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Bike touring is when you ride with all of your living requirements packed on your bike in panniers -- small bags that attach to your bike. Because you will be traveling long distances, every pound you can shave off of your bike will make a difference on your legs and lungs. The bare essentials that you should not skip on include:  Water bottles and purification tablets. Hand pump. Extra tubes and a tube patch kit. Extra brake and derailleur cable. Bike multitool. Headlamp. First-Aid kit, especially if spending time away from cities/towns. You should always use rear panniers first, but if adding a set of front panniers will reduce stress on vital bike parts and potentially help you climb. While it will not actually lighten your bike, it will keep your back tire, frame, bike rack, and rear brakes happy and help stabilize the bike. Bring an all-purpose camping cleaner instead of separate dish soap, shampoo, body soap, and detergent. Bring small items you like to do to use during your downtime, such as books to read. Just be sure not to overpack them or else it will add unnecessary weight.  While a full sized bike pump may be easier to inflate tires with, a hand-pump is much lighter. Instead of bringing dishes, pack a metal mess kit. You can cook with it and save leftovers inside. Bike touring is not a glamorous affair, and you shouldn't expect to dress like it. You should have 2-3 pairs of bike shorts and jerseys, a lightweight rain jacket and/or pants, depending on climate, and a set of clean clothes to change into after riding. If it will be a cold night then a pair of pants, a hat, and gloves are all light and easy additions.  Opt for wool clothing since it doesn’t retain as much much water as cotton. Keep one pannier for dirty clothes and one for clean clothes, and re-wear dirty stuff as often as you can before switching to something clean. Unlike back country hikers, clever tour bikers don't have to carry all of their necessary food supplies on their back each day. If you plan your tour to run through at least one town every 1-2 days, you can cut down a ton of weight by purchasing your meals late in the day. If you stop near or in a town each day, buy that night's dinner and the next day's breakfast and check a map. If you'll be in a town sometime early the next day, buy lunch later and eat it on the side of the road, then repeat the next day.  Make sure you always have 3-4 high-carbohydrate riding snacks (granola/protein bars, fruit, dried cereal, etc.) on hand. When biking long distances you need to eat something every 30-60 minutes. Always have one lightweight "emergency meal" available in your bag. Granola, dried rice or pasta, beans, etc. in case you cannot find a place to stop. You can also try to forage for food whenever you stop. The same lessons learned by backpackers, who carry everything they need on their backs, can help you keep weight off your wheels. Lightweight tents, sleeping bags, and sleeping pads may not be quite as essential for a biker, but they will still make a big difference. Every camping brand offers light-weight variations on their equipment, but here are some hints for making do with the best you've got.  Instead of bringing a tent, just bring a tarp and a collapsible pole that you can fashion into a shelter. Foam pads, including foam mattress toppers for twin beds in a pinch, are incredibly light, though they may be bulky. Use the lightest-weight sleeping bag you can get. If you're touring the California coast in the summer, for example, your 0-degree bag is likely too heavy for your needs. If you’re serious about really making your bike as light and efficient as possible, all the little safety/convenience gear can be stripped. Don’t bring a cycle computer or your cell phone if they add too much weight.  Be smart when packing -- will you really read 3 books on the trip? Do you need the pair of nice, heavy jeans, "just in case?" Touring is about living simply and enjoying the scenery as you ride, not packing your entire life on a bike. Never remove reflectors from your bike since they help other drivers see you.
Understand that you should carry as much as you can safely carry on long tours. Consider getting front panniers to distribute your weight evenly. Be smart about your accessories. Get used to dirty clothes. Whenever possible, buy food day-by-day. Invest in lightweight camping equipment. Get rid of the minor stuff.