What types of wheels and axles on a bike?

If we look at a regular bike, it has two wheels, and each has an axle. The bottom bracket is also a wheel and axle assembly. The crank arms and/or chain ring are the wheel, and they're mounted to an axle. Lastly, the steering tube (some call them "forks") qualifies as a wheel and axle as it rotates in the headset. Just because it doesn't turn all the way around doesn't mean that the principle doesn't apply. And consider the freestyle bikes that have a steering tube that does spin in the headset. In parting and on a related note, we should consider the full suspension bikes. That rear suspension assembly is actually a lever, as it has a pivot point on the frame with the shock as the fulcrum. There is a whole world of very basic and interesting physics in the bicycle. And it can be as simple or as complex as your analysis will permit it. When you get to the physics of the spinning wheel, the gyroscope, it can really be fun.