Stem size for switch to mustache handlebars from drop bars
 

I ride a 56cm Cross Check with an eTT of 570 and a 100mm stem, for a total of 670mm reach. My REI sizing said my cockpit length is 660. I've often felt that my frame is a little long for my comfort, so I'm switching from drop bars to mustache handlebars. I've heard that m'bars afford a more upright riding position. I've also heard that they elongate the reach. Contradictory statements, right? Well, I don't know which to believe but I heard it on BFwhere everyone's opinion is truth. Anyways, sizing down...I want to know what size stem would be most appropriate. I think my drop bars would be okay with a 90mm stem or even an 80mm for more of an upright position when riding the tops. Since I don't know what to believe about m'bars, I don't know what size stem I should get. I was thinking of getting a 70mm for a reach of 640mm, 30mm less than my current set up. Is that going to be too short?

The primary hand position for moustache bars is in the hooks, as opposed to on the hoods for drop bars. You'll probably want to have the stem much higher and a little closer.

If the current bars are serving you well, other than that you feel a bit stretched out, what about keeping it and shortening the stem by 10mm since that'd put your overall reach in line with your fitting?

Yes, the bars bend forward, significantly and a very slight drop..
I have used a tall up angled stem with a fairly short extension .
Consider level with the saddle a starting point for height..
though you may think better of your drop bars
if they were higher and nearer, as well .

My Bike with the mustache bars on it , has a threaded fork.
the stem is an 8 Nitto Dirt drop
off a MB1 take off from the 80's
the 8 is measured up at a 45 degree angle, following the stem Rise,
so its even shorter horizontally than the numbers suggest.

Yea I could just shorten the stem and keep my drop bars but a part of me really wants to give my bike a make over with complete different handlebars and chrome accents. So between a 70, 80, or 90mm stem?...90 giving me my 660mm cockpit length.

you are there , you choose..

go to a bike shop with all 3 in stock, and try them ... ride around the block a few times
threadless stems are a quick swap.

QBP/Dimension has angle adjustable stems .. they're black , maybe that will suit.