heat from brazing often distorts the seat tube. The reaming is done to ensure the seat tube is round and the distortion caused by the seat cluster brazing heat doesn't obstruct the correct diameter seat post from sliding into the seat tube.

So yes, Branimir, as a general rule: the thicker the seat post, the thinner the tubing wall, which generally means better tubing. Assuming a standard 28.6 mm seat tube diameter, a 27.2 post means Columbus or 531 butted tubing; 26.2 to 26.8 might be straight gauge 531 or another high quality (often Japanese) tubing. Anything from 25 up to 26.4 or so can be garden variety hi-ten steel.

But there are always exceptions. My Raleigh Record Ace seems to be the lightest frame I own, made of butted 531, and it takes a 25.4 post.

Reynolds 531 might be anything from 26,0 to 27,2. The Peugeot PX 10 used Reynolds 531 double butted. I have owned two from 1977 - while both came with the original seatposts, one of them was 26.6, the other one 26.4.

I think, these differences might have just occured during production. Here is a nice little info on french rear spacing:

"Also appeared as Peugeot Trophy and Spidel. Top quality French hubs. The spacing could get weird. The front hubs often measured less than the standard 100mm. The 120mm (5-speed) rears often measured out 124mm. Gotta love the French. Weight given is both hubs with skewers

EDIT: Some additional information. While pursuing another issue, I learned from an informed CR lister why the rear hubs were spaced 124mm for the "standard" five speed hub. It seems that in combination with certain frames, some brands of 5-speed freewheels simply would not fit. So many French builders compensated by spacing the rear at 122mm and spacing the hub to 124mm. A simple, elegant solution. Gotta love the French still applies.""

The lightest frame I ever owned was a 60s Carlton with 531 double butted and a 26.8 seatpost.

After some digging, here's what a Reynolds representative wrote to forum veteran miamijim in 2007:

this is really difficult to reply to.

You see when we supplied Peugeot we did supply tubes that were metric size (28.0mm outside) and imperial (28.6mm outside).

Then in each diameter the thickness of the tube was also supplied, 0.7mm or 0.55mm.
The frame builder would also ream the inside of the tube, so really it could
be a 26.6, 27.0 or 27.2 depending on the seat tube and reaming done.

TUBING size Reference Charts on FB
 

hey guys, just a heads-up for you, in case you might be interested:
I've started a collection of TUBING SIZE CHARTS from the various TUBING MANUFACTURERS on facebook, to help people who deal with vintage bikes.

I don't know how to direct-link the post itself, but here is a link to the album that's part of the post:
FB Tubing Charts Gallery on FB

You'll find outside diameter listed, as well as the wall-thicknesses. Finding out the seatpost size is a simple mathematical operation (OutsideDiameter - (2*WallThickness)), and for single-butted tubing, it would be by subtracting the non-butted section's thickness.

As someone mentioned, I am skeptical that builders went through the trouble of "reaming out" seat tubes to merely increase the diameter, since, seatposts were available in all diameters needed for the various seat tube sizes. Polishing/refinishing however, is a different matter. (and then there is the matter of doing it for the wrong reasons: because you don't stock the correct size seatpost).

This may have been necessary for seam-welded tubing, that wasn't finished on the inside by the tubing manufacturer (and thus needed to have the weld-seam removed), or cheaper "gaspipe" tubing. But it was not needed for quality tubing.

I've seen mentions that reaming was necessary from distortion after welding... but for the most part, in these forums, we are usually talking about better quality bikes made from better alloy tubing, which required brazing, so as not to overheat the tubing.
If any reaming was needed to correct significant deformation from welding, then it would either have been over-heated in the first place or made from very low-grade tubing that is thick enough to allow for such temperatures and more leisurely manufacturing methods.


Some seat tubes were double-butted, while others just used more economical thicker straight-gauge tubing. However, some bikes (like an old Peugeot that I once had) seemed to use smaller outside diameter tubing as well. But most seat tubes were single-butted, with the thicker butted end at the bottom-bracket shell, and the larger-opening straight-gauge end at the seat-cluster.

I have personally watched hundreds of Bassos and Rossins being uncrated and reamed. Those are usually counted as better quality bikes and definitely brazed. They needed reaming badly. Have seen a new Hetchins that needed reaming. A new Colnago built for Eddy but sold when he switched to De Rosa that needed reaming. Some builders at the level of Chris Kvale build so precisely that Columbus SL frames might use a 27.4 but even in that rare air it is not assured every time. Reaming is normal. Big question is does the builder do it or does the store do it. If the builder expected the store to do it and the store is not prepared to do it anything might happen.

And then there are liners. For all sorts of reasons builders decide to use liners. I've never personally examined a seat bore on an old RRA, first guess is that for a frame built of 0.7 straight gauge in the 30s there were few if any seatposts available so they lined it for a normal post. But there are all sorts of other explanations for why rhm would have a light frame with a one inch post, my guess is only a starting point for an investigation of facts.

Probably half of all the top quality bikes I have owned came with an undersized seatpost fitted. Presume many here have had the same experience. Still there are collectors and perfectionists who have obsessively maintained bikes and no clue why that binder bolt is bent or why the post is stiff going up and down or why the post keeps getting so scarred. General public, average LBS, they just put a post in a hole. They come in different sizes?

Sheldon Brown has a fairly extensive database of setpost sizes for various bike models on his website, for those who may not know.

.................................................................................................... .........................

You are putting down the "average LBS" as not doing it right - but could it be that the one you were in contact with was the faulty one, doing something no longer necessary, if he was reaming out every high-end bike he received? There is no reason to ream out Columbus SL so that it requires a 27.4mm instead of a 27.2mm seatpost as you suggest. That's just nonsense.

All the bikes we come across in our sales group, that use Columbus SL for example, use a 27.2mm seatpost. None are reamed out to 27.4 - I don't even recall seeing any 27.4 seatposts appear in any of our members' listings.

As the letter from Reynolds points out, there were standard inside diameters delivered that some may ream out, but that does not imply that all, nor even most, reamed them out - it sounds more like an exception than the rule.

I do know that the bikes we see in our FB sales groups ( Velo76.com for example) seem to use a consistent given seatpost diameter for a given tubing gamut. For example, Columbus Zeta uses 26.4, Aelle 26.6, SL 27.2, just to name some off the top of my head.

As for bent tubes at the opening - well, you should know what that is from - it's from tightening it around a post that is too small. It proves nothing else. And posts that are scratched badly - that's most often from inserting them into a tube that was clamped down onto a seatpost that was too small.
You buy the right size for the tubing - simple as that.

Regrettably, in my experience, it is true. The frame prep on many of the high end European frames, especially Italian brands, could be quite bad. Personally, I've seen lots of cases where the seat tubes, had burrs, scale or distortion that prevented use of the proper size post. The Japanese were much better in this respect, even on lower grade bicycles.

Since frame tube ODs a re standardized (or can easily be measured) the post diameter is a direct indicator of tube wall thickness, which is half the difference between OD and post or ID.

That in turn can offer some guidance about the type and overall quality o9f the tubing, but that's about as far as it goes.

Indeed, it appears that Ishiwata tubing was as well finished on the inside, as it was on it's outside.

Deburring however, is not the same thing - it refers to only the edges. That's part of the frame-building process.

I do agree that some builders might have over-heated the tubing, causing distortions. But I don't think that was symptomatic of the better builders or the high-end bikes.

Since my reference to my 1948 Raleigh Record Ace (RRA) has been quoted a couple times above, I'll add to that the little I know.

I have had three 531 Raleigh made bikes over the years: the 1948 RRA that I already mentioned; a 1951 Lenton Sports; and a 1958 Lenton Grand Prix. The RRA was butted tubing; the others plain gauge. All three took a 25.4 mm seat post. In fact the only Raleigh-made bikes (as opposed to Carlton-made bikes) I've seen that take a seat post other than 25.4 mm are the RSW-16 and the Twenty, both of which take a 28.6 mm seat post. I assume the 531 bikes had a sleeve brazed into the seat tube so they could take the standard seat post.

Also worth mentioning: my 1960 Allegro, full 531 butted tubing, takes a 26.6 or 26.8 mm post. It came from the factory with a 26.0 post and a shim. The post was a very nice one, chromed steel but quite light, fully domed over at the top. I guess 26.6 posts weren't that easy to source at the time.

I concur, except I would have said, "especially French brands." In the upper-mid quality range, my Bianchi has much better fit and finish than my Peugeot PKN-10 did. The frame were about a year apart in production date.

From working in a bike shop through the 70's and again for a number of years in the 80's, very few "name" frame sets did not need reaming to get the seat tubes round.
Masi, Serotta, John Howard, Fuso, KHS, Nishiki, Centurion, from my experience were pretty good.

Always be ready to correct, Colnago, Gios, Pinarello, Bob Jackson, Raleigh ( not the team bikes) almost all French, ( upper end Motobecanes were pretty good) in general, it was test, don't guess.

Heat distorts, almost without fail. It is very hard to fight physics.

Reynolds 753 will forever be associated with 27.4

Except for 1st generation 753 in the 70's which was had metric diameter tubes and took a 26.8mm seat post. ;)

Brazing can also distort the tubing. Brass in particular requires the steel to be hot enough to become quite soft and thus susceptible to distortion. This is particularly an issue at the top of the seat tube, for a number of reasons. First, there are several tubes that all join in the region (seat tube, top tube, and seat stays), so the seat cluster gets quite a bit of heat while the frame is being brazed together. Second, seat tubes, as you note, are frequently single-butted, with the thin end at the seat cluster (because a seat post will be inserted and reinforce the area). The thinness of the tube make it more susceptible to distortion. Finally, the top tube is fixed at both ends, and when heated to brazing temperature expands, gets longer and exerts pressure on the softened, thin seat tube, distorting it. That's why seat tubes frequently need reaming. This is less of an issue with silver brazing, but silver is more expensive and requires closer tolerances for the joints.

Thanks @JohnDThompson Excellent description of contributing factors!