I bought this bicycle second hand - Cannondale Synapse 105 Alloy 2013... size 58cm

The fellow before me was 6'4'' and I'm 6'2''. I hadn't made any adjustments other than change my seat height. After a 20 miler the lower back was screaming a bit at me and I feel as though I have to push my ass further back to meet the seat and be a bit over extended to get to the bars. The picture you see now has the bars pointed a bit further back to me since the last ride, and I moved my seat a bit further forward as well.

I'm wondering if the fellow before me put an extended stem length on this bike and that might be where I'm having the issue.. for me a slightly shorter stem?

Attached is a picture of the stem:

https://gyazo.com/6b674510b7f0b30fb019c80353078fe4

Just take apart and inspect.
Don't forget that modern stems can be mounted angled upward or downward.

I'm doubting anything was extended. New steer tubes tend to be long, and customers may cut off if they choose.

Stems (the part sticking out) can be cheaply swapped for longer, shorter, or more angled.

I guess what I'm asking here then is - how should I go about measuring it to figure out which I have versus which I need to order / replace it with. What is the actual 'measured' distance of the stem? From absolute front to back or just the tube part itself without taking the front clapsing area into consideration.

I am sure I am over-complicating this, but that's how I am

What would be the name of the replacement stem that I would be looking for? "cannondale synapse 2013 stem?"

It most likely has the length marked on it. Look on both sides and the bottom.

I see the torque spec stamped on the top.

Stems aren't specific to bikes, they just have steerer/bar clamp diameters that need to match up. Fortunately most modern road bikes have the same dimensions, 1 1/8" for the steerer and 31.8 mm for the bar clamp. There are 2 other measures that you can adjust to taste- the angle (which can be up or down depending on how you have it mounted), usually something like 6 or 7 degrees, and the length, usually between 80 and 120 mm.

Your current stem should say the length measure on it somewhere. Look for a little tiny number between 90 and 120 in multiples of 10, that would most likely be the length. If it's too long, get one that's a bit shorter. Very easy. You can find cheap alloy stems on Ebay and the like for $20 and less.

Here's an example-

Bontrager Race Lite Road Bike Stem 7D 110mm 31 8 OS | eBay

Not specific to your bike, but it would work. Probably going to sell for less than $15.

It's a Cannondale C3 stem. There's probably a hundred of them on ebay. If the length isn't on the stem just measure it from the center of the bolt on the steerer cap to the center of the bars. It's not a zero degree stem but you'll get close enough, looks like maybe a 110mm to me.

100 or 110mm I would say just by looking at it. If you do decide to change it out, don't make too huge of a jump, like from 110 down to 70 or 80. I would go in 10mm increments, so 110 to 100, 100 to 90, etc.

I believe stems are measured from the middle of the steer tube to the middle of the handlebars. Round to the nearest 10mm.

I have a 54cm Synapse and my stem length is 105mm. I guess it's 110mm according to [MENTION=392454]CliffordK[/MENTION]

Yours seem longer than mine so i think it's a 120mm stem.

It's hard to say by looking at the photo, but I believe my cannondale stem on a size 56 was 100mm, of course, shops could swap out stems for a better fit at the time of purchase, so it's hard to say what the bike was sold with.

You can get the c3 stems for about $20 on ebay if you wanted to try a smaller size. I'd guess it's a 110 or a 120, so I'd start out by trying a 100mm.

However, if you're scooting your butt back to get a proper position on the saddle, I wonder if you have a setback seatpost, which might be the problem.

Or the saddle is slightly nose down. Could be either. Without seeing a picture, we are both guessing.

The stem length as it is should be fine for most 6-footers on that 58cm frame, but you need to get a completely comfortable reach to the bars when you are squarely on the saddle. So firstly I would skoot the saddle forward until the reach to the bars is sustainably comfortable while riding in the drops of the handlebars. From there, you will want to test a range of saddle height adjustments.

I see a lot of road bikes these days fitted with the saddle too far back, which rotates the lower body rearward, thus forcing the rider to bend more sharply at the waist. I assure you that road bikes are not best fitted that way, as it makes it impossible to ride in the sporting manner that the bikes were made for.

Moving the saddle forward rotates the lower body forward, which may require a slight tilting down of the saddle to where only the front half of the saddle is level, assuming the saddle top has at least some concave curve to it (kicking slightly up toward the rear).

A more-forward saddle makes for a quicker and lower-effort "heave" up to a standing position, as when attacking rolling terrain, so helps one maintain pace despite changes in grade.

Here is one of the larger bikes that I've set up for myself, and which is comfortable for me to maintain speed on though the hills here while on training rides. It's an old criterium racing bike, with as big of a frame as yours, but I'm only a long-legged 5'9" (175cm) tall and 55 years old.
Again, note that I have a comfortable reach distance from the saddle to the bars, by way of moving the saddle forward.