What is it about a bike that contributes to stability at high speed? For me, high speed is over 40mph. I never went over 40 mph for years but now my new weekend route takes me a long down hill where I could hit 50mph if I wasn't constantly on the brakes. I feel the rush just like you guys do when going fast, but I nudge the brakes at 40.

The bike, a 58cm allez, doesnt feel too stable at that speed, but that maybe coz I'm not accustomed to going that fast. So what contributes to high speed bike stability? I'm guessing it's rider position, tires, wheels and frame, pretty much the whole package. But what is it in the frame that aides stability the most.... head tube ? Would an Allez E5 be more stable?