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Comment by crazygringo | Hacker News Reader

Can you explain or provide a link? Thinking about it all seems pretty straightforward to me so I'm curious what I'm missing, what's the complicated part.

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Rough approximation is reasonably straightforward, but it quickly gets complicated if you want more precision. At a high level cars want to roll because momentum is resisting a force being applied by the road. But, dig deeper and things get even more complicated.

An uneven amount of weight is distributed to each tire through a turn. Tire temperature, tread, pressure, etc impacts the surface in contact with the road. Even slicks don’t have a uniform pressure across the surface in contact with the road. etc etc.

These conversations often remind me of the power of the human mind to intuit but not understand something conceptually.

Someone like myself loves knowing the details of how stuff works, and even with a head full of fundamental knowledge, someone who just does it and feels it can drive better. You find it in biking too, tonnes of deliberation goes on online about suspension and ergonomics and technique etc, then someone who doesn't think about any of that blows past you on the trail.

The same with music. You can debate the finer points of music theory all day long, but some kid will just walk up, pick up an instrument and start playing. They may not be able to use the right terminology for what they do but there is a fair chance they'll play a lot better than those that do know all the right terminology. In part this is because the one is 'just' knowledge, the other is endless practice to get it right. They are two different skills and really gifted individuals are good at both.

https://en.wikipedia.org/wiki/Ackermann_steering_geometry

https://www.sae.org/news/2018/10/2018-yamaha-niken-chassis-t...

I'm just a software engineer, not a mechanical engineer, so any explanation would fall short. However, I've been surrounded by world-class Mech E's for decades, so I can remember these conversations being out of my depth!

I've also done urban commutes on bicycle and motorcycle for decades, so I can say given my experience, this is much easier to feel than reason about. You can tell by watching a person ride at licensing whether a person has integrated this feeling or is still reasoning about it. Hope this helps.

The key to understanding car steering is a concept called "slip angle".

That is, the amount of "misalignment" between the wheel relative to the direction of motion.

Imagine a wheel on a treadmill, with the wheel angled one degree to the right, but held in place so it's kind of slipping slightly. This would have a slip angle of 1 degree.

The fundamental thing is that for a tyre, the lateral force generated by the tire is directly related to the slip angle (also the vertical load and the camber, but these can be removed for a simple model). Slip angle is the main thing.

If you know the relationship between slip angle and lateral force, you can model the movement of a car given the angles of the 4 wheels.

The front and rear wheels have different angles of attack on the vector of the car, so they have slightly different friction vectors.

The result can be oversteer (when the back slide more) or understeer (back slides less).

And it can change depending on road conditions, tire inflation, velocity, acceleration/deceleration. And of course changes radically from vehicle to vehicle.

Covered in depth in the movie Cars "You need to turn left to turn right". :-)

Of course the same thing applies to two wheeled vehicles also - see trail braking.

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