Is there anyway to know how much height or lift I will get from flipping a 12 degree 100m stem?
Is there anyway to know how much height or lift I will get from flipping a 12 degree 100m stem?
Yes. It’s called trigonometry 
Algebra my son… remember soh cah toa. The length of your stem is the hypotenuse, theta is 12 degrees and you want to calculate the length of the opposite side if this hypothetical triangle. sin12=x/100
I work out x to equal 20.79mm
HOWEVER, assuming you’re already running this stem in the reverse position, the total height gain would be double that… about 41mm
Discalimer: I haven’t done math like this in 10 years since I left high school. Feel free to correct me noble slowtwitchers.
Should have known that Todd would have a cool little applet like this to figure this out. Thanks for sharing.
All right. Next question…and yeah, I admit this is dumb but is a “flipped” 12 degree stem then become -12 degrees?
That’s what I would assume.
No. The stem angle is measured relative to the steerer tube, so until you take into account the angle of the headtube, you’re still dealing with all of your angles in a rotated coordinate system. Since most road bike head tube angles are pretty close to 73 deg, a stem of -17 degrees will be close to “flat” (i.e zero degrees in the earth coordinate system), whereas a “0 degree stem” would actually point up 17 degrees above horizontal in the earth coordinate system. You can do trig equally well in both coordinate systems, but it’s just much more useful to have the “stack”/“reach” deltas in X-Y than it is in some system using the steerer tube as the reference axis.
The fit calculators that others have posted will do this for you.
On a 100m stem? It will go up a bloody lot.
http://yojimg.net/bike/web_tools/stem.php
Bookmark this site!!! Very helpful. David K
or you could just not leave the site. amazing everything here on slowtwitch if you just nose around a little 
Assuming a 72° steering tube and flipping from 12° up to down
39.55mm Down
12.85mm Longer
.