Sweat Rate - Indoor vs Outdoor results

Because it’s relatively simple, I’ve been tracking my sweat rate for my indoor trainer rides in order to gain a better understanding of how much fluid I should be looking to replace when on long outdoor rides. I’m also correlating it to kJ/hour to see if the intensity correlates with sweat rate.

As might be expected, the correlation is fairly strong: I.e. my sweat rates on recovery and lower intensity rides like Carter, Taku, Pettit, and Bald Knob averaged ~26ozs with an average 555kJ/hour work rate; and on the more intense interval workouts the sweat rate averages ~34ozs with an average 626kJ/hour work rate. When I regress sweat rate to kJ/hour of work, the correlation coefficient is ~69%, which I consider fairly decent. However, the standard error is fairly high at ~4.8oz. This is all based on indoor training rides.

This weekend I went outdoors for a hard 2 hour ride where I put 1,544 kJs of work through the pedals, or 772kj/hr. Using the regression data above, my estimated fluid need would have been 38.4ozs/hour. However, my actual sweat rate was only 26.6ozs. Indoors I ride with 2 fans and the temperature is probably ~72 degrees. The temperature when I rode was probably ~65 to 60 degrees.

I’m curious to know if others find this large of a discrepancy with indoor vs outdoor sweat rates?

started getting interested in sweat rate, I also want to keep data (eventually make a google doc for everyone to enter data).
I was surprised how much I lost in water (at ~138lbs I’m going 1.9L/hr)
I wanted to point out that kj is absolute but IF or TSS is relative, I would think that a 200w ftp rider going 100% for an hour sweat rate more than a 400w ftp rider noodling at 200 (which would be z1 for him)
I was thinking something along the lines of sweat rate / hour and people could put the workout they did (and the ideal software would pull data , IF, kj’s…)