I used to get bad cramps after about 50 miles of riding in my left calf only. Thinking it was due to injury or scar tissue from a previous injury I went to see a sports masseur who could find no trace of scarring or damage.
He reckons (and I don't know if it's true) that asymmetrical cramp, i.e. in one calf rather than both at once, is down to an electrolyte deficit whilst symmetrical cramp is down to an energy deficiency.
As a result I use the Elete Water, as per the instructions, along with regular stretching between rides and that has sorted it right out.