You can’t come into a ride in a chronically dehydrated state and expect to perform consistently. Keep yourself hydrated – before, during and after.
Your body is mostly water. The human brain is 70 percent water, your lungs are 90 percent water, and blood is 83 percent water [source: USGS]. When you sweat, you lose water. Hydration is necessary for the distribution of oxygen and nutrients throughout the body, and it allows the body to cool itself by sweating. When you are dehydrated, you might use poor judgment, run out of energy and even get dizzy on a trail — none of which makes for a good ride.
To maintain an optimum level of hydration while on your bike, you need to drink water constantly, whether it’s hot, cold, sunny or cloudy (although hot and sunny calls for the most water consumption).
Everyone is different, but as a general guideline the following is a good minimum:
16 ounces before the ride
20 ounces per hour during the ride
16 ounces after the ride
For longer rides, a couple of water bottles won’t cut it — you might want to strap on a hydration pack.