Even on a short, half-hour ride, it’s important to stay hydrated when you’re mountain biking. How much you should drink varies on several factors, including the duration of your ride, the intensity, the weather, how much you hydrated before the ride -- you do drink up beforehand, don’t you? -- your fitness level, your gender and size. In general, though, you should drink a full water bottle (about 20 ounces) during a one-hour ride. So the question isn’t whether to drink, but how: water bottles or hydration pack?
Ride Duration and Trail Conditions
If you’re riding for an hour or less, a lone water bottle should suffice. As for conditions, remember that water bottles attach low on your bike — on the downtube and/or the seat tube — meaning they’re close to the earthy action. If the trail is dry, expect that dust to kick up and stick to a sweaty water bottle. Thus, expect that first sip to result in a spit. A sweaty, dirty, slippery water bottle can also be a challenge to grab and hang on to. If you like to ride especially rough trails, you’ll need a more sophisticated cage, one built to better grip your water bottle and keep it from bouncing out. For longer rides, the greater capacity of a hydration pack, or bladder, is a big plus.
No Room on the Frame
You may not even have the option of mounting a conventional water bottle cage. If you’re riding a full suspension bike, you’ve lost the seat tube option: That space is taken up by your rear suspension. On some newer frames — again, especially with full-suspension bikes — the top tube and down tube are so close there's no room for a water bottle. That leaves less desirable options, such as a cage that attaches behind your saddle, which may work on a smooth triathlon bike but creates some awkward fumbling behind you on a rough mountain bike ride.
Maybe on a road bike you feel safe letting go of the handlebar with one hand and reaching down for your water bottle, but not on a jolting mountain bike trail. Hazards come out of nowhere: If you’re reaching for your water with your right hand and a sudden obstacle appears, you have only your left-hand-grabbing front brakes to rely on, and most of us know what happens when we quickly apply only the front brakes — automatic ejection! Steering on rough terrain is also a challenge when one hand is occupied with a water bottle.
Advantage, Water Bottle
On a hot day, you can fill a water bottle half to three-quarters full, freeze it, top it off before heading out and have cold-to-cool water for most of your ride. That may sound like a luxury, but your incentive to drink is greater if the water is cold rather than lukewarm. Also, if you have two water bottle cages, one can hold a bottle for water, the other a bottle dedicated to a sports drink. That's a popular option, since once you’ve put sports drink in a water bottle, the taste tends to linger. And despite efforts to redesign hydration packs with suspension systems that elevate the pack off your back, they still tend to make your back clammy on a warm day.
Advantage, Hydration Pack
Hydration packs can carry 100 ounces or more (with two water bottles, you’re looking at less than half that), enough to keep you hydrated on your longer rides. While hydration packs vary in sophistication, most also provide a convenient place to store your spare tubes, tools, food, car keys, cell phone and a layer. Some riders aren’t crazy about the extra weight at first, but once you get the fit right, you barely notice it’s there.
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