Sweat is the cooling process of the skin. The sweat glands produce sweat once the body reaches a "high" temperature where it needs to cool off the heat by sweating and using the evaporating aspect of the water to cool the body. The "high" temperature is reached by how much intensity is being done at the time of exercising. So the "less" fit cyclist will start sweating first because he/she is out of shape and will attain a higher intensity work out before a "well" fit cyclist. Air temperature and humidity also play a small role.