I noticed The Bridgestone RB-1 has a longer top tube: 57.5 cm. This is the longest I ever seen.most frames are 55-56 cm. Any out there with 56.5 or 57 cm?
Some current Soma/New Albion frames have a 57cm TT. I might do one of those if I do not get a vintage frame.
I should mentioned "small diameter steel frame" originally because some of these large tubes at 56 c-t-c creates a larger frame The only mention of a C&V frame is the Peugeot..
My question is only about top tubes parallel to the ground.

I seem to recall @wrk101 had a thread on just this topic. I tried to ride a 56cm Allez but I could just not get it to work which is why I tend towards 58cm frames but if I found at 56 with a 57 cm tt or better that might work.

I bought a 56cm Salsa Casserole, top tube was long, too much so for me and I sold it on... nice bike though.

Most measure seat tube center to top. So your 56cm c-t-c may be what others call a 58cm size. Not unusual to see bikes this size with square geometry, where top tube center to center is very close to seat tube center to top.

The thread I posted earlier were frames well beyond square, like a seat tube of 53cm center to top, with a top tube of 58cm c-t-c.

On vintage road bikes, as sizes got larger, say 61cm CTT ST, top tubes didn't grow much. So less square.

Lemond and Bridgestone are the ones I have seen with longer top tubes, but I ride a larger frame than 56cm.

Peugeots tended to be long in the tube tube, at least all of mine have been.

Note that the seattube angle can effectively lengthen or shorten the frame's forward REACH dimension to the tune of roughly .9cm per degree (and in proportion to frame size).
So my Lemond Zurich has very normal/average REACH for it's frame size.

Also, the headtube angle tends to affect how long of a stem will provide best handling. I find that, generally, a shallower headtube angle favors the use of a shorter stem, and vice-versa.
The fork's rake would also be expected to play some role here.

I have a Detel Marathon road frame with 50cm C-C seattube and 56cm level toptube. Angles are on the steep side. It fits me well (and handles well) with a mtb riser stem and long seatpost.

Never got to measure it, but I remember, my younger brother's 1983 Gitane Criterium had a very long top tube for the size of the bike. Not sure if the design was influenced by Gitane team manager's (Cyrille Guimard) concept at that time to have his pro riders back flat (horizontal) when on the drops. As Lemond had.

Look for aero Sakai or SR Triathalon bikes fom 82 to 84. My Sakai has 53 seat tube and 56 top tube.

There was once a "fit" that many preferred, with a longer TT than "square." I have a 53x56 frameset that really does seem to fit well. I also have a 53x53 that I've yet to fit with any kind of comfort level, and I had somewhat the same problem with an '87 Paramount.

If I recall, the early Merckx were somewhat like that, longer than tall in smaller sizes. I think once you get to around 57cm or 58cm "tall," the top tubes kind of level off, or at least much less incrementally than most current models.

The general condition seemed to be a "square" model in the lineup, then for about every 1.5 to 2 cm in height increase, the frames seemed to go about 1 cm longer in reach. Again, I've seen earlier models that were longer than this in the smaller sizes, and it appeared to be more prevalent in the 70's and early-mid 80's than later.

I'm sure someone will chime in with their 52x56 1990 bike, but I'm speaking in general terms of what I've seen.

I do not find center to top numbers people give accurate because they are taken to the top of the lug. So if you use a lug that goes up higher, you get a 58 instead of a 57?
A 56 c-t-c plus the top half of the top tube (12.7 mm.) is 56 + 12.7mm= 57.27 cm or rounded to 57cm, which is the true size of the frame.

The smaller sizes need a longer top tube for some top clip to front wheel clearance.
I noticed almost every frame (~56cm) that i have seen with a longer top tube, is built in Asia and not Europe.

My '92 Univega Via Carisma's top tube measures 59cm CTC, over a 58cm seat tube. It felt too stretched out with the original flat bar, a little better with a riser bar and much better with an albatross bar -- the latter is the keeper, both more comfortable and more aero with more hand positions.

No idea whether that applies to other Univega frames. This was one of their Bi-axial Power Oval frames.

I have a steel Bridgestone (Anchor) track frame with a 56.5 cm seat tube and a 58 cm top tube.

My '89 Centurion Ironman (Expert) frame measures 57cm (C2C) on the seat tube, 59cm (C2C) on the top tube.
My '92 Specialized Crossroads has a 53cm (C2C) seat tube, 60cm (C2C) top tube. It takes wide 700c/28"/29" tires.
I've got a '94 Diamondback Parkway frame around here somehwere with a 56cm (C2C) seat tube and a 61cm (C2C) top tube.
My '83 Bianchi Grizzly frame has a 57cm (C2C) seat tube, and a 60cm (C2C) top tube. Unfortunately it's designed for 26" (ISO 559) wheels.
I seem to recall the Giant Boulevard hybrid has a longer top tube as well.

Top tube length varies with geometry. I hava a '89 Peugeot Montreal Express and the top tube is 60cm but dumps the bars at my knees, on my favorite bike the top tube c-to-c is 56 and I feel a little bit stretched out until I relax in to the fit.

Most actual measurements are fudged to give the buyer some idea of fit on a hypothetical standard. TT bikes measure Seat post to air in most cases due to steep seat tube angle. In that case any buyer looking for measurement x or y yould be buying a frame many sizes too large if they walked into a store with a tape measure.. It's best to just put on a sticker with a number the buyer is familiar with.

I do not follow this. I have not seen fudged TT numbers, but you may have seen more frames than I. The mentioning of TT frames does the opposite of clarification.

I have converted a 26" MTB frame to use 700c road wheels.

A bit wide at the bottom bracket, but otherwise it works well. The end result was a long frame with a low top tube.

Oh, sorry. TT frames have super agressive geometry, so you would expect a longer top tube. It's not really so weird though, because the frames are so short. Imagine in your mind a standard road bike. Keep the crankset and head tube where they are. Then make the seat/seat tube more vertical and move the rear wheel forward. The rider is shoved forward a lot. So for the bike to fit the top tube length in actual measure is shorter than you would think because the bars are meant to be under the riders elbows.

I think I see your confusion. In my post above, I mis-typed head tube, I meant seat tube. I edited the post above.

Thank you for this post. I neglected to put down my post is for lugged frames. I should not have to as those are the only ones that fit this Classic and Vintage forum.
I looked at the 55.5 cm Peugeots of the early 80s (versions of the PN10) and they seem to have the slightly steeper seat angle to fit me.They have a 57cm top tube.