I have been riding my SLT just a little over three weeks ago and I'm loving it. the only real issue I'm experiencing is tightness/pain at the base of my neck on rides longer than 20 mi./45 minutes.

I have the stock build with the default stack height and the (swapped it for a 90mm) stem upturned to the widest degree (not sure what that position is?). The seat is perfectly set for me and while it is about 2-3 inches higher than the stem, I'm reticent to adjust that.

Technique wise I shift hand positions often and look down at the pedals to stretch it...

Should I have my LBS add another spacer? What sort of consequence will this have with bike handling? Is it something that will ease over time with more riding?



 

Pain like you are experiencing after only 45 minutes is not good.  You should definitely try to get fitted properly if that's possible.  Typically this pain is caused by being too stretched out (top tube and stem combo too long) and/or too hunched over (handlebars too low). 

I've already had a fitting with the LBS and would be willing to make more adjustments to accommodate a better position. However, they aren't too likely to know/care about consequences related to performance/handling specific to the SLT.

 If I'm going to go down that option, I'd like to make an informed choice and determine what the best corrective action to take.
 

 

Is this your first road bike? If so, the lower front end and resultant strain on your neck caused by having to hold your head up is not that uncommon to begin with. 2-3 inches of drop isn't by any means excessive, so I'd suggest toughing it out for a bit longer. If the pain gets worse over time, then obviously revert to the LBS to have your position tweaked. But hopefully it will get better over time.

I apologize if some of this is basic to you. I don't know your level of experience. 

Gregg66's remarks ring true. If you are new to the road bike, and particularly if you trust your initial fitting, you should give it a little more time. Some avid road cyclists work through minor stiffness and aches at the beginning of every season. It would be that much more pronounced for you if you're new to it. It gets better. In the mean while, you might be able to do a couple of things on and off the bike to help.

First off - work on relaxing those neck muscles while you ride. This might be just a tensing up issue rather than a strain issue, especially if you're new to the bike. Just try to be aware of tenseness.

Check you posture on the bike - if your torso is hunched over with a lot curve to you spine, try strightening your back a bit. This will roll your shoulders up and to the rear and may provide some relief at the shoulders and neck. However, note that this may also rotate you pelvis down and to the front, which might affect your shadowy parts and/or your back. You have to find a balance.

Check your arms - if they're locked out, you might be in a permanent "shrug" at the shoulders, which will add to the tensing up thing. Your arms should be bent at the elbows slightly, and your shoulders should be in a slightly rounded relaxed position. It helps to loosen up the hands so that your grip is light rather than tight. Really, it's about finding as relaxed a state on the bike as possible. Tensing up creates aches and stiffness, and robs power and energy.

This might sound funny, but check your jaw. Clinching is a sign of tension in the neck and head.

Off the bike you can work on stretching and relaxing those muscles with neck rotations and rolls.

Hope this helps. Good luck.

Gimme Shoulder:

Check your arms - if they're locked out, you might be in a permanent "shrug" at the shoulders, which will add to the tensing up thing. Your arms should be bent at the elbows slightly, and your shoulders should be in a slightly rounded relaxed position. It helps to loosen up the hands so that your grip is light rather than tight. Really, it's about finding as relaxed a state on the bike as possible. Tensing up creates aches and stiffness, and robs power and energy.

 

I totally agree w/ that.  I've found a lot of times when the elbows are locked out it is due to being too stretched out (combo of too long top tube and too long/low stem).

I also agree there's definitely a component of how used to it you are.  Early in the year if I push the ride times above what I've been doing, I come crying to the wife for a little massage in the upper back.