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Re: steering force



On 11/09/2003 09:12 PM, Ludwik Kowalski wrote:

I am supposed to say that the force is provided by the road; it is a
familiar static friction force.

I like to call it quasi-static. Details
http://www.av8n.com/physics/car-go.htm#sec-rolling-friction

But this is not enough; something essential is missing to make this
acceptable.

Indeed yes, there is more to the story.

The sidewise force would not appear (ideally), if all four wheels
were suddenly locked, no matter what their orientations are.

If I understand the question, that's not where the
answer lies.

The magic of steering comes in part from quasi-static
rolling friction, but there's more to the story. In
particular, if the "tires" were rolling *freely* like
the ball in a ball-point pen, there would be plenty
of quasi-static rolling friction, but no steering.

So we need to think also about what goes on at the
*bearing* at the axle. That is what allows a proper
wheel to roll freely in one direction, while exerting
forces of constraint in the other direction.

If you want to analyze the bearing, it is an
interesting but not entirely elementary exercise.
For students who don't yet know how to analyze the
dynamics of a car in a turn, analyzing the bearing
is not appropriate. It would be better to just
say that the bearing is designed to create forces
of constraint that allow the wheel to roll in the
"good" direction but not in the cross-direction.

We note in passing that it didn't have to be that
way; consider a disk penetrated (through its
center) by an axis that is mis-aligned with the
axis-of-symmetry. The result will be a wobbly
wheel. Not good. This illustrates that you
can't take wheels for granted ... rather you must
engineer them to do what you want.