Originally Posted by gastro
Now visualize if you will a bike sitting at rest. If you remove material from the bottom of the head tube the attitude of the frame will rotate forward around the rear axle. This will reduce the absolute height of the bottom of the head tube from the ground, shorten the wheelbase, and steepen the head angle. Got it?
Now visualize removing material from the top of the head tube. What will happen to the absolute height of the bottom of the head tube/crown race from the ground? That's right, absolutely nothing. Which, not coincidentally, is the exact amount of head angle change you will see.
This is the perfect explination Pete.^^
Now lets take this 1 step further and I think you will see where I've been coming from on this, and hopefully we can put this to rest.
Visualize the same bike, but with a triple clamp fork.
Now cut some materiel off the top of the head tube, then loosen the clamp bolts on the lower clamp and slide them upwards until the head tube sits where it should in relation to the top triple clamp as we did not move it.
Obviously there is more to it than just this. Cutting the top of the steerer tube or spacing the stem, etc, but it shows the same theory at work.
When we slid the lower triple clamp upwards, what did it do th the frame?
It rotated around the rear axel in an upward direction.
The only difference is that I started from the upper starting point as that is normally where I've set up my forks with the triple clamps, as slack as possible, and thus, the top side is the chosen starting point my general use.
There fore, Gastro did actually validate my original point with his description here by making a visual of the theory at hand.