So you want to commute by bike? There's all sorts of ways to do it. Some use a rack and panniers, some a messenger bag, some a backpack from kmart. Some use a 3-speed, some a roadbike, some a fixed gear or mountainbike with street tires.
There is no easy answer one way or the other as your situation may vary, but here is the way I do it:
Bike: I take a road bike converted to fixed gear, or a maintainbike. Sometimes an older roadbike. But mostly the fixed gear. The fixed gear is an old 12-speed running a 43/17 gear, and one front brake. When it rains I equip the bike with a clip on rear fender.
Commute: My commute is about 12 to 14 miles round trip at the most, although I often stretch it on the way home to get in more riding. My evening job, when I ride to it, is 9 miles each way.
Gear: A rack would be something else on my bike and I'm a minimalist about that sort of thing. A conventional backpack would leak in the rain. I find a messenger bag of the large to medium size is best for commuting by bike (even on longer rides, such as 20 or 40 miles)
For daily use I use either my RELoad small Civilian or my Chrome Ransacker, both of which are large enough old-style messenger bags and substantially water proof, in that I've never had the contents of either get wet when riding in steady rain. For everyday riding and errand running I use a smaller messenger bag such as a medium Timbuk2 or small Chrome.
Inside the bag:
You are going to want to take water. You can have a bottle cage on your bike but I prefer to keep the bottle in my bag. SIGG makes excellent water bottles and they are expensive but last and keep your drink cool. They also won't leak in your bag which is a plus! Or, you could use a conventional plastic water bottle of the type you'd put in your bottle cage.
Locks: If you park your bike indoors you may not need any lock, or only see fit to take a small lock for if you stop either coming or going at a store, to grab a cup of coffee, etc. A small cable or chain with a decent padlock should do. If you are locking your bike outside I advise at least two types of locks and both should be somewhat sturdy. This may make them heavy. Either leave one at your destination if possible so you don;t have to haul both, or stop whining because the extra weight makes you stronger. I use two locks, a u-lock and either a chain or cable. I lock the frane and the wheels. The u-lock is a Kryptonite. I've used other brands and never have problems with them but I prefer the Kryptonite. Also when I lock the bike, even though it is under an overhang, I cover the seat and bars to protect them from blowing rain, grime, etc. A plastic bag from the supermarket will do for either.
Tools: I cheat on tools. Inside my messenger bag I have a pouch. (I actually have two pouches, one for the mountainbike and one for the fixed gear since the tire sized defer!) The fixed gear one has a small tire pump, tire levers, at least one road tube and a 15mm wrench for the rear wheel in addition to a mini-tool with allen keys. The one for the mountainbike has a pump, a 26" innertube, and a mini-tool with allen keys. When I use one bike I stick in one pouch, when I use the other I stick in the other. No fumbling with individual items. I keep my chain in the pouch too along with the plastic bags, so if I want to go for a ride after work and don't want to haul the extra lock with me I take it out and begone.
Clothes: A regular haberdashery, my bag is. My work does not allow us to wear shorts so I wear pants made of a lightweight material and roll them up to the knee like shorts. When it's rainy or looks like it might rain I wear cargo pants that look like everyday pants but are water resistant. They get damp after a long ride but won't soak through and they dry real fast. Or I use my regular pants and trust that they dry quick... hopefully! I take a light jacket, depending on the temperature either my Louis Garneau windbreaker or my light jacket from Cannondale. The cannondale is slgihtly heavier for cooler days but the arms unzip to make it a vest if it warms up on the ride home; both repel water to some degree. For the real warm weather I take my Mountain Hardwear rain jacket, it's a pullover with a half zipper but its the lightest jacket on the market they say and rolls up to smaller than a short sleeve cycling jersey. You could almost stick it in the watch pocket of an old-time suit vest. The Garneau and Cannondale are red; the Mountain Hardwear is orange. I find that reddish colors are most visible to me on the road; I see bikers wearing them quite easily. While mayhap not everyone will be able to do so I feel it makes it safer than wearing black. Which is why on days that it is really warm and don't wear a jacket, I wear a orange, yellow or red cyclign jersey -- some bright color -- and change my shirt at work, carrying the other shirt in my bag.
Winter jacket: In winter I typically take a jacket that's warmer, or wear a longs sleeve jersey or soft shell underneath one of the lighter jackets. Either will keep you warm, and the important thing is not to sweat too much. You have a backpack, so you can always take somethign off and stow it. fold and roll your jersey or jacket -- it takes up less room that way. For winter riding I either use the shop jacket with the "fat men" logo or my Pearl Izumi. But I recently scored a Blackbottoms Park City jacket in safety yellow, it's obnoxiously bright AND has a plethora of pockets, although oddly enough no external back pocket. Over a winter jersey this thing is warm enough and bright.
Helmet: wear a helmet. You only got one head. I use Bell cause they are inexpensive, comfy, and my local shop sells em. For road helmets they can't be beat. In winter or in coooler weather I use a hard-shell bmx type helmet, either a Bell or Protec.
Gloves: Ditto on gloves. You don't want torn up hands if you fall or have to fend off a car hood. I prefer the ones with the knitted back, you can still find those. I have ablack pair made by Push and a tan one made by Airius.
For winter gloves it varies. Decent winter gloves with cheap knit inner gloves, keep you warn enough on long long rides. For around town, commuting, etc., I find regular leather gloves -- provided they have insulation -- work fine. The leather breaks the wind, protects you if you fall, and they are usually cheaper than cycling specific gloves.
Music: People rip on riders with headphones. Yeh, well I don't see anyone complainign about soundproofed cabins in cars or drivers listening to the radio? In a group ride headphones can be dangerous buit a competant rider cna ride and listen to music in traffic -- in fact, he can still tell by sound when a car is about to overtake him. I carry an MP3 player after finally setting aside my walkman tape player after many years. I keep it loud enough so I can hear it, but not too loud to hear the street, and turn it down or off on particularly dangerous stretches if I feel it might give me an edge to do so. Even if you don't ride with music on, you might want somethign to listen to at work. No harm there. Take your 'phones.
Take lights with you. One on the back of your bike one in front. I use a minimum of two in back on my commuter, and usually an additional one on my backpack or bag. I run a minimum of two front lights as well. I keep em off the bike less I'm using them to discourage people from swiping them as they clip on and off, but I also hang a light on my messenger bag. Even if your commute doesn't take place at night, take lights. You never know. You may take a detour or end up talking to a friend and find it's getting dark. Don't take chances... take lights.
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