Originally Posted by
electrik
Think about this, the centre of mass moves backwards with your shoulders when you shorten the stem, by moving the stem upwards you relieve more pressure off the hands. By, shifting the seat forwards you move your hips and centre of mass forward so you can try to get back how the bicycle was balance previously. If you simply shorten and raise the stem the bicycle will handle differently due to the centre of mass difference. This is one reason why bicycles come with different sizes, HTA and STAs.
The center of gravity is at the top of the hip bone when standing. As you hunch over that moves forward. THe goal is your CG be far enough back that you can lift your hands off the bars when your butt is on the saddle. That allows you to shift the weight around between your feet, hands and Butt.
Simply moving the seat forward moves your hips forward in relation to the crank center. The forces more weight onto your hands and if you get the seat far enough forward if you attempted to lift your hands off the bars you would go over the handle bars.
Yes if you raise the stem it will move the CG back to some extent but the saddle it to adjust your hips in relation to the BB not to adjust reach.
Sheldon's site has a good bit of info
http://sheldonbrown.com/frames/index.html
This is the part about saddle placement.
http://sheldonbrown.com/frame-sizing.html#frontback
Good quote from Sheldon:
For recreational riders, who don't tend to pedal as hard or as much of the time, a more relaxed position, with the saddle farther back is likely to be more comfortable. As the saddle goes back, the handlebars will generally move back and up to avoid an excessively sharp bend in the torso.