There is a great article for you to look at on Proper Hydration for Distance Running, written by the USA Track and Field Advisory Group. I have applied many of its recommendations to this web article. The link to their article is http://www.usatf.org/coaches/library/hydration/ This article I think has the best science about hydration for long distance events and applies equally well to cycling.
We lose salt plus water in our sweat. If you do long workouts or races there is a danger in replacing these losses with just water. You can develop a condition called “hyponatremia” which means the body has too low a salt concentration in the body fluid. It is hard to believe but athletes can die from consuming too much water. In one case a high school football team practiced in full uniform in very hot weather. Obviously they lost a lot of fluid weight. The coach told one player to go drink several quarts of water. A short time later he had a seizure and died due to this condition called “hyponatremia”. He lost a lot of sodium and potassium in his sweat, and when he drank a large amount of plain water, his body’s sodium concentration became dangerously low .Endurance athletes must understand that fluid losses from extended exercise must be replaced with a sport drink that contains sodium and potassium. It is also important to understand how much fluid you lose per hour and not over replace it in order to avoid hyponatremia (low sodium in the body fluid).Recreational athletes or competitive athletes doing only short work outs do not need sports drinks and can just use water. If you are just doing a short criterium you won’t drink during the race so you need to focus on pre and post race hydration.