Hey Crazycanuck, how long roughly does it take you to ride 35km??? 3.5 litres of water sounds like a lot to be drinking over that distance even in the hot weather. I drink about 750ml per hour.

In this kind of weather you should really be drinking a sports drink. I would recommend trying the powerade or the enervit powdered drinks. I never make the drink to the specified concentrations they recommend on the container, I always dilute them quite substantially.

Perhaps you should try taking a combination of both water and sports drink on your next ride (ie. carry water in your hydration pack, and put a water bottle with sports drink on your bike), and see if that helps. This weekend will be perfect for you to test to see if the sports drink helps since it is going to be over 35 degrees celsius.