does seat tube length really not matter?
 

so im one of those guys who bought a 56cm IRO from the group buy who thinks he should have maybe gotten a 59. so my bianchi pista is a 56 and fits pretty good. does the shorter tube mean i'll just have a few more cm of seat post sticking out? will handling change? i feel like bikes that are longer rather than taller should be less "twitchy" less agressive.

someone fill me in.

seat tube length is pretty irrelevant with the longer seatposts available a bigger concern should be top tube length

As long as you can get the bars high enough then you are fine. You may have to raise the stem or use a positive rise one... hopefully you got the threadless.

so far as i know, twitchiness has mostly to do with trail, which is the distance between where a straight line through your head tube intersects the ground, and a point on the ground directly below your fork dropout. Other aspects that affect handling are wheelbase, stem length / hand position relative to the headtube, and saddle setback (?). All these factors influence body position, which is the main factor.

Seat tube length matters a bit, as it will affect your stem height. Moving a stem up also moves it back, decreasing effective reach, and effective stem length.

The best way to pick out a correct frame size is to ride lots of different bikes and find one that works.

If you want to pick out a frame by numbers alone, I suggest checking out how pro road racers set up their bikes. Keep in mind they're pros, so they'll be a lot stronger and more flexible than you or I, so you'll probably be comfortable with less saddle-bar drop (or none at all). They pretty much all have long stems, lots of post showing (though this is in part because smaller frames are stiffer and lighter, and because they can deal with more drop), and lots of saddle setback. They're literally hanging over the wheels.

Compare these setups with your current ride, and adjust your frame pick accordingly.

seat tube length is pretty important if you don't want a stem setup with tons of rise..
with a traditional frame (no compact geometry/sloping top tube) the head tube gets taller along with the seat tube..

no matter what the ladies say seat tube length is very important

You might need a longer seat post and taller stem. The stem part is easier if you got the threadless fork.

The main thing is the top tube length. Even that can be fixed somewhat with your choice of stem.

I'm not sure if toe overlap is affected by having a frame too small. I guess if you bought a 50cm frame, installed 175 cranks, and had size 13 feet, then you'd probably have toe overlap problems. I doubt you are that extremely too tall for the frame you bought, though.

Edit: The way I chose my frame size is to measure the top tube length of my favorite road bike and choose the IRO frame with the same top tube length. If the TT on your IRO is smaller than your favorite road bike, then you need a correspondingly long stem.

Unless you have freakish proportions (really short legs, really long torso/arms. My new custom has a 59 tt and 52 st and still with a 140stem) you shouldn't have to worry about seattube length.

A shorter wheelbase will also be more "twitchy". Bikes with longer seat tubes tend to have longer wheelbases. If it is the same model, the wheelbase is always larger.

If you want a racing bike, and handling and responsiveness are most important. If comfort and stability is more important, do the opposite, a la Rivendell.

First off since the question was basically is st important or just tt this answer is nonsensical. It is also factually incorrect. There have been many models of bike(not so many any more) where nothing changed between st sizes except the length of the st and the length of the ht. As if that wasn;t enough due to the contortions required to make small bikes work with big wheels you will find that sometimes a smaller size in a model line has a larger wheelbase.

Take a look at the pictures. The C-T Seat Tube length is exactly the same on the Bianchi Pista and BF IRO. While the C-C ST on the IRO is 4 cm shorter than the "size", the seat tube goes up 4cm past the top tube, so the same exact amount of seat tube will be showing on both. This does mean that the standover height will be slightly lower, as will the top of the head tube, so you might need a couple 1cm spacers or a more flexible back.

I wouldn't be too attentive to the whole "twitchy" perception. Nothing wrong with a lot of stem to accompany a lot of post. Otherwise, your 5th lumbar will let you know it.