Thanks, FarHorizon and LastPlace.
I've pondered this question for many months and you have explained the geometry of the problem. I sure wish that I had that old Schwinn back! Been looking on ebay the last couple of weeks, but haven't seen anything that looks either in good enough condition to restore or the correct size. And yes, my son is getting tired of hearing about the "old Schwinn."
I just bought him a Trek1500 today. Got a good deal on it and it is straight 105, which for a 20 year old's first road bike isn't too shabby. And he does like speed! I was in his slipstream at the local trail coasting at 25mph until I said how much I was enjoying being pulled along. Then he started pedaling and I fell out of the slipstream and had to pedal furiously (26mph) to catch up.
And this was when he was riding an old steel frame Gitane....he's gonna leave in the dust with his new ride!
Thanks again for the explanation!