There are a lot of advantages to an “adult tricycle” over a bicycle. For one thing, a tricycle often has a basked at the rear big enough to hold an entire shopping trip’s worth of groceries. For another, a tricycle can be fitted with a weather shell. (See the Swiss “Twike” – http://en.wikipedia.org/wiki/Twike – which is actually electric-assisted.) But, at least where I’ve lived, one never sees an adult trike ridden by anyone but senior citizens. Why is the tricycle not more popular? And why does it seem to be slower than a bicycle? Both are driven by muscle power.
A bicycle only has two wheels, so there’s less friction. It’s also more maneuverable and easier to park.
Oh – bicycles are lighter as well.
Speed, size, weight (important when you get to hills…). Also, there’s a good chance that when the mechanical bits were being figured out, all they were making were bicycles rather than trikes with the same mechanisms, for the above reasons (a trike from 1940 with a bicycle mechanism would have weighed a ton, and nobody would have wanted to pedal one around town), so there’s a huge amount of inertia into producing bikes rather than trikes.
Also, if I have to dodge cars in traffic, I want a bike, for the maneuverability’s sake.
Plus the fact that tricycles make you look like a dork.
I think they used to be more popular inthe early days, but never as popular as bikes. Much heavier and much harder and slower to turn.
What if you wear Bermuda shorts, black socks, sandals, a plaid short-sleeve shirt, and a visor or pith helmet?
It’s extremely difficult to whizz on a tricycle like you can on a mountain bike with more gearings than metal components.
Plus, tricycles are more dangerous than bikes on car-filled roads. A bike is about eye-level with most (non-semi) traffic, whereas a trike is pretty much level with the ground. People put flags on, but flags are easier to miss than someone half-standing on something that would fold up like origami if your Volvo so much as bumped it.
Advantages of a bicycle over a trike?
[ul]
[li]Weight, trikes are heavy. This is important when you are supplying the power.[/li][li]Less wind resistance. Try pedaling into a headwind with a trike[/li][li]Cornering / Stability Trikes are scary to corner on at all but the slowest speeds. If you doubt this ask yourself why Honda doesn’t sell 3 wheeled ATVs anymore. 
[/li][li]Storage. You can store a bike in aobut a 20" wide space. Not to mention you can carry a bike up a set of stairs[/li][li]Portability, mounting a bike on top of a car is fairly easy. Try lifting a trike up there.[/li][/ul]
I’m guessing most trikes are too wide to slip down the side of roads the way bikes do. The Swiss Trikes drive along with the cars, and that’s only really possible because of the highly efficient design. I was surprised to see a picture of one in the link - I’ve never thought of them as an alternative to bikes.
Watch people cycling sometime - the handlebars hardly move at all, unless they are moving really slowly. Most of the cornering is done by leaning.
Full-time pedalist here (no car). I see some trikes, and newer models aren’t so wide as those of even just a few years ago. Still, they’re wider than bikes, which is a big issue if you’re riding on streets. Also, as others have said, heavier and slower. If it’s a question of which to get, go with a bike. (Which, I think answers your question of why trikes are so much less popular. People energetic enough to pedal usually go with bikes because they’re better.) If it’s get a trike or nothing, go for it. But get a narrow one.
BTW, there’s a straightforward solution to the cargo problem on a bike. Get a trailer. I prefer the Burley cargo trailer, but the Bob also works.
Visor = dork
Pith helmet = cool
Narrower also means even less stable.
True. And haven’t ridden one, so can’t speak from personal experience. But, as someone said, that’s mostly an issue for cornering. The simple solution is to slow down.
You can’t lean the trike to pump up hills, either.
I’ve owned several recumbent trikes, and currently own a Catrike Pocket. These address many of the above-mentioned shortcomings. Specifically, the low center of gravity gives it very good stability, even with a very narrow width. This trike is only a couple inches wider than a typical MTB.
But I have to admit it’s still slower than a bike with similar riding position. Maybe 10% slower. I’d say it’s because the two front wheels are never perfectly parallel. There’s always some alignment error, and further alignment shift as the frame deforms under stress.
Actually trikes tend to do better on steep hills, because you don’t “stall” (lose balance because speed is too low). And leaning the bike while “pumping” dosn’t do much anyway.
Yeah, and ain’t THAT embarrasing when you can’t get your new cleats out of the pedals? :D.
At faster speeds, trikes tend to be more unstable, hence trikes built for speed end up being built more like carriages.
As for stability, I’ve seen a lot of 3-wheeled scooters (mostly used by delivery companies and resturants) that seem to use a very flexible rear suspension. The driver can lean into the turns like on a 2-wheeler, but both rear wheels stay on the ground.
Here’s one from Honda’s Japan website. They don’t appear to be available in the states.
http://www.honda.co.jp/motor-lineup/gyro-canopy/index.html
This kind of suspension may well be too heavy (or too fragile if lightened) to use on a human-powered vehicle.
There are a few tricycles (human-powered ones) with a leaning mechanism. Avantec (Japanese only site) sells upright trikes with a leaning mechanism. I tried it and it was interesting - you have to force it to lean, but it does give you better maneuverability. The Stites Cameleon has a different leaning mechanism, and I’m told it handles very well. The Tripendo is another example but this one is very wide, which IMO negates the advantage of a leaning trike (i.e. that you can make it narrow and still have good cornering ability).
But all these are very complex and heavy machines. For most uses and most people, a 2-wheel bike is more efficient and better handling. I mostly use the trike in wet weather so I don’t have to worry about hitting a slippery spot and going down. Any other day I prefer the 2-wheelers.
I agree this is a definite advantage. The one you linked to is more of an electric car with pedal assist, but there are many fully enclosed human-powered vehicles out there, commonly referred to as velomobiles. You can get more info on the Velomobile USA site. It’s also possible to use a simpler fairing - less coverage, but less weight and much better ventilation. My trike is equipped with a front fairing which keeps my lower body dry in rain, and minimizes the cold wind on my whole body. I could put something like that on a bike too, but it’s easier to deal with on a trike.
