Will it give about 20mm difference in bar height? Can't seem to wrap my head around the maths.

https://www.brightspoke.com/t/bike-stem-calculator.html

This tool says that for a 73 degree head tube angle, flipping a -6 degree 100 mm stem up will result in a 20 mm taller stack and 6.1 mm shorter reach.

Junior high math. What distance will twelve degrees subtend at 100 mm? Think about it...

I don't think that simple approach accounts for the tilt of the head tube. The distance you calculate is not vertical. Close I am sure but not exactly the distance subtended by 12 degrees at 100 mm.

i like this calculator:

Stem Comparison

You are right. But it's close enough I think for most people to compute the new position.

I can do long division by hand too, but I don't.

I just played with a 90mm 6° stem and a 73.5° HT.

Computation not necessary - go by feel. Use the Force, Luke.

Right.

Often when I flip the stem, I move the spacer from the top to the bottom - a 5 mm spacer on a 90mm stem is almost ther same height.

That was forty years ago... forget that crap. Thats what the interwbz are for

What if I have an adjustable 100mm stem?

Just clarify: the subtend length is always exactly the same. Only it's parallel to the steerer. I know this is what you meant.

Right. If I am not mistaken, the correct value is the subtended length X sin(head tube angle) which comes out to about 95% of the subtended length. Thanks for the clarification.

For crying out loud, this isn't a root canal. Just flip the thing and see how you like it. It's a simple procedure that shouldn't require more than about 3 minutes to complete. Take the bike for a ride around the block or set it up on a trainer. If you don't like the new position, make a spacer adjustment or flip it back.

The collective time invested in this thread already exceeds that required to have conducted the experiment.

Collective time required for replies, is collective and not mine

Besides, if I get that it is 20mm difference, I don't 'need' to do a test to see if I like it or not. I can get a fair idea without actually flipping it.

To simplify the math:
The angle of your stem, relative to a level plane (the ground), is 90*-(HTA)+/-(Stem angle). A 0* stem on a bike with head tube angle of 73* really has an angle of 17* up--most cyclists understand this part.

Your 6* stem, therefore, is really either an 11* stem or a 23* stem depending on how it's flipped. Since stem length is the hypotenuse of the triangle that determines the vertical height of the stem, the height difference is (100mm)*(sin23*)-(100mm)*(sin11*). The difference is almost exactly 20mm.