I'm looking for an 80s Chorus clamp FD. My seat tube diameter is 28.6. I bought one that was listed as 28.6 - but inside the clamp is stamped 28.5. Did the seller make a mistake? Was there also a 28.6 version? I checked the 80s Chorus catalog but I didn't see any info about that.
Do Record/C-Record FDs have clamp sizes?

No mistake. I’ve seen those and basically they are the same. The metal flexes a bit to fit snuggly and securely.

That's wacky! Some Campy engineer's idea of a joke? No front derailer ever made could tell the difference of a tenth of a millimeter.

28.6 or 28.5, those are nominal dimensions really, more like a category than a measurement. And they fall in the same category! They just indicate that it's not for inch (25.4) tubes (Schwinn Varsity et al.), or 1-1/4" (31.7), or 1-3/8" (34.9) "OS" tubes.

Most 1-1/8" derailers will happily clamp on an old French frame with 28.0 mm tubes and vice-versa, with a caveat — a Simplex delrin mech made for 28.0 will probably crack if you try it on 1-1/8". Other than that though, the 0.6 mm diff between metric and "English" isn't enough to require its own category.

You can also use a 1-1/8" mech on an early Schwinn Super Sport or Sports Tourer with their slightly oversized seat tubes. I forget, what's the diameter on those? Schwinn got Huret to make 'em specifically for that diameter, but many people have "upgraded" to a Suntour, Shimano or whatever, sometimes requiring a longer bolt and a little persuasion, but no big deal.

Thanks for showing that pic, I got a laugh out of it.

Clearly a collector's item.

I just measured again more carefully. 1 1/8 inches converts to 28.575 mm. When we say 28.6 we are rounding up a little. So Campy indicating 28.5 is correct too.

just keep an eye on the gap where the thru bolt clamps down on the seat tube. on french bikes with metric tubes you may need to make a little shim out of brass to ensure you get a good fit.

/markp

Well maybe in an unscientific, regular-joe way, but 28.58 absolutely does not round to 28.5, ever, in an engineering sense. That's like saying a price of $15.99 is $15. People do it all the time but it's infuriating to me. OK maybe "fury" is putting it a bit too strongly...

I wonder if the guy who made the forging die (or casting or however they make these) mis-read the spec sheet. A 6 can look like a 5 if the light's not strong, or if he had a liter of Chianti on his lunch break. After it was "chiseled in stone" (metaphorically speaking), they may have decided to let it slide since there's no real-world impact to speak of and it would have been expensive to fix. I'm imagining Peewee Herman doing an endo off his bike, getting up and saying "I meant to do that!"

No, it's like saying the $15.75 rounds to $15. In the engineering sense.

To paraphrase Art Linketter, engineering departments do the darndest things. That photo reminds me of the 26.7 seatpost on my old Miyata 610. The difference being that such a small differential really does matter on a seatpost--I had to hone the seat tube to 26.8 to install a different post.

But everything gets rounded off at some point, right? Otherwise it would be impossible to build anything. A machinist friend ot mine used to make dies for medical devices that were specified to millionths of an inch. But I assume that they rounded off the ten millionths.

But I like your mis-reading of the spec sheet theory. It sounds plausible to me.

maybe the ID is 28.5 when fully closed?

I would have hoped it was labeled ~28.5.

Yes but there are rules for rounding. You don't get to choose whether to round up or down on a whim, and the number of significant digits shouldn't exceed the precision of your measuring device.

Rounding is done for a few reasons, one of which is the spurious extra digits you get with certain measurements that are beyond what the measuring device can resolve repeatably. Like the specs I see for bike geometry that give, say, the chainstay length as 17.375", when they measured it with a tape measure. Clearly they meant 17-3/8" and converted that to decimal, but the choice of three digits after the decimal implies they are measuring to the nearest thousandth of an inch, which is highly unlikely. It's fake precision. Or the old balloon tires called 26 x 2.125", are they seriously telling us they measure the width of their tires to a thousandth? Highly unscientific! But we know it was never a measurement, it was a marketing category.

Another reason for rounding is brevity, when you have the extra precision, it can be measured repeatably, but it just doesn't matter. For me that's why I always round prices like $15.99 to the nearest dollar. I'm throwing away valid precision, intentionally being less precise because that last penny doesn't matter, and it annoys me. Clearly, I'm easily annoyed! Does anyone mentally add another penny to the posted prices at the gas pump, since they all have an extra 9/10ths of a cent tacked on? No, the bastards count on you not doing that.

That's why I usually refer to tube sizes in fractional inch. I would call that derailer 1-1/8", which doesn't imply any extra precision beyond "the size that's smaller than 1-1/4" and bigger than 1"." That's all the precision you need for this application. That's why to me, 28.5 is a joke, either intentional in which case that's one hilarious product manager, or more likely accidental in which case I'd advise them to read the blueprint more carefully. But no harm done, we know what they meant. (or do we? This thread might indicate otherwise...)

Unlikely, since it almost certainly works on 28.0 metric frames, and probably isn't fully closed up even then — can go a smidge smaller than that.

There may be a derailer made for 1-1/8" that will not clamp down on a 28 mm tube, but if so, that's malpractice and whoever designed it should be fired.

Maybe it's made for installation and riding in a really cold location.

Paint thickness may be more than 0.1mm, so 0.2mm or more bigger diameter.

Thanks, Mark, that's useful and interesting. The difference between accuracy and precision of measurement is not new to me, but somehow I had never thought the implicit difference between--as in the example you use--between 17.375" and 17 3/8". They're the same thing, but at the same time they're not the same thing.

Wait until you guys come across a 25.8 stem!