Apologies if this is a silly question and everyone already knows how wheels work. But a few days ago I realised I was missing a key piece of the invention, and that it's not quite as simple as I thought.
So you have your wheel, and it rotates, which makes it easier to move in one direction. Great—this is something easy to notice. The first caveman who pushed a rock or rolled a tree-trunk probably noticed this, and it couldn't have been much longer from there to inventing the roller sledges that were used to transport the rocks for stonehenge. But with a sledge you have to keep moving the logs falling off the back to the front again. How do you go about attaching your rollers to a stable platform so that you don't have to do this?
The key invention is the axle and the bearing. A loose fitting rod through the centre of the wheel, that allows the wheel to turn around a fixed shaft. You also need some kind of a collar to keep the bearing in place.
The point where the wheel meets the axle is called the bearing, in this case it's a plain bearing—no additional mechanics involved. You can either have the wheel loose around the axle, so that as the wheel is turning the axle is stationary, or you can have the wheel tight to the axle so that they turn together, and you can fit another wheel on the other end of the axle. In this case you would need a different bearing to attach the axle to the actual vehicle. You might want a stationary axle to make the vehicle easier to turn, or a rotating axle to constrain two wheels to turn together and make it easier to move the vehicle in a straight line.
Once you've got that, the how of the wheel is done. But I still had another question, which was a why. Why is the wheel more mechanically efficient? The friction against the ground is greatly reduced, because the wheels now roll rather than sliding the whole vehicle, but what about the friction within the bearing itself? Where the body of the vehicle presses down on the axle, or where the axle pressed down on the wheel, won't it get stuck there?
The answer is yes, there is some sticking there, and that interface still has sliding friction where the axle rubs against the wheel. But the sliding friction here is between two much smoother surfaces that the sliding friction would be against the ground - smoothed wood on smoothed wood, to take my primitive example, is much easier than wood against earth. In theory, you can vary the contact area and size of the bearing a lot, since a larger contact area will give a reduced pressure, which should balance out.
As the tech advances better and better bearings are possible. Smooth metal has less friction than wood, machined-smooth metal even less, and then you get things like ball bearings that turn much of that sliding friction into more rolling friction. Lubricant in the joints can also reduce sliding friction. But you don't need any of that for the fundamental idea!
Today, I learned.