My calves periodically cramp (not severely) on rides also but i generally recognize the signs of impending cramps and deal with it before they become full cramps. I usually have pretty good success stretching the calves while riding - if you have clipless pedals, just put your foot down at the bottom of the pedal strike, lower your heel as much as you can and enjoy the stretching sensation you get. Repeat with the other foot. Once that's over with, I generally try to avoid riding toe-down (which is another way of saying I'll try to focus on keeping my heel a little lower than normal) and that seems to help as well.