Any bike, really, any bike, is suited for commuting.
OK. I'll make a couple of exceptions. A bike that is very highly specialized for a particular type of use, such as road racing, track racing, triathalon, downhill racing, etc. is less well-suited to commuting. That doesn't mean one can't commute on it. However, chances are that you've spent too much money on a bike like that to give it a beating on the streets of Chicago.
But let's examine some more common types of bicycles, and take look at ways they are particularly well-suited to urban travel:
I firmly believe that were it not for the invention of the hybrid bike, urban cycling in the US would not be what it is today. Thanks to their versatility, utility and ease of use, hybrid bikes made urban biking accessible and convenient for all. And, unlike bicycle shop personnel, and other nutty enthusiasts, most normal people tend to only own one bicycle. For these normal people, a hybrid is a great choice, because it adapts well to a variety of uses. Hybrids usually offer a fairly upright (or adjustable) sitting position, and come equipped with smooth street tires. They readily accept racks and fenders, and are increasingly available with commuter-specific accessories. Though many hybrids focus on comfort at a reasonable price, performance models are also available.
If ordinary bikes give you freedom to move around the city with ease and sidestep traffic woes, folding bikes crank that freedom up another notch. For the urban commuter, they offer the ultimate in versatility and flexibility, allowing you to combine biking with transit, taxi cabs, and giving you access to storage and security options unavailable to standard bikes. Because they are easy to store at home or at the office, and relatively easy to take in and out of buildings,they never need to be locked outside. They fit a wide variety of riders, and can be shared by members of the same household, or loaned to out-of-town visitors. And that's just for local commuting. For the traveler with a folding bike, the world is your oyster.
3-Speeds and Cruisers (including thrift-store finds).
Inexpensive, even cheap, tough, low maintenance no-frills transportation. Because these bikes have completely enclosed gearing and braking systems, they are highly weather-resistant. If you are not too concerned about speed, a basic bike of this sort will carry you dependably over just about any distance in the city. In style.
Unsurpassed for comfort, period. This characteristic makes recumbents fabulous for longer commutes, especially for those who use cycling paths. Additionally , recumbents allow us to fill the needs of those customers who would not
be well-served by a standard upright bike. We are approached by people
who are older, who have had to give up cycling because of back or other
injuries, or who cannot get comfortable on a standard bike for reasons
of their own.
But let me dispel the notion that recumbents are for the aged and infirm. The main reason our customers are attracted to recumbents is because they combine good ergonomics and high performance with spectacular success. They are very fast, and thus capable of keeping up with motorized traffic in many urban situations. They are also surprisingly visible, which makes them great for people who want to draw attention to themselves. For even more visibility, and weather protection, use a front fairing.
"City bikes" (including European bikes and their look-alikes)
Perfected for daily urban use by millions of European bike commuters, such bikes offer simple, weather-protected gearing and braking mechanisms and durable, often heavy construction. They are made to be ridden in ordinary street clothes, in a very upright, dignified position, allowing for good view of the road. These bikes usually include commuting essentials, such as fenders, chain guards, racks, locking devices, dynamo lights and bells.
Touring and Cyclocross bikes
These lend themselves well to longer commutes, giving the rider a more athletic and efficient riding position and multiple hand positions thanks to the drop bars. Though not as light as performance road bikes, they are still built for fast-paced riding, and sturdy enough to carry significant loads. They are durable and versatile, with wider frame clearances to accept a wide variety of urban tires, and eyelets for easy installation of fenders and racks. They typically come with stockier wheelsets that can withstand the beating they'll get on potholes.
Single-Speed Road bikes
Ideal for the minimalist urban commuter; light, fast, uncluttered; easily thrown over the shoulder for the fourth-floor walk-up. If you install a light fender set, skip a rack and carry a messenger backpack instead, you'll have the quintessential no-frills personal transportation bike. (Stay tuned: Rapid Transit will be rolling out it's own line of RT Bikes just like this, handmade by Waterford).
Trail bikes and Hardtail Mountain bikes
(Yes, this is a mountain bike. Take my word for it. It's mine.)
For many riders, there is no bike as comfortable as a mountain bike. The smaller wheels and tighter main triangle of these bikes give the rider a different type of control in stop-and-go city traffic. These make great, tough, bouncy urban vehicles once you change out the knobby tires. It you don't actually ride off-road too much, throw on some fenders and a rack. You can also lock out the front suspension if you want a little more control on a city street.
A word about quality
Within any of the above categories, there is a wide range of quality and prices. Most bikes are not priced higher simply for prestige. The differences may not be obvious to an untrained eye, and that's why as a bike shopper, you'll want to put yourself in the hands of trustworthy bike shop personnel. Generally, they should be able to explain to you what upgrades in frame materials and construction, wheel construction and components you get as you go up the price ladder, and what the benefits of those upgrades are to you as a rider. You, in turn, need to honestly assess your riding needs, and evaluate if those upgrades are simply icing, or indispensable for your demanding commute.
A word about department store bikes
Bike shop personnel are notorious for looking down their noses at mass-merchant bikes. Yet the reality is that mass merchants put bikes withing reach of people who could not otherwise afford them. As I said above, any bike can be used for commuting. But before you buy any type of bike, take the time to honestly and thoroughly evaluate your cycling needs: the frequency and distance of your commute, potential savings over other transit options, maintenance costs, accessories, etc. If you go through this process, you may discover that the initial savings on a department store bike are more than offset by higher maintenance cost over a couple of seasons of use.
Having said this, I would just like to add that the kind of bike you ride does not make you a cyclist. It's the act of riding that does.