Originally Posted by adriano

'twitchiness'

twitchiness would be based on distance of the point of steering from the axis of rotation along the plane perpendicular to the axis.
a setup with 42cm bars and a 50mm stem has the hoods 28cm from the top cap bolt.
a setup with 42cm bars and a 100mm stem would have the hoods 31cm away.

while this is a sizable difference, a 50% shorter stem will result in 9% not 50% faster steering. shoulder to palm and shoulder to shoulder measurements vary by a range much greater than that.

to say that using stem shorter than x mm will be twitchy is ridiculous. provided that the reach is correct, moving from a shorter top tube with longer stem to a longer top tube with a shorter stem should take a day at most to become accustomed to.