7/27/14 3:30 PM

58CM Top tube is as short as I dare go. But, I did test ride a bianchi infinito with a 57.5 top tube at some point since I have been here. I purposefully requested, and watched as 125lb or air was topped off in the the 23C tires. It was a short ride of less than 10 minutes. But I did not feel cramped, but I did not get low and fast either ['relative' fast ;0]

I will say it absorbed chip seal rattle and deep cracks/broken pavement better than anything I remember riding. It felt like my Roubaix does with 80/85 lbs in 25C tires. but with a more solid feedback of course. But I have been running the Roubaix of late 90/95lb with the Vit Paves. Take some of the sponge out of it. ;)

7/27/14 5:00 PM

my normal range is 57 to 58.5, what works also depends on the STA. every bike i have is setup with 59.5cm reach and constant saddle-setback & handlebar drop.

steeper STA i can get away with a shorter TT to get the same reach. shallower STA then i need the higher TT range to get the reach i need.

7/29/14 5:40 AM

You definitely need to take both measurements into account in order to get the right fit. What I find for myself is that a 72.5 STA works fine with a zero setback seatpost and anything steeper requires a setback post and a shorter top tube to get achieve the same position. IIRC, for my saddle height (~79cm from the center of the bb) a .5 degree change in STA translates to ~1cm change in saddle position.

7/29/14 7:19 AM

i use the sin on the STA delta to approximate that lateral shift. its not technically perfect, but certainly close enuf for gubment work!

sin (0.5°) * 80cm = ~0.7cm change

7/29/14 11:05 AM

Since we're measuring top tube length at the actual height of the top tube, the effect of a 1-degree change (going from 73 to 74 degrees) is 0.9cm for each degree, assuming a frame size near 60cm (x=55cm vertical).

Or roughly 1.1 degree for each cm of effective top tube length change.

Head tube angle also plays it's part with respect to fit in that a very relaxed head tube angle may not permit such a long stem to be fitted without causing "steering heave" when riding out of the saddle.
Inversely, a steep head tube angle usually results in "flighty steering" if the stem is on the short side, and made worse by a taller handlebar height.

Moral of story here might be to not ride too large of a quick-steering racing frame or too small of a slow-steering cyclocross frame, as in both cases the stem length options won't correct the fit without handling consequences.
Unless, that is, the rider doesn't mind a very-forward or very-rearward saddle position, respectively, for each of those extremes.

As an example, this one worked out well, at least for hard-charging ~shorter rides, simply by moving the saddle forward. This frame has steep angles but the 120mm stem that it came equipped with calms it nicely: