i seem to see a lot on BF that it isnt okay to just get a longer stem for your bike, vs buying a bike with a longer top tube.

my bike is comfortable for standover but I feel a little scrunched up riding it & it fatigues my lower back. I'm otherwise happy with the bike. I'd rather not have to sell it (I've only had it for a year & a half) and deal with buying a new bike.

what's the real reason not to just get a longer stem? is that not part of fine tuning your fit anyway?

Within reason, there's nothing wrong with it. i have seven bikes, and i think only one of them has the original stem. One of my mountain bikes (now a commuter, with road tires and fenders) has had stems from 60 to 150mm, as I switched from using it for crosscountry riding to downhill to city riding, where I wanted to be upright so i could see the traffic. No problems.
some riders say they have trouble adapting to the steering feel when they change stem length, but it's never taken me more than about 10 seconds. You can get off one bike and get on another that's completely different and not crash. Why should a small stem change be dangerous?
Old-school wisdom says to adjust your saddle first (the fore-and-aft adjustment) to get your knees where they need to be, then buy a stem that puts your hands where you need them.

Most of that comes from serious riders, who spend serious money to get a seriously good, long-term ride.

Now, a longer stem is generally just a band-aid fix for a poor fit; it CAN BE part of the fit process, but your bike has to be close to right to start with. More than 20mm of stem change, and you're on the wrong size bike.

Now, a casual rider can make use of a stem change to salvage an otherwise acceptable bike; since the casual rider doesn't go out and do 50-milers, club rides, centuries, or charity events (usually), a well-fit bike is less of an issue. If you like your bike aside from the stem issue, you likely fall into this group.

It's an inexpensive way to try; you can always keep the extra stem(s) if you have to buckle down and buy new. (I have about 4-5 extra stems laying around, just in case a need arises; that, and they're sweet pieces of alloy!)

i was thinking i would buy a 110 & a 120 and see if either fixes the "problem" I currently have a 100mm. While I'm at it, I'm also thinking of getting new bars & upgrading to a 31.8mm clamp on that bike. Not sure if it really matters but I seem to like it on my other bike, the bars feel a little thicker. Who knows, now more than ever, I could probably be easily convinced to sell it & buy a new bike... we'll see... trying to avoid shelling out the extra cash

i see no problem with it... i was not comfortable at all with a 61cm bike with 6* 120mm stem, so i got a 64cm bike and have trialed a 20* 90mm stem and really like it... so basically i have a 'nearly' identical reach, but i don't have to have the seat post maxed out to get an appropriate seat height.

i see no problem with it as long as your not going gigga short or mega long

The issues are about where the steering axis sits wrt to your weight. If you go too long you'll feel like you're operating the tiller on a boat.

By changing the stem you just move your hand position relative to the steerer, with the top tube size you move your hand position and steering axis together. Too much or too little stem can make for a quirky ride the designer didn't intend.

On a new bike, the stem length should be proportional to the top tube, so bigger bikes have longer TT and longer stem. Most med size bikes come with 10cm.
With your own bike, you can change it however you like. I replace the 10cm with an 8cm and it works for me. Im sure it could be changed to a 12cm and work just as well.

thanks for all the tips guys!

I'm kind of fighting myself back & forth about whether I should try to sell my bike & buy a different one or take the extra money I would have to spend to buy the new bike & use it toward upgrading what I have... There arent a lot of upgrades that I want but a CF fork would be nice, compact double (I have a triple & I have never used my granny) as well as different bars & stem are on my list.
Just changing to a double will cost me in the neighborhood of $150 assuming equal level components to what I have & the fork will be another 100-150. Another 50-75 on bars & stem. Just there, I've spent almost $400 and thats almost the difference between the new bikes that I've looked at & what I can sell mine for.

Maybe, maybe not.

You really need to evaluate your fit on the bicycle as a whole. Larger bicycles tend to be larger all over - longer top tube, longer seat tube and longer head tube. There's certainly no reason not to make relatively minor adjustments in stem length. If you find yourself making a major change, however, you're almost surely going to be screwing up something else.

Since different people's proportions are different, you may find that your personal comfort differs from the default approach. That's fine, just be sensible about the whole business.

I think I may have a solution to my discomfort on my bike that involved spending no (extra) money.

I had to replace the tape on my bars anyway so I moved my hoods forward a little bit, took it for a very short jaunt up the road and it felt MUCH better. Hopefully thats the fix