Why one bike? Well, aside from my general tendency to avoid accumulating stuff, I believe it's not about the bike — it's about riding the bike. If you have zero bikes, you probably won't find yourself riding very often. If you have more than one, you'll have to spend more time fussing with your bikes than if you had just one. So, optimally, if you want to spend your time riding you'll have one bike.
If you're going to have one bike you'll have to think carefully about what sort of bike it should be. I think there are a few things that hold true for any cyclist that doesn't race for money (especially those living in Seattle):
- You almost certainly want a road bike of some kind. Most practical riding is done on the road. So you want a bike that rides well on roads. Despite the efforts of the American bike industry to equate road bikes with cheap imitations of racing bikes, these are not one and the same — road bikes take many forms. A few things are constants. One is tires. They should be smooth. Tread hurts your traction on pavement, especially in wet conditions. There is a special place in hell reserved for salesmen that try to sell bikes with knobby tires to urban commuters. Most people that will regularly ride more than a few miles at a time will want handlebars that allow a variety of riding positions. In Seattle we have climbs, descents, and flats; some trips are long and others are short. Flat handlebars are popular with outright beginners, but they don't give you enough choices to tackle long rides or varied terrain.
- Fenders and a rack. This is Seattle; you need fenders on your bike at least nine months out of the year. Take them off in the summer if you really want to, but remember, you're not racing for money, so don't get too bent out of shape about weight or aerodynamics. Fenders make it hard to fit a bike into a car, but why would you want to drive your bike somewhere when you could ride it? Racks are pretty nice. They allow you to carry all kinds of useful things for commutes and utility rides (clothes, computers, tools, locks, etc.) without awkward backpacks. And they let you do all-day and multi-day rides without vehicle support — why make someone support your ride in a vehicle when that person could have more fun riding along with you? Even if fenders or a rack don't have a permanent place on your bike, your bike should be able to support them. This has consequences for your fork and brakes.
- Carbon fiber is a stupid material choice for any part of the bike that supports your weight. Quality carbon fiber isn't, to the best of my understanding, more likely to fail than other frame materials, if treated with due care. However, due care is hard to exercise; it's really hard to tell when carbon is damaged. Supposedly a few experts can determine this, but the cost of enlisting their help is significant. Many cyclists recount carbon components failing without warning; others will say they probably should have known their parts were damaged, but let's be honest: you are probably not expert enough to know this. Finally, the failure mode of carbon fiber is shattering. This typically leaves the part unsalvageable and the rider in serious trouble. Aluminum seems a little better, but has its own disadvantages. Rather than shattering, it shears, which is less than ideal. As it's employed in typical bike frames, it's considered uncomfortably stiff, leading to aluminum frames paired with carbon forks (or seat posts, which ended badly for me). And, so I've heard, it gets stiffer and more brittle as it ages. Titanium is ludicrously expensive (though supposedly great in every way). Which leaves us mere mortals with steel. In 2012 steel is still the best material for a practical bike. Yeah, steel is heavy. If you ride carbon your bike is maybe a little less than 10% of your total weight, and if you ride a typical cheap steel frame it's maybe a little more than 10%. You're not racing for money. It just doesn't matter.
There are other things that can vary considerably.
- Frame geometry. There are many right answers for frame geometry. In Seattle's hills and traffic, recumbent is probably not one (unless it's dramatically more comfortable for you). Mountain and cyclocross frames have road handling characteristics compromised by off-road capabilities (for one thing, high bottom brackets raise your center of gravity relative to your wheels); how much you care is up to you (my current bike is a cross bike; I care a little bit about this, but... more on this later...). Cargo and “Dutch” bikes are great for mostly short-haul riding, or for carrying cargo! They tend to have sweeping handlebars that have a better default position than flat bars for most road riding, and even allow a more forward position when you want it (I rode an old Schwinn cruiser in college, loved it).
- Freewheel or fixie. If you can ride fixie in Seattle, more power to ya. I'm not sure I could handle steep Seattle descents on a fixie. If you ride fixie, at least have a front brake there just in case (this PSA brought to you by your mom, unless your mom is a crazy trackie/messenger).
- Brakes. Whatever stops ya, as long as it doesn't get in the way of your fenders. Compared to rim brakes, trendy disc brakes are still mounted such that super-hard braking can pull the wheel out of the front fork, but I'm not sure that overwhelms their advantages: easy power and great rain performance. They require less maintenance but the maintenance is more difficult. They can still work if your rims are damaged, and don't put so much stress on the rims. This, in turn, frees you to get all kinds of crazy, lightweight, structurally dubious rims, but you're not racing for money, so you don't care. Weight and aerodynamics of brakes are negligible for all but the most serious of racers — even more so than frames. All brakes yet known can have overheating problems on mountain descents; the only solution is to use them less. Coaster brakes are pretty lousy in every way, but if you're not going fast or down mountains you might be able to get away with them.
- Gearing and drivetrain. There's almost no wrong answer if you're comfortable with what you have. After initial skepticism I have come to like my cross-style gearing on Seattle's hills, though I'd certainly prefer a bigger little ring. More traditional road-style gearing works fine, too, though I'm pretty sure I wear my cogs and rings more evenly with the cross-style setup. If you hate maintenance and care even less about performance than I do try a full chain case and hub gearing — word is that there are some great hub gearing systems out there. Some riders with hub gears or single-speed bikes are going to belt drive. I have no opinion on this.
- Pedals. Whatever. You. Like.
- Age and price. Spending money does not make you cool.
So... you have to make some compromises to find a bike that's adequate for all your needs. I have a long commute, have to carry a lot of stuff, and like to go on long rides for the fun and challenge of it (challenge, to me, involves pushing my body pretty hard), so my ideal single bike is probably something like a touring bike. Unfortunately mainstream American bike manufacturers don't promote the stodgy, long-lasting touring bike the way they promote racing bikes (whether on-road or off-) and beach cruisers, so they're not always easy to find. And this led me to my biggest problem with my one-bike strategy. When my one bike went down in 2011 I had no spare to fall back on. To get back riding I needed a new bike quickly, and ended up (somewhat unwittingly) with something closer to a cross bike than the touring machine I really wanted.
There are good reasons to have more than one bike. Having a hot spare might be a good one, especially if you want to avoid having to buy a bike in a hurry in the event of a catastrophic failure. If you do two different sorts of riding that can't reasonably be served by one bike, that's another. If you really like to go off-road and get dirty, for example, but also have a commute too long to use an MTB, you probably want two bikes; or if you do lots of long rides where you want pretty aggressive geometry and clip-in pedals, but also quick errands where you want slab pedals (I have clip-in pedals and just push 'em with my tennis shoes for this sort of ride, but I mostly walk over those distances; if making trips like this on bike was important to me I might want a Dutch bike for them). Maybe if you regularly have to park in a high-theft area like UW it's a good idea to have a bike you don't care about. On the other hand, saving your best bike for dry weather, in Seattle, is utter nonsense.