I know the rule of thumb is a 24 oz bottle per hour and that may work for some people in some climates but not for me. Did a test today.

Before I started my ride this morning I dressed in my normal gear, filled my 70 oz camelback and tanked up by drinking a a 24 oz gatorade. I then weighed myself with full camelback, about 3 pounds of Shimano sandals and biking clothes: 210.8 pounds.

The ride: The temps ran from 82 to 88 degrees (8:30-10:30 AM) with a dew point of 72 degrees (humid as h***). The ride was 30.01 miles in 1:50 for a ride average of 16.4 mph, wasn't pushing, just trying to duplicate an average ride for me. There was a modest amount of climbing at 470 feet and a 7 mph wind.

BTW: I drained the gatorback during the ride.

When I got back, went straight in a weighed in the same configuration (camelback + sandals + biking clothes) as earlier: 203.0, or 7.8 pounds less than where I started. Should have thought about it but one variable that I didn't take into account was the weight of the moisture in my shorts and shirt, it was significant. One gallon of water weights 8.3 pounds

Conclusion: Today at least, if I had restricted myself to 24 oz/hour, I would have been BADLY dehydrated. Correcting for the moisture in the clothing, I sweated out very close to or slightly over a gallon of water in less than two hours. This is very close to 3-24 oz bottles of water per hour.

The Florida humidity is often high enough that there is very little evaporative cooling.

Certainly an eye opening experiment. We are all wired differently Tom. I'm a lot like you and am a sweater' and when on the trainer or rollers it looks like I leaked due to the puddles of water under me where the guys next to me, doing the same workout, have a small wet spot. When it is hot and humid I will drink a bottle every 45 minutes at a minimum. I also mix Gatorade at the 1 quart serving per bottle and open 3 electrolyte capsules into each bottle to attempt to replenish the minerals lost in the sweat.

Two weeks ago I was in a race with a time trail, road race and crit. On Friday evening we had the TT and it was 94 degrees. During my 85 minute warm-up for the TT I drank 2 bottles of water and a 1/2 bottle of Gatorade just before the start of the TT. I rode the worst race of the season and barley finished the TT and averaged only an upper Z3 watt average for the short 22 min race. After the fact thinking is that I was depleted of electrolytes due to the sweating and heat during WU and didn't bank ahead on minerals or replace any lost during the WU.

I remember the humid days in NC. Florida has got to be worse. I now ride in the Sacramento heat. Not much humidity, but it can easily top 100º
Something that works for me to remind myself to drink. I use a Timex or a Polar watch with a countdown/repeat timer on it and set it for three minutes. Everytime it went off, I suck down fluids. Sounds weird, and is a pain in the butt at the start, but you don't get dehydrated. It is easy to miss a couple of fluid intake intervals if you are in a race, or being competitive. Miss a few in a row and you won't be able to recover. Set the interval for whatever works for you

What a great experiment - I am going to start doing the same and seeing what happens - I need about 1 liter or a quart 32oz per hour and thats all year long - During the summer even more with salted fruit (yes in Texas we too salt our water melon) - Local sprinklers in the yard are also free game for me if I need a quick cool down...

Odd thing is that sometimes I am just burning up and other times even with the same conditions not as hot - Oh Well...

The next morning, after the TT, we did our 44 mile road race with a 1.5 mile steep climb about 30 miles in. I usually would carry 3 bottles in a 40+ mile race but decided to make a go if it with 2 bottles to save weight on the climb. Luck would have it that 1 minute into the race as we crossed a RR track one of my 2 bottles launched from the holder leaving me one bottle. One can only imagine how the rest of my race went. I attempted to ration the bottle over the first 2/3rds of the race and totally blew apart 1/2 way up the hill with no recovery for the next 8 miles or so.

Keeping the entire racing weekend at "fail" was the fact that my hamstring cramped up on the 4th lap of Sunday's crit due to the total abuse I put my body through.

Actually 24oz/hour is not quite enough, you need 1L (33US oz) per hour, under normal riding conditions, with a temperature below 25℃/77℉, my personal experience is to increase that about 20% for every degree C or 11% for every degree F above that level. If there is a humidex or heat index, use that temperature in your water requirements, here today it's 28℃ and feels 39C so 39-25 is 14 degrees at 200ml per degree that would be close to 3.8L /hour or about 1 US Gallon/hour. Personally when the humidex is more then 35C I simply do not ride.

My hydration rule when riding in the warm months is one bottle per every 10-12 miles.
I fill one bottle with Gatorade and drink it while I'm riding rather than drinking before my ride.

Whew! Other than that, how was the play, Mrs. Lincoln?

+1

I tend to sweat a lot so I'll drink 2 24oz bottles in an hour when it gets really hot. Your figures sound about right for Southern Ontario. I'll drink somewhere between 24-33 oz/hr. when it's about 25C.

I can get away with water only for rides under an hour and if it's 25C or cooler otherwise I'll use Endurolytes or Nuun's. If I don't have either handy I'll use orange juice with a pinch of salt in it. I find that electrolyte replacements like Endurolytes or Nuun's to be far more effective though.

I use the same formula when kayaking as well.

Lets not do this - Wear and tear and age are enough to deal with...
You really do deserve thumbs up for knowing whats going on with your body though...
I don't follow races anymore - Are there rules about picking up water along the course?

Some races will have "feed zones" where the rider can take a bottle or food from someone handing it to them within a designated area. My race did, however I was all alone and didn't have any family or team support to rely on.

Fluid replacement will vary for each individual. Any "rules" for what one should drink are likely based on a conservative average. If I consumed only 24 oz per hour, I wouldn't make it back home on many rides during July and August. I've learned to adjust my routes during these months to ensure that I have a place to refill my two water bottles at least once, sometimes twice.

If it's in the upper 80s or worse on a ride, I will drink as I can force in. I'll end up with a belly full of water and still dehydrated by the end of the ride.

I guess not being a racer I don't quite understand... 85 minute warmup for a 22 minute race??? It seems like you overdid the warmup-- sort of like the year Armstrong warmed up outside in the heat, and Ulrich warmed up in an airconditioned room... I have done triathlons, and your hear is already screaming when you get on the bike, and the first mile or so is tough, then you get into your groove. I could see a 20-30 minute warmup, but 85 minutes seems excessive-- and you are sweating throughout, so it puts you behind unless you hydrate efficiently and effectively. At any rate, since I can't race (I am on bloodthinners and cannot fall), I admire those who can.

train safe-

In California there's the ultramarathon race of 135 miles, Badwater ultramarathon. Amanda McIntosh has a good piece about the kind of preparation it takes to run that distance through Death Valley at 120 degrees.

https://www.badwater.com/training/mcintoshlessons.html