It occurs to me that this post would have been more helpful 3 months ago, but until 6 weeks ago I couldn't have written it with authority, so I guess you can file this under "Better late than Never".
I have tried about every possible way to manage the two, and only two bottles in the cages of my Pinarello, and this method is the clear winner, so I am happy to share it now that I have something to share.
1st, buy the best possible bottles, which are hands-down the Podium Ice bottles by CamelBak. Yes, they are expensive, and they don't hold as much liquid, but they are MUCH more effective than Podium Chill, or **gulp** God forbid, uninsulated bottles. Also, turn the temp down in your fridge and freezer as low as possible. A small mini-fridge can be helpful in this regard. Remember, water ice can be any temp between 32F and -459F - absolute zero. You want that ice as cold as possible. For a special event, like a Century, you might even consider using some Dry Ice, which is frozen CO2, and is at least -110F.
It's important to have only water, or water and Emergen-C in the back bottle, so you can drink the water or pour it into the front bottle to make more Gatorade by adding Gatorade dry powder to the water. I find if I eat GU, rice, or energy bars, I need water to balance the salinity and sugar in my stomach, thus the dedicated H2O bottle. Do don't want to get yourself into a situation where you have fluids on the bike you can't use because they have too much salt in them.
Early in the season, May & June, I tried using Zefal's Magnum bottles in the rear, cock full of ice at the start, and full of very, very cold water, and that kind of works on dry days where you drain the bottle in under 2 hours, AND, move water from the back bottle to the front as quickly as possible. Like when you stop for GU every 15 minutes, drink water from this bottle too. In all cases, LARGER ice-cubes melt slower.
It turns out the bottle on the down-tube, at least on the Pinarello, is better protected from moving air that pushes heat into the skin of the bottle than the one on the seat-tube is, so my strategy, in addition to buying a Silca bottle cage for the seat-tube which has long, slotted bolt holes so it can be seated within 2mm of the bottom of the downtube, is to drain the liquid out of the back bottle ASAP as I ride, because it's the liquid which conducts the cold from the ice to the heat against the bottle's outer skin.
For really hot days, which we have every day in July, Aug, and most of Sept, use only Podium Ice bottles, and PACK them with ice. That means put cubes in the bottles and then slam them down onto a hard counter-top (not so hard as to breach the inner surface of the bottle, but hard) to shatter, crush, pack the ice into the bottle. If you have a cold spot in your fridge, store a bottle there so the water you use is as close to freezing as possible. The Zefal Magnums work great for this.
Now make a Gatorade & Emergen-C mix in a 3rd UNINSULATED Podium bottle using that water about 1/3rd full of ice. Shake the bottle vigorously and put all 3 bottles in your freezer. What you're avoiding here is putting "red hot" Gatorade from 75 degree tap water into those very cold, ice-filled Podium Ice bottles, because that will raise the temp of the ice and the bottle, and you won't have enough time while getting showered and dressed for those insulated bottles to shed that heat into the freezer. The 3rd bottle solves this problem.
When you are all ready, roll your bike out to the kitchen, lights flashing, fully clothed, helmet on, Garmin started, and squirt the slushy mix from the 3rd bottle into the two Podium Ice bottles you are taking with you. I use a 25oz uninsulated or Magnum bottle and aim to have just a bit of it left in the bottle when done. Drink 12-24oz of water (or Gatorade) just before you get on the bike, and anticipate having to pee somewhere 10-15 miles into your ride. Also, expect the Gatorade to get frozen into a slush the first 30-45 minutes as you ride. Don't worry, you won't have depleted your electrolytes this early into a ride, and it will liquefy soon enough.
As you ride, try to find the coldest water you can to replace the water in the back bottle, which should still have ice left in it 2-3 hours into a ride. Sacrifice the cold in the back bottle to chill the water in it before squirting it into the front Gatorade bottle you add powdered Gatorade to. Do NOT take the lid off of the back bottle to squirt water into the front bottle. Having both bottles open at the same time wastes cold and usually results in spilling water & ice that you can't replace. You, of course, will have to take the lid off the back bottle while refilling it with cold water, but only then should you risk a spill.
If you follow this procedure, you can probably still have cold Gatorade 4-5 hours into a ride, even when starting out in 70F temps and finishing up in the low 100Fs. I find the water cooler at the Apple Valley airport very helpful in implementing this strategy, but ice and chilled water from a convenience store can also be fantastic. Unfortunately, the one here in the right location to help doesn't allow bikes inside, so I'm SOL there.
In short, this strategy has two parts:
- Make everything colder than everything else at home
- Sacrifice the cold in the back water bottle to keep the front Gatorade bottle cold
NOTE: Do NOT, NOT, NOT freeze water in Podium Ice bottles. They absolutely WILL rupture after a few freeze cycles and you're SOL with your $25 bottle.
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