Hi if I change my -6 120mm stem to a -17 120cm, will it increase or decrease the reach due to the angle? or no change?
Also how much would the drop be? Im not very good at angles and that stuff. I would of just guessed 1.1cm…120
If you do a Google search, you’ll find links to a few calculators that will allow to make the stem sizing comparison that you are asking about. Here’s a link to one:
Nice… 23mm lower drop…
Hopefully my current stem is a -6… and not a 0 angle, otherwise its a 35mm drop
Reason i wanna change is because I think my currnt fit on road bike, is too high, my shoulders, shrug up… and my neck goes forward, resulting in neck/shoulder pain… and extra weight on the bars…
If you do a Google search, you’ll find links to a few calculators that will allow to make the stem sizing comparison that you are asking about. Here’s a link to one:
http://yojimg.net/bike/web_tools/stem.php
if you go to the navbar just above you can find one 
120mm, -6°
rise: 42mm
run: 112mm
120mm, -17°
rise: 19mm
run: 114mm
this is the rise and run from the distal terminus of stack/reach to the handlebar bore.
Unfortunately the calculators you’ll find online have a bit of a systemic error in using a fixed pivot point instead of a floating one
The diagram below gives a better reflection of what happens for a 100mm stem
Dropped clamp stems (FSA & Syntace) go lower
For a 120mm, here is what would happen on my bike
19mm lower, 1mm longer
Yes, it’s a small difference from the 23mm figure generated by the standard calculations, but accuracy is important!
Having said all that, your issue sounds more like having the saddle too far forward rather than bars too high
I had my stem at 140 and found my self too stretched… so I shortened by 2cm to reduce the neck pain…, moving my saddle back would just bring the same issue back ?
weight on your hands is from not being supported by your core, get that sorted then find a comfortable reach
.
So I hoping I’m not doing anything I shouldn’t by directing him to the site that, if I recall correctly, you created. I find it a wonderful resource for more comprehensive fit-related info.
Slowman won’t tell you off for linking that 
I left the business, but the tool retains the formulae generated by my obsession with calculating anything fit related. As far as I know it’s still the only tool with floating pivot based formulae
Can you elaborate on your statement regarding core support and weight on your hands when riding? Is it to do with riding position or strengthening of the core? TIA
As a fitter I deal with the rider that is in front of me. So that is about setting position for current shape and conditioning.
If the rider works on core strength, or rides a lot to improve bike specific conditioning then position may be able to change
If you get sore/numb hands or shoulders, then moving the saddle back is the most likely avenue to follow in order to shift weight from hands to butt/core
Trigonometry.
weight on your hands is from not being supported by your core, get that sorted then find a comfortable reach
I pass the balance test in any saddle position already
.
Advice. In 2022.
I think I got a 5-6mm increase in legnth from this 120mm -17stem… from the 120 -6… the original must be neutral…
The difference between the stems you originally posted, if they have the same base height are:
Original stem -6° 120mm
-17° 120mm
22.9mm lower
2.4 mm longer
If the original stem was 0° 120 mm
-17° 120mm
35mm lower
5.6mm longer
.