Why do bicycles have two wheels?

Jeff's answer is correct.... But I suppose what you want to know is why the two-wheeled configuration is essentially standard. There are human-powered vehicles in many configurations, of course, but the two-wheeled "bike" is pretty much standard. One wheeled unicycles are not practical. read more

If the two paired wheels are drive wheels,, then the machine must have a differential to allow efficient cornering. Without, one wheel tends to "skip" and the axle may break. So, these machines are heavy. The three-wheeled recumbents use the paired wheels in front, to steer, and do not require a diff. read more

having longer arms a bike like this would be great to get proper drop. but carrying twice as many spares would be a pain. I had a bike with double 650c's but I like the cornering and suspension of 700c better. read more