Up to a certain temperature, well into the 90's I am fine on rides, and have even done a 65 miler where the temperature was recorded at 105F. What gets me though is Humidity. I have been shattered with temperaturs as low as 80F but with very high Humidity, until plenty of salt was taken. Then I managed to do the 35miler and actually won my class as I was the only one to take salt.