Keep in mind that EVERYTHING changes with a smaller frame. Shorter head tube means a more aggressive drop and as someone else has also mentioned check toe overlap. Chances are you will be fine but there are other variables other than the top tub length.

If I have learned anything over the years participating on this forum, I have learned that on the question of up or down for framesize, there is no right answer. For example, I believe always size up. This is because more drop is generally less comfortable for the vast majority of riders. Further larger frames don't change as much in reach. Plus a taller head tube shortens reach on a larger bike...handlebars are closer to shoulder joints...reach is comprised of both horizontal and vertical components.
I see the mantra constantly of sizing down. I couldn't disagree more with this. I always size up.

At 6'1 - 34" inseam... I'm "supposed" to ride a 58, which I have before. Sat on a 56cm and it just felt right. That being said... My seat height is about 1 cm away from the max recommended on the frame. The stem length was just fine at 110mm. I went to a compact handlebar and needed to increase it to 120mm though.

One caveat... the 56cm bike came with 172.5mm cranks... the 58cm with 175mm cranks. It's going to be a little extra cost at this point to get that changed to 175mm with my inseam length.

All depends on what works for you...

This is a big part of what I was getting at. Screw this up because you don't know what you want, and even a great deal becomes hundreds or thousands of dollars wasted on a bike that is compromised for you.

I can't abide this kind of nonsense - that proper handling is only important to racers. Slot that in with light weight, nice components, etc. It's baloney. If you are riding a road bike, proper handling is hugely important. Even if you aren't pushing the limits. A bicycle that handles right is more pleasant to ride, and safer. You don't need to be a racer to enjoy those benefits.

I've ridden road bikes that weren't optimally set up. Sure, it can be tolerated, and you might not know what you're missing. But you're still missing something. When I finally got on a bike that fitted and handled right, it was a revelation, and I wouldn't go back by choice.

So what your saying is that you made the wrong size frame work for you. Having a seat post almost all the way out and having to use a 120 would be a clue to me (I have been a fitter for 5 or 6 years now) that maybe your on the wrong size bike. I guarantee to you that I can duplicate your position that you like so much now on a 58 without having the post and stem almost maxed out.

gotta shake my head...

"wrong frame size"...
"seat post all the way out"
"having to use a 120 would be a clue for me"
"I guarantee you"
"post and stem maxed"

maybe a 'pro fitter' might refrain from statements with knowing almost nothing about a rider other than height and inseam.
Yes a 58 is certainly a very reasonable choice for the 6'1" 34 inseam guy, but maybe not the only one. And Prolly a good START point. But maybe he likes the smaller size and handling of THAT bike...?
a 120 stem is hardly an indication of a 'mine field ahead' fitting...
nor it is a 'max', for many it is the absolute norm/average/median/just right
'guaranteeing' a fit because you can move things around is not a guarantee that he'll like the ride.
in fact guarantees should be limited to parts durability, at best
reasonable suggestions couched in 'absolutes' often come across as dogma

We are talking about FIT in this thread and if he has to have the post at 1cm from the max AND has to use a 120 stem then I stand by my statement that it is the wrong size frame. Would a 56 work for this guy the obvious answer is yes it will work but it is not optimal. I also GUARANTEE that I or any other fitter could exactly duplicate the fit that he has now on his 56 on a 58 and be much closer to the bullseye. You are correct that a 120 is hardly out of the ballpark of norm but when you take that AND the fully extended post the evidence does not lie.

I went to a very compact handle bar which is why the stem was taken to 120. Anything close to standard and the original bar were just fine. I don't think I chose "the wrong size frame and made it work"... Everything that was part of my fit was within the specs of the bike. The 172.5mm cranks work just fine... the move to 175mm again is by personal choice... the 172.5mm again is perfectly within the specs...

And sure a 58 wouldn't quite be maxed out... but the top tube is only 1/4" longer I believe... the 120mm stem might needed with that size frame as well.

My point is that going with a smaller frame size is just fine as long as your fit works with it. Plenty of options for changing bars, stem lengths, crank lengths, seat setback etc... etc.. regardless what frame size you go with.

All that matters is that your happy with your choice and it works for you.
I agree with you on the cranks. It is a personal call.

First...I agree with you as a 6'1" guy with long legs. I would never be on a 56. I am on a 58 with 130mm...so I can't understand how a guy my size could ride so cramped anyway. I could easily ride a 60cm bike and have comfortably.
The dynamic I see common on this forum is guys riding 1 frame size down emulating racers...ONLY...with short stem. Most top racers ride with a stem 130-140mm...there are exceptions but most size down frames sizes for more drop...top racers aren't like average road bike riders....BUT...they still need to satisfy reach and hence a long stem.
At the end of the day it doesn't matter. An average rider will never be transformed to a strong rider by changing a frame size. Also, most aren't willing to own a number of different frame sizes to learn what works best for them. The only way to know is to try every permutation of drop, reach, setback, bar shape, width etc. I have, like a few here I am sure as well. When you ask the question like the OP has...you are just starting out. You have to go there to know for sure and other's word on it isn't really good enough if you want to ride with the best geometry.

That's crazy talk. It's almost like saying jacking up the back tires on my car won't make it faster.

now ya tawkin!

for the geez...

I know...heresy...even flipping the air cleaner doesn't help.

Frame sizing on the 41 is almost as devisive as Washington gridlock.
Maybe best advice is for us all to get mtbs as we go over the fiscal cliff together.

Agreed.

Now wrap your heads around this one - 5'7" middle-age woman and her 5'11" teenage son swap bikes all the time and do not adjust a thing....

Actually, you miss the point. Switching bikes has nothing to do with what is optimal for either rider.
This underscores a basic tenent that transcends cycling. If you don't know, it doesn't matter.
PS: putting another spin on it...you are optimizing something. It just isn't comfort or speed for either rider...one fits perhaps better than the other.
What you are optimizing is...the economy of two riders sharing the same bike. Nothing wrong with that at all. Road bikes are expensive. If you can share a bike and both are happy with the arrangement...all that matters really.

A BMW M3 handles better than a Hynudai. That doesn't mean that there is anything wrong with the Hyundai. You are implying that a bike that isn't the exact right size is dangerous. The handling characteristics between bike sizes is so minute that most people would never notice. I'm not talking about someone 4'11" jumping on a 62cm bike. We are talking going up or down one size. I submit that you will never notice a significant difference once adjustments are made.

OP: There's always another deal. I would never compromise my desired sizing/geometry just to save some money. In fact, I just turned down the opportunity to save $1,500 on a Cervelo R5 because the fit wasn't right. Off by less than 1" (2 cm), but I'd rather wait to get exactly what I want rather than a compromise. Good luck.

What I am saying that one bike size can fit range of heights. Conversely, there is a range of bike sizes that fit one height. So - OP may never know until he tries it.

I love how people have to obfuscate bike fit by using 'new' terms. Reach really isn't 'reach'. It's the front center distance from the bottom bracket to a perpendicular line from the middle of the steer tube. It's proportional to the top tube length but the top tube length is an easier measurement to make. "Reach", as most people would understand it and now called 'handlebar reach' to further obfuscate the issue, is the distance from the saddle to the handlebars which, unlike the top tube length, can be adjusted somewhat to fit individual tastes.

Stack height, which for years has been defined as the distance from the fork crown to the top of the steer tube, has now morphed into 'frame' stack height which is really just a gussied up version of the seattube length or the frame 'size' which is now a measure from the center of the bottom bracket to a virtual horizontal top tube along the seat tube. Yes, there will be a slight difference because one is vertical and one is measured at an angle but the difference isn't all that great and you can easily convert from one to the other or see that they are proportional to each other. A bike with a certain 'stack' isn't going to have a seat tube length that is radically different...at least not when measured to a virtual horizontal top tube.

From what I've observed lately in bicycle fit, it's appalling. I see far too many people who have been fitted to bikes that obviously don't fit them. I see lots of small people doing the Superman thing on bikes that are too big and I see far too many others who are riding bikes that make them look like they are trying to ride the bike their parents bought for them when they were 14. They look like BMX bikes with the rider folded in half between the saddle and the bars and the angle of their knees changes from 90 degrees to 85 degrees as they go from the top of the pedal stroke to the bottom.

Stack really does have some value to it. It totally eliminates fork length and BB drop from the comparison. If all you do is compare the head tube length, then BB drop and fork length differences could lead to unanticipated changes to the spacers required under the stem or the stem angle, to get the desired setup. Headset height still needs to be considered, since some frames may have a 20mm minimum headset top section and another may allow as little as 8mm.

The idea behind reach is to eliminate the seat tube angle from the fit comparison, which it does, but I've never seen a manufacturer explain that reach comparisons are only valid at ONE stack height. You can find frames that differ by 20mm in stack height and list the same reach, but they will not fit the same, when the handlebar height is set to the same distance above the BB. The "handlebar reach" of the smaller frame will be about 6mm shorter, with the same stem.

Measuring the horizontal ("effective") TT lengths is fine, but a buyer has to understand how to correct for any difference in the STA, in order to figure the true difference in reach. Many buyers don't understand this and only compare TT lengths.

as I understand it 'reach' and 'stack' came from the mtb community, in an effort to find some std reference for the many variations of frame specs. Much of this was to allow riders who either wanted to have a reference for point of balance for climbing and especially for DH, while off the saddle.
https://www.pinkbike.com/news/08ReachStackStandard.html

Then, I guess the Tri crowd found it a good way to get some same std for their spec-ing...
https://www.slowtwitch.com/Bike_Fit/C...er_One_95.html

these dims do eliminate ST angle and BB, but just because they do, not because they actually take those into account. As you say 'stack' can vary without affecting reach.
BB Drop/height has an affect on bike handling and has always been a consideration in frame design, as relates to those intangible qualities of 'ride' and was always a discussion when it came to 'English' vs Italian and French, back in the day. The fact that is rarely mentioned in rider discussion is that TT clearance is almost never an issue these days and relating it to a 'road' position serves only to confuse most, and is predetermined once you pick a frame.
And even though some think STA might not be important, other than provide the right setback for a given rider/leg length; it does have an appreciable affect on the 'ride' of a bike (for which I could give numerous examples).
In all I find stack and reach to be just another set of numbers in the roadie world, which promote no better understanding of the overall package. Ultimately you have to pick some reference points to do your setup (if you know what you're doin for yourself...) for which someone could include stack and reach or just call it TMI.

This is indeed true. Simply put, two parameters aka stack and reach do NOT encompass total frame geometry. They may be two more important dimensions...but only a different way of describing the sizing of frames for decades. Plus...stack and reach do not encompass sta or hta...sta is critical in determining the position of the rider relative to BB center. With traditional effective TT length...sta was included in this dimension implicitly...taken from the top of head set (stack) with horizontal line drawn to intersect with the center of seat post. In this regard, effective top tube length is more inclusive.
As to stack, most road bikes have a std. fork crown to wheel center dimension. For this reason, head tube length is effective.

At the end of the day, stack and reach together is no more effective than virtual top tube and head tube length. You have to look at sta as well which again is part of effective top tube. Basically what reach does is separate these two dimensions.

And eliminating the fork length does what for fit? You can't ride a bike without a fork and the length of a fork is an important part of how far up the front end of the bike is pushed. If as cyclezen states the reach and stack comes from mountain biking, eliminating the fork length from the fit is even sillier. When you can have forks that runs from 80mm of travel to 300 mm, that's a lot of variance in the head height and it will have a huge impact on size frame the rider needs.

The fit of a bicycle isn't that hypercritical. If it were, we'd have stems that are available in many more lengths than what are currently available. We'd also find bikes with much more variance in frame dimensions which really don't vary all that much from manufacturer to manufacturer when comparing bikes for similar uses.

Again, if the seat tube angle varied much, you might have a valid point but the seat angle doesn't vary all the much within classes of bikes for similar applications.

In the end, fit is all about getting into the right ball park and then adapting the flexible part of the system...the rider...to the inflexible part of the system...the bike. Fit matters but only up to a certain point.

In my frame size, the STA can vary from 73 to 75 degrees and that makes a 2cm difference in reach. If I ignored the STA, I could end up needing a stubby 90mm stem instead of my normal 110mm. I want my stem to be a certain length and I don't want a high rise stem or a lot of spacers, so stack is also just as important to me. Sure you can make a poorly sized frame fit, but it will look like the buyer didn't know what he was doing.