I ride a 55cm Cannondale, swapped out stock 110 stem to 100.
Looking at new Cannondale, but 55 not available, so I either size
down a tad to 54cm, or size up a tad to 56cm. Knowing that I
had to get a shorter stem, my gut feeling is go with 54cm. What ya'll
think? The geometries are almost identical (Six13 & SystemSix).

What is the rule of thumb in this type of scenario?

I'm 6ft, with slightly shorter legs and slightly longer torso.

Thanks.

The simple answer is that the 54cm SystemSix will require 6mm more spacer or 3-4 more degrees of stem angle and one size longer stem than the 55cm Six13.

A 6' tall rider using such a short stem seems quite odd. The total reach on my bikes is only about 1cm less and I'm 6 inches shorter with long legs and a short torso! I can't imagine that you're not cramped up on this bike.

Do you have some other relevant info, like a saddle height, measured from the center of the bottom bracket to the top of the saddle, along the center of the seat tube? How about some info on the stem setup, like the amount of spacer and stem angle?

Do you have the saddle extremely far back to achieve the proper knee over pedal relationship? That would help to explain the short stem. Ever check the horizontal distance from the tip of the saddle to the center of the BB?

C-40,

Thanks for the information. I have those measurements, but I have to find them.
Otherwise, I'll do some measuring tonight and post them for you to review.
It seems like the majority of the riders my height are all riding bikes a little
bigger than mine. I had the bike fitted by LBS and it seems spot on, and
very comfy. I don't feel cramped at all.

A buddy of mine is a Cat 1 racer, which to me means he knows what's up
with bike fit etc, so I told him I got a 55cm bike and he said that is too small.
But when we met up for a ride, he saw me and said "Sh*t.... so weird... but
that bike fits you just right." LoL!

I'd be curious to review your dimensions. While the fit may be comfy, it's probably what most would call a recreational fit with a short reach.

Here's an on-the-bike test to take. Ride with your hands in the drops, with your fingers in reach of the brake levers. Position your upper back close to horizontal, or as low as you can tolerate for an extended period. If your knees and elbows interfere in this position, then your reach is short. If your elbows must be bent quite a bit to get your back low, that's another indicator of a short reach, which also cause knee to arm interference. I use just enough stem length to avoid any knee to arm interference.

One other possibility, if you have a large vertical drop from the saddle to the top of the bars (like 12-15cm) then you're substituting drop for reach (and the frame is too small). In that case, the larger frame might be wise, in order to better balance reach and drop. It's more common to have an 8-10cm drop.

C-40,

Saddle height from center of BB: 75cm
FSA OS140 Stem (100mm) angled downwards
2.5 cm of carbon spacers under the stem
8cm drop of saddle height to top of handle bar

I did the simple riding on drops test, back as parallel
to ground as I can get while riding... forearms are
about parallel as well. The way I pedal, my knees
come up to just inside of my elbows, so I never
get any elbow/knee interference, but if I tuck my
elbows in and point my knees out when the knees
come up, then yeah the elbows and knees to touch.

I'm just thinking if I go 1 size up, I'll have to shorten
that stem by 20mm..... since on the 55cm bike, I
had to shorter the stem by 10mm for a more
comfortable fit.

Any idea if I will notice big different by going from 55cm
to 54cm size bike? (Ofcourse this is until I go test ride
one myself.)

Thanks!

With a 75cm saddle height, you definitely have short legs for your height, no more than 3cm longer than mine (and you're 15cm taller).

With the smaller frame, you would need 3cm of spacer, which is too much, IMO. If your cycling inseam is 86cm, then you should definitely get the 56cm frame. It will reduce your spacers to only 1.5cm. To duplicate the fit, you would only need 7mm less stem length, since the TT length is 7mm longer and the seat tube angles are the same.

I would suggest that you carefully review your saddle fore/aft position and be sure it's not too far back. I just can't imagine someone of your proportions needing a stem that short. Most riders of your proportion would be using a 120-130mm stem.

Based on the riding test you did, I'd say your stem is too short. The forearms should not be anywhere near horizontal. I keep my arms relaxed when riding in the drops, but my arms only have a slight bend when I have my upper back approximately horizontal. If I have my forearms hoizontal, then my nose is only a couple of inches above the stem and my knees hit my arms well above the elbow. It's not a position I could tolerate riding for any length of time.

You may also want to experiment with more drop. 8cm is a significant drop, but larger riders often use more drop than smaller ones. I have no problem with a 9cm drop and I'm a lot smaller rider.

Another thing to consider is the placement of your brake hoods, relative to the handlebar. If the area where the crook of the thumb rests is far below the top of the bars, you should consider a different handlebar and/or relocating the brake/shift levers. I see some pretty goofy setups where the brake hood is 3-4 centimeters below the top of the bars. Sometimes it's due to the bend of the handlebar and sometimes its due to the placement of the brake/shift lever and sometimes both are at fault. If the brake/shift levers are mounted too far down on the bar, it increases both the reach and the drop, resulting in the need for a short stem.

Here's an example of how NOT to do it.

http://www2.trekbikes.com/bikes/bike.php?bikeid=1424000&f=4

I'm 5'11" with and my saddle height is the same as yours -- 75 cm from center of BB to top of saddle. I ride a size 57 (c-c) frame. However, I am not very flexible with neck issues, so need to have my handlebars about even with the saddle height.

Nevertheless, a 55 cm frame seems small for someone your size and 54 would seem ridiculously small -- unless you like riding with handlebars about 6" below the saddle. I would think a 56 would be a better, but if the shoe fits, wear it.