tkamd73 

I’ve bought most of my bikes online, on a CV frame, with a horizontal top tub, I just look at the head tube to gage the fit. Any frame that is 25” to 27” will fit me, 26” being about perfect. I know frame angles vary, but I can fix that with an appropriate length quill stem.
Tim

John E

I am 5'8" tall, trouser inseam 30". My consistent C&V frame size sweet spot is 55cm C-T. The 53cm C-T Peugeot UO-8 also works well for me because of its proportionately long top tube.

As for frame angles, the Bianchi is 73 degrees parallel, whereas all of the other road bikes are 72 degrees parallel.

My 19" mountain bike (Schwinn KOM-10) fits me well, has 71 degree seat tube and 74 degree head tube.

smd4

There isn’t a stem available that can change the angles of a frame.

alexihnen 

Different than a previous comment, I tend to go by the top tube length and then if the seat tube is 53-58 I consider it. That said, I ride a lot of different geometries/frame sizes. For the odd ride here and there it’s just fine. I have a 56/53 bike with 130mm stem I really enjoy riding, as well as a 55/59.5 with a 90mm stem, also a nice ride. For a regular riding vintage bike, I go with one somewhere near the middle of those extremes.

One thing I learned early on is that I need a shorter top tube frame when using modern brake levers. The reach is 30mm+ more than on old brake levers. Of course the geometry of bars can compensate a bit.

shoota 

You can't understand what he's saying?

tkamd73 

Obviously, but that wasn’t what I was saying. The different quill lengths will give me a similar view of the front wheel when seated on the bike, and that’s what I’m comfortable with. The quill stems on my bikes vary from 70 to 110mm. In simple terms, I can adjust the fit reach wise by the horizontal length of the stem.
Tim

smd4

l can assume (and he clarified) that he can adjust his reach with different stem lengths. But that’s not what he wrote and it’s poor form to assume.

shoota 

Everyone knows what he’s saying, go troll somewhere else.

smd4

Yet another person who has no clue what a troll is.

Germany_chris

I go by top tube length also

smd4

Weird.

smd4

It’s exactly what you wrote. I appreciate your clarification.

RiddleOfSteel

In many cases, being on the very tall end of the production size offerings of companies makes things easy, at least as a place to start. For vintage bikes and their trusty horizontal top tubes, I look primarily at seat tube CTT as using normal stems and not Technomics all the time is feasible with, say, a 66cm frame, but not a 65cm or shorter. At a 65 or (definitely) 66cm frame, the top tubes can get long and that's where I need to be careful. 60cm is my limit more or less, given a 73° seat tube angle. As others have mentioned, and I will expound, stem length affects things, as does handlebar reach, as do brake/shift lever body lengths, as well as their position on the bars. Immense variability from the steerer forward.

For modern frames, the stack and reach numbers are pretty much the first things I look at. My 1985 trek 620 in the 25.5" size--65cm CTT seat tube, 59cm TT (with seat tube 'normalized' to 73°) will net a stack of about 625mm and reach of almost 400mm.

So I do all this work, have my crank-spindle-to-saddle-sitbone-point, saddle-sitbone-point-setback (from crank spindle), and saddle-sitbone-point-to-brake-lever-hood-notch (where it kicks up from flat to vertical) numbers, and then ride a 66cm (good) Cannondale SR with a 61cm TT (too long). But wait, Cannondale shifted the seat tube forward of the BB centerline, so it's a 62cm TT (definitely too long). So I put a Cinelli XA stem at 110mm long (105mm actual), which is fine, but somehow the overall reach from saddle sitbone point (~35.5") is nicely below my max of 36". So I find a longer "120mm" XA stem (115mm actual), make the bike look better, and it's a money fit. Even out of the saddle feels completely natural. The steering is also calmed, which I wanted. The bars are Cinelli 66s, so not a short reach. But lever position isn't weird.

If you can find catalogs for bikes you're interested in, that's ideal as you can compare numbers. Also knowing about how a vintage frame for your height is proportioned is extremely helpful. Plenty of online sellers are poor at measuring things initially or measuring them when requested, so doing as much work beforehand is often a good idea.

repechage

from the beginning.
rethink your saddle "position" assumption.
define it in a way so that it works for different brand of saddles, they are not the same overall width, length and mid " girth "
evaluate the two you have in relation to the crank and pedal center with the crank horizontal and forward, to a point at your knee, you chose and mark it with a sharpie. Measure displacements from that, assuming equal crank lengths on both bikes, if not? Just another item to note.

get the bikes set up on a trainer or rollers, have a helper take images from off to the side at a specific height and distance away.

you really need to see what you have equal now and what is not, this mid saddle stuff I am very skeptical of

52telecaster

I know very little about frame geometry except that even though I ride very upright I much prefer my 1971 Raleigh professional to my 1973 Raleigh super course. Also when it comes to stand over height I do enjoy many bikes that are supposedly too tall for me. If I went by stand over height my three favorite bikes would be too big. Learning to mount and dismount is part of riding a bike.

Doug Fattic 

The reason you like a bigger frame is because they have a longer head tube that allows your stem to be raised up higher so your handlebars can be near or at the height of your saddle. The reason you like your 71 over your 73 I would guess is because your 71 has a shallower seat angle. Siting further back balances the weight of your body more over your pedals. That means your arms and hands are not needing to support the weight of your body as much.

These are not issues with young fit riders with 5% or less body fat. They want a lot of drop between their seat and handlebars to me more aerodynamic. This rotation of their body forward puts their saddle further forward requiring a steeper seat angle. As we age and gain weight and get less flexible we want to sit more upright to be more comfortable so the angles on the frame need to be more relaxed. The conclusion is that skinny fit riders can ride almost anything but will prefer smaller frames with steeper angles. Those frames dimensions can be miserable for the older crowd. That is why after trying a bunch of bikes they will prefer a bigger 1971 Raleigh that as a more relaxed geometry.

Road Fan

How do you measure standover? How big is the difference between the two bikes? I measure it vertically from the top of TT to the ground, preferably smooth and level. If the two TTs have the same diameter, and the vertical portion of ST length (measured c-t) are the same, then the only possible difference between the two vertical dimensions is the height of the BB center above ground. There's no more magic here than plain old Euclidean geometry.

mhespenheide 

I obsess about it. I try to find posted geometries online because the seller usually doesn't know them and you can't trust a random seller to measure it properly anyway.

Then I do a bunch of trigonometry to see if I'm in the right ballpark, because the seat tube angle and top tube length can offset each other once the saddle is in its position relative to the bottom bracket and pedals. And the stack and reach offset each other.

And then I might take a flyer on it, or I might not. If I do, I'll usually be close. And some manufacturers from the 70's weren't all that good at following their own specifications, either! But sometimes I'm just wrong and then have to take the time to re-sell a frame, sometimes losing money in the process. If you don't want to risk that, you're stuck buying from your local market. Or from sellers who will accept a return, I guess.