[devnullbrain]

>You see, the shapes of roads in real life come from an underlying essential fact: the wheel axles of a vehicle. No matter how you drive a car, the distance between the left and right wheels remains constant. You can notice this in tyre tracks in snow or sand. Two perfectly parallel paths, always the same distance apart maintaining a consistent curved shape.

Emphasis mine - that's not really true

- Cars have differentials, so the wheel speed can differ between wheels

- Steering geometry isn't parallel! The tyres turn a different amount when you turn the wheel

- Unless you're going in a straight line, cars don't go where the tyres point! Tyres 'pull' the car towards their slip angle

What you will actually see in tracks in snow or sand is non-parallel tracks, describing curves of different radii. You can also see this in racetracks, where the path more closely resembles the limits of car physics without care for passenger comfort or tyre wear. The example 'fail' image looks not dissimilar from a hairpin turn.

[Ef996]

Author here: i agree the article oversimplified here and in practice vehicle tracks can sometimes be non concentric. But it has nothing to do with differentials. Differentials just allow wheels to rotate at different speeds (as perimeter of a smaller radius circle is smaller the outer one). Non parallelism usually comes from the Ackerman steering or slipping. But rear tracks of a vehicle going slowly through sand will always be concentric. Parallelism is important for engineers because it also allows multiple vehicles to go in parallel on a multi lane road. You can see how that is failing with bezier paths in the CS2 ss in the article. Also hairpin turns are usually designed with arcs in mind as well

[JackFr]

Railroad cars don't have differentials and travel on parallel tracks. How?

https://www.reddit.com/r/Skookum/comments/47sbri/richard_fey...

[pixl97]

For those that don't read the link, by having conical wheels.

[JackFr]

It’s much cooler when Richard Feynman explains it.

[iggldiggl]

See https://www.thecontactpatch.com/ for some interesting reading material related to that.