I've read threads on 'why pro's ride smaller frames' but I'm wondering/asking for a little more on the pro's and con's of going down a size. From those who have done so.

Some measurements below.
I'm 5'9" , 34" cycling inseam w/ 6'1" wingspan
Bike has 57cm effective tt, 78cm bb to saddle, 110 stem and 12cm saddle to bar drop

Going down a size would mean a 55.5cm tt and running a 120-130 stem.

Not broken so why fix it? I know I won't know until I try it but... I can't help but wonder what I have to gain or lose by doing so.

Given my reach and flexibility am I better suited for the longer top tube? I also don't plan on dropping the bars any lower as I can bend my elbows pretty low if I need to get lower. So that means more spacers (15cm head tube from 17cm) for those who care about that.

I don't care much for showing more seat post for "that pro look" but interested in handling and performance characteristics that can be gained.

Thanks for reading.

The only reason to go smaller is your current frame doesn't fit. Longer stems can also change the handling.
The saying 'the Pro's do it" is cause years ago (and some now) frames were custom built for them, the frame could be
smaller (seat tube measurement) and still have a longer top tube. Pro's also ride much lower than 'other' cyclists,
which can make a frame look smaller.
If your current fram fits, you are comfortable on it & it handles like you want, there is NO gain going to a smaller frame.

Thanks for the reply.

While searching for answers I started looking at different setups. IE: Pro bike: Dan Martin's Cervélo S3 | BikeRadar

He's taller but rides one size down, slightly lower saddle but longer stem (reach), so I would love to know why not go a size up and run shorter stem (110-120)? I'm guessing I'll get a million and one answers but I've found other setups show similar if not the same as above.

Pros are all over the chart on fit. There's so much variation that I wouldn't base anything at all on how the pros ride.

The smaller frame is supposed to be more nimble, whatever that means, and incrementally stiffer. And has a smaller head tube allowing more saddle to bars drop, which you said doesn't interest you.

You're the same height as me but your legs are much longer. With a relatively long torso and shorter legs I have a tougher time getting a flat back. The only reason to go down a size would be if you were already riding with a -17 degree stem with no spacers and wanted to get into a lower position.

Pros go smaller to get there stems lower. Boonen gets a custom frame. Dan Martin slammed stem. So unless you need to get your stem lower you would be using allot of spacers and not looking very Pro.

pretty much what i do it for. nothing else. i can get the fit i want with setback/straight/setward seatposts, adjusting saddle on clamp and various stem lengths.

if i have a frame that has a headtube that won't allow me to go as low as i want, and don't want to purchase a new frame, i've resorted to a 25 or 35 degree stem and then flip it down, so as to get a negative drop. it has a few drawbacks though...

It's apparent to me that there is a lot of confusion on this topic because people have a lot of preconceived notions that just aren't true. For example, OP, sizing down does not mean going to a 55.5 ETT. That's totally random, because a smaller, more compact frame could offer the same ETT, it just depends on the design. Same for the smaller headtube comment; depends on design.

The main reason to go to a smaller frame, from my perspective, is to get the perfect fit. The secondary reason would be to tighten up the handling on a frame design that would otherwise be too flexible, specifically "standard" (i.e. smaller or traditional) diameter steel and titanium. The tertiary reason would be to perfect front/rear balance via a shorter wheelbase, if in fact more reactive handling is desired.

But again, it all depends on frame design. I don't think I'd take a hit on ETT, though, just to run a longer stem. There would have to be compelling features in a frame in order for me to consider that. Rather I prefer to work around ETT, starting there, and finding the most compact frame with the right numbers. I understand you're approach may be different, however, but I'm just uncomfortable justifying the choices proceeding from compromised fit.

Not sure we're on same page.

My TCR is a M/L which has a 57cm tt and a 17cm head tube, going to a medium (same make and model) would give me a 55.5cm tt and a 15cm head tube.

If I measure my bike from a stack and reach perspective, all I have to do is duplicate it on the smaller bike given the saddle height and setback would be the same in relation to the bb.

I only assume what length stem and know that I will run more spacers given I don't really intend to lower bars or run a different angle on stem.

Based on my height it's suggested I ride a medium or a 54 in most makes, I seem have slightly longer legs and arms than most paired with being flexible.

Other than lowering stem, I'm curious to know if there is anything to gain other than a few grams in weight. Could there be some characteristic changes/improvements that would justify the switch. That's what I'm curious about.

Im in sure I can lower bars to not run a spacer on smaller bike but I prefer running a traditional drop which allows me a higher position on tops but a low enough position on drops paired with bending elbows. Assuming I had to have a bike with no spacers for aesthetics.

In others words would there be an improvement of some sort riding a bike with a shorter top tube, shorter wheel base and head tube compared to a slightly longer top tube in terms of performance assuming I can duplicate the fit (which I'm sure I can).

I could also pose the question in reverse and ask if there would be a reason to go a size up and run a shorter stem given I can still clear the stand over requirements of a larger frame.

note: my Fixie has a 58.6cm top tube and I run a 100mm stem with track drops

At the risk of stating the obvious: You ought to get a real fit before you change frame sizes.

It seems unlikely that you would be in that magic spot where you could use either a M/L or a L. It's much more likely that one will work better for you than the other, especially given the modern penchant for fewer sizes.

In theory, the smaller bike will be stiffer and perhaps a bit more responsive. In practice, the effects are small -- and getting smaller. Bike manufacturers are starting to make the ride feel consistent across sizes. E.g. Jamis has used size-specific tubing for years; Specialized is just starting to make the Tarmac's stiffness and handling uniform across all sizes.

And any theoretical advantages will be outweighed by just using the bike that's the right fit. A slightly stiffer bike won't help you if it turns out to be the wrong size, or requires a 140mm stem.

I've had a real fit and am quite happy with it, I'm not questioning fit.

Im sure if I went across town to every "fitter" they would each set me up slightly different based on their school of thought/practice, but not my question.

For what it's worth, every fit calculator would have me on a 57cm and everyone who looks at me says 56cm but once again it's opinionated. As someone mentioned, pro fits seem to be all over the place and I'm sure they'll all tell you they've got the correct fit. I think that's the beauty of it and why I'm curious to know why.

Your last sentence is what I'm looking for although I think "right fit" is relative.

Thanks.

Oh, I didn't realize you were talking same make/model as you already have.

In that case, I'd say no, there's probably no improvement to be had going smaller, because the elements most germane to handling are the same, e.g. head and seat tube angles, bb drop and chainstay length (talking Giant TCRs, here).

Yes, you're weight will shift forward over the front wheel some, due to shorter wheelbase, which could be positive or negative depending on where your balance is now. It sounds that you have long arms, so perhaps out of the saddle efforts might pull your weight a bit further forward, perhaps making the bike feel less stable during those times.

A smaller frame will have better handling (there is a margin of diminishing returns, and a margin of negative returns with this) but better handling means you will need to watch your foot/front wheel clearance a bit more often.

But if its not broken don't fix it.