mcoate

Road bike from the early 70's.. top tube looks to be about 25.6mm and the seat tube is 28.8mm although my calipers aren't great so could be off by a tenth.

branko_76 

The inner diameter of the seat tube would be the place to start. Does the seatpost have the size engraved on it?

mcoate

The seat post measures about 26.2mm

martl

Short answer: no sure way of telling by the outer diameter, those were the same for many 70ies 80iies tube sets because available lugs sort of dictated them.

As has been pointed out, seat pin diameter *is* a hint, the high end stuff usually had 27ish like 27.2mm. 26.2 would be unusual but 26.4 or 25.8 was common for lower/mid range tube sets.

If you know nothing about prevenience etc. The sort of lugs used can provide clues as well.

Wileyone 

My B. Carre is 26.4 metric 531. I would not consider it a lower/mid range tube set.
Exceptions abound.

verktyg 

You need to add a little bit to the measured diameter for paint thickness: 25.6mm works out to 25.4mm which is 1" diameter and 28.8mm is 28.6mm or 1 1/8" diameter. The tubing is "inch" or "imperial" size so it could be a lot of different types of tubing.

The simplest, best way is to post pictures of your bike, we are not fortune tellers!



Here's a guide of what to photograph:



Good luck...

verktyg

RobbieTunes

No.

mcoate

Ha I know, I just cant attach things yet. Thanks for your help though I'll do some more research when I can take stuff apart this weekend.

branko_76 

Is there a brand name or other identifying marks on the frame?

Do the front and rear dropouts have a name stamped in them?

Does the rear dropout have an integral derailleur hanger?

Are there braze-ons?

Does it have lugs?

T-Mar

Those tube diameters are, within measuring variance of standard imperial tubing of the era, which are nominally 28.6mm for the seat tube and 25.2mm for the top tube As noted, the seat post diameter will give you an appreciation of the grade of tubing. Assuming the post is actually 26.2mm and is correctly sized for the seat tube with a 0.2mm diametrical clearance, then your seat tube has a wall thickness of 1.1mm. This is typically indicative of a lightweight hi-tensile steel, of which there were dozens of manufacturers.

branko_76 

If the i.d. of the seat tube is 26.4mm, it's probably Reynolds 531 Plain Gauge tubing.

semroc

Great thread. Reading about bike steel is cool. Great post on what to photograph verktyg.

bulgie 

I disagree. Of all the frames in the world with a 26.4 seat tube ID, probably a tiny percentage are 531 plain gauge.

branko_76 

That's good to know, can you name some others?

bulgie 

Well, depends on what you mean. Do you mean frames that take a 26.4 mm seatpost? Probably most of them (historically) are French frames with 28.0 mm seat tubes with a 0.7 mm wall. That's a theoretical ID of 26.6, which makes the normal amount of clearance to fit a 26.4 post. (You know a 26.4 post can't go in a 26.4 hole right? Except by being pounded in with a very big hammer, never to be removed afterward.) There are exceptions to the "rule" of 0.2 mm theoretical clearance between tube ID and post size, but that's most common.

If a 1-1/8" (28.6) seat tube has a 26.6 theoretical ID (26.4 post), that's 1.0 mm thick, which is thicker than the plain gauge 531 I remember. Maybe they made it in more than one thickness, but I remember it as 0.8 or 0.9 mm wall, which would result in seat posts of 26.8 or 26.6 respectively. My 1971 Raleigh Super Course, which is 531 PG according to the catalog, takes a 28.6 seatpost.

If a 531 1-1/8" seattube is 1.0 wall at the top (26.4 post), then I expect it's most likely a 1.0 - 0.7 double-butted tube.

Other brands of tubing (Tange for example) came in 1.0 plain gauge and 1.0 - 0.7 DB, another reason you can't ever assume 531 based on a seatpost size.

If you meant literally a seat tube with a 26.4 ID (to take a 26.2 post), that would be a theoretical wall of 1.1 mm, though in some cases (Cinelli frames for example) that's a thinwall tube with an internal sleeve at the top for the post. You can feel for presence of a sleeve with a bent poker, such as a J-bend spoke. (Usually need to sharpen up the spoke head a little to catch on a thin sleeve like a Cinelli). If the bottom edge of the sleeve is relieved with a taper, then the bent-poker method wouldn't work, but that's rare. Cinelli didn't do it, their sleeve has a bottom edge you can detect.

I know Reynolds made 531 in 1.2 mm wall, but I doubt there was a 1.1 mm wall.

Mark B in Seattle

beicster 

My 79 Super Course which came with plain gauge 531 used a 26.4 seat post. The catalog shows it as 26.6 but a 26.6 did not fit and the 26.4 was perfect.

bulgie 

Hmm, interesting.

Anyone else with a SC with this size post? I did measure my '71 today, I'm not just going by memory, definitely 26.8 with a high-quality Mitutoyo caliper.

Makes me wonder if they subbed in a different tube on yours. They were known for using whatever was at hand when they ran out of what the catalog said.

Mark B

beicster 

I can't quote a particular post but based on my reading of the "Show us your Super Course" thread, 26.4 seat posts are fairly common. I could be remembering that wrong.

branko_76 

I have two 1972 Raleigh Super Course and two Falcon bikes from the same era. They all have Reynolds 531 Plain Gauge tubing and 26.4mm seatposts

EDIT: I just checked my Falcons, one takes a 26.8mm seatpost, the other 26.4

branko_76 

You are correct

bulgie 

OK thanks, I'll adjust my mental database of 531 offerings.

I gotta say though, that's sad -- there's no need to use a tube that thick and heavy, in 531. It makes sense in cheap weak carbon steel tubes, but 531 was expensive high-strength alloy (for the day) and only worth it if you used it in thinner gauges.

I'm glad my earlier Super Course used the thinner gauge (26.8 post). It's a 25-1/2" frame, and I am big and heavy (clydesdale+) and a sprinter by nature (fast-twitch muscle). I have owned it since '71 and put a lot of miles on it, included loaded touring. Never wished for anything stiffer or stronger. I can hardly imagine anyone who would benefit from a seat tube (and maybe the whole triangle?) being 25% thicker than that.

But, all my whining aside, facts are facts and I was wrong.

MB

branko_76 

My 26.8mm seatpost Falcon may be of the same era as your SC. I have no idea when plain gauge 531 was introduced, but maybe during the bike boom of the 1970's, with its high demand and resultant lower quality, the Reynolds engineers decided it was time to beef things up to reduce their own liability. Just a guess.

Road Fan

Mark, it should not be a new discovery, British companies have from time to time used straight 531, certainly back at least to 1952. My '52 Rudge Aero Special and the twin sibling Raleigh Super Lenton of that year were straight gauge 531 "throughout." Butted had been used on that product line before and after that production year, so I don't know what they were up to. It could just have been a supply chain problem, such as the contractor who took raw tubing from Reynolds and profiled it and then shipped it to Raleigh Nottingham, was short some labor? In the early 1970s, Raleigh, Dawes, Falcon, and others were supplying bikes with non-butted 531. Back then I just thought it was to get to a lower price point. With what I know now, I think it was technically pointless, considereing the ride quality Peug provided using straight gauge whatever steel, at least up to 1973 or whenevr they steepened their frame geometries. At the same time, Raleigh changed the frames from 590 wheels to 630, FWIW. Maybe they were uncertain about the stress analysis with the taller wheels a longer stays, so maybe they compensated (made management have less heartburn) by adding in some structural meat, thicker tube walls.

Or am I missing something?
.
BTW, the Rudge frame I mention takes a 27.2 mm post. The OD is 28.6. 28.6-27.2 = 1.4 mm for a tube wall between 0.6 and 0.7 mm. Does this make sense, to have such a thinwall tube back in the dark ages, with no butting? .6/.9 was known.

Or maybe the tubing broker got the specs wrong and sent a year's wortth of straight gauge 0.6 instread of a year's worth of .9 / .6, and Production just said "we'll just build through it!"

When I get it built, I think it will ride very nicely!

bulgie 

.
Haha, hardly a new discovery. I knew my SC was PG when I bought it in '71. Actually it has the decal with 531 on a diagonal, usually meant to indicate butted, but the words "butted tubes forks and stays" were painted out with black paint, by hand. They must have run out of the right decals. Others who bought a SC around that time have reported the same thing, so they made some number of them with the hand-painted decals.

That Rudge sounds like a pretty special frame, take good care of it. 0.6 mm plain gauge is very unlikely, so if you have a 27.2 post, they probably substituted a single-butted seattube, or it was thicker at the top and got reamed to 27.2. Either way, an upgrade from PG.

I had a Rudge "Aero" for awhile but sold it, too small. I liked the graphics though, and workmanship was very good for a factory-made frame. There was nothing 'aero' about it, but back then I guess the word was just meant to evoke aircraft technology or some such -- not streamlined as we think of it today.

Mark B

branko_76 

I checked some of my seatposts that are marked 26.4mm, they vary between 26.25 to 26.35mm, same manufacturer (SR Sakae) but different dates. Does your seatpost fit snugly into the seat tube without forcing it?

Also, I see you have 12 posts so you can now upload some photos of the frame.