How can vehicles wheels rotate that freely eventhough there is a massive weight resting on them?

Question

Edit: thank you so much for the clear explanation! It makes so much more sense now!

In: Physics

Answer 1

This is really simplified and probably not as accurate as another explanation of the same concept, but here goes.

First, imagine a car with wheels that were cubes or rectangular prisms. Assuming you could get enough people to push, it would eventually either 1) just slide along, or 2) start the cubes tipping up and forward; you’d need to push forward and lift a bit to make this happen (or if the corner digs in have the “wheel” do that for you), but eventually the car would reach a point it would balance on the edge of the cubes; push a bit harder and gravity takes over and you come thudding down to the ground.

Now let’s look at two other “wheel” shapes: the equilateral triangular prism and the octagonal prism (these may be inaccurate terms, Geometry was a lifetime ago: the shapes I am referring to are what you would get if you took a 2D equilateral triangle and regular octagon and extruded them out such that it made a cylinder-like shape but instead of two circular faces you get two (triangular/octagonal) faces and (three/eight) rectangular faces)

With the triangle, you’d have all the problems of the cube but more pronounced; you’d have to lift more and longer to get to the balancing point and it would thud down harder.

Conversely, the octagon would have a reduced need to lift and shorter to go when force was applied.

This trend should continue as you add sides to your n-gonal prism wheels, until you reach infinity where you just need a push forward since on a rounded surface placed on a plane, you are (mathematically) always balanced on what is effectively a tangent line to the circle.

Now, in reality, our tires aren’t perfectly round/ they deform and are generally imperfect. But the principle is basically the same: you don’t have to push too far to find the point where the shape of the wheel gets “unbalanced” and wants to fall forward. It’s a really short fall to the next “side” but since that side doesn’t have as much resistance as a cube or octagon, it won’t just stop there, it likes to keep rolling until friction slows it back down.

Again: this is incredibly simplified.