A hydration pack is a small backpack that contains a plastic bladder for water, with a tube and mouthpiece to get the water to your mouth.
Hydration packs have largely replaced water bottles in mountain biking because they enable riders to carry more water (typically 2-3 litres rather than the 1.5l of two large bottles), and they don't jump off during high-speed descents.
The first hydration pack - the original Camelbak - appeared in the early 1990s. That system was very simple: a pair of shoulder straps, a two-litre bladder, a bite valve and a bit of insulation to stop the water getting warm too quickly.
Other manufacturers quickly realised the potential of the idea and started making sleeves that added pockets to the basic Camelbak. Camelbak responded with its own packs that incorporated padded shoulder straps and cargo space.
Since then, the number of manufacturers of hydration packs has exploded, and Camelbak's own range has diversified hugely. There are minimal packs for mountain bike racing, strapless insulated bladders that fit into regular backpacks, day packs with enough room for food, clothes and spares and scaled down packs for kids.
Things to look for
Hydration pack fluid capacity varies from about a litre to three litres. How much do you need? That depends on how long you ride, the temperature, and the availability of refills. A common recommendation is for an 80kg rider to drink 600ml/hour in cool conditions, rising to 900ml/hour for racing in hot conditions. Racers still tend to use bottles, and those that go for hydration packs - in long-lap endurance races, for example - use small ones and swap them every lap. For anything but racing, we recommend a three-litre pack. You can always half-fill it in the winter.
A hydration pack backpack is a handy place for spare tubes, food, tools, clothing, a camera and all the other toys you can't live without. Too much weight on your back gets uncomfortable, though, no matter how well-padded and shaped the pack is. In some circumstances, you can live with that: riders in two-day mountain bike orienteering events almost always use backpacks even though they're carrying tents and sleeping bags. For less severe circumstances, think comfort and move some of the weight to your bike if you can.
Straps and harness
Almost all hydration packs are built along the lines of a modern rucksack, with padded shoulder straps, a hip belt and a sternum strap to pull the shoulder straps together. Where the pack sits on your back, look out for channels to get a bit of airflow in there and help you stay cool, or even a suspended mesh back.
You'll be using this part of the pack every time you take a drink, so have a close look at it. Is it made of soft materials that are comfortable to bite? Does the water flood out when it's opened or does it trickle? Does the mounting put it close to your mouth? Can you lock it closed so it doesn't leak into your bike gear when it's in a holdall in the car?
Adjustability and fit
The straps and hip belt should be adjustable to keep the pack snugly on your back and under control. For most riders this isn't an issue, but very tall or short riders should check the range of adjustability suits them.
A few hydration pack manufacturers make their packs so that most of the load is taken by the hip belt rather than the shoulder straps.
This works well for larger packs, long rides and tall riders whose centres of gravity are high enough already. In the last few years hydration pack makers have begun to tailor their packs for women. Happily, these aren't just men's regular packs in pink Cordura. A women's drinking pack will be shorter and have narrower shoulder straps that are shaped to go round the bust. The sternum strap will be higher, and the hip belt will be shaped for the flare of a women's waist and hips.
Built-in body armour
It didn't take long for riders to notice that a couple of litres of water is quite useful spinal padding if you crash. Some manufacturers incorporate body armour-style padding and plastic plates into their packs to enhance this protection.
Hydration pack bladders use a variety of closures to allow you to fill them. Screw-caps are the most common, and usually have a tether to keep the cap attached to the bladder. Take care not to cross the threads or damage the o-ring. Roll-top closures use Velcro or an external clamp to keep them closed. Push-fit caps are less common than they once were, presumably because it's possible for them to pop open in a crash.
Care and feeding
Cleanliness is essential to hydration pack bladders. They should be emptied immediately after use and hung out to dry. Several companies make drying inserts to keep the internal surfaces apart, and many riders modify a coat-hanger for the job. A regular rinse with baby-bottle sterilising fluid helps prevent mould growth in the tube.
If you have a big fridge, sorting the bladder there also helps keep it free of nasties. Our advice is to only use hydration packs to carry water. Cleaning out carbohydrate sports drinks thoroughly is almost impossible. One day you will forget and return a week later to a mould culture that's preparing to take over your whole backpack.
Hydration packs vary in how you get at the bladder itself. Some put the bladder inside the main pack, which saves weight, while others give it its own compartment so you can fill it without having to unpack everything else.
Hydration pack makers add all sorts of features to their packs: special pockets with soft liners for sunglasses; hooks for keys; whistles; elastic hoops for carrying soft gear; helmet cradles for when you're off the bike; expandable compartments; iPod pockets and cable channels; and many others. It's worth reading the detailed specs to find out what's on offer.