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Ok, I go try to explain this.
See it this way, draw a circle, thats the turn your car makes.
Now the "Y" rotate you mention is the center of the circel, in other words outside the car.
Its NOT at the inner back wheel. Take a look at a car when it makes a turn, the inner back wheel is not standing still. This would happen if the turning point is there.

Yup, you're right there.  However, the centre of rotation must change, since a car does not execute a turn all at once.  The front wheels eventually go to their "turn" position, then straighten out again.  I confess I have absolutely no idea how to represent this, though.

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*With regards to diferentiated axes of a car.
Look at that circle again and place a car on it, you see that the inner wheels make a smaller circel than the outher wheels. This means that the outher wheels have to rotate faster than the inner, this is where a diferential is used. A "fixed" rear or drive axel is only used on terain cars, and normaly can be turned off and on.
Using a fixed axel on a hard road will couse damage to the drive work and tires since one wheel will be forced to to rotate the same speed as the other wheel in a turn.
In soft terain this is not a problem since the soft material like sand compensates for this.
This "fixed" axle takes away the disadvantage of a diferential in terain, like if one wheel gets stuck it stops turning and the diferential only drives the other wheel.

I hope this all made a bit sence :smile*:

 

Well, yes.  And that's my point.  The vehicle I mentioned (I honestly cannot remember who makes it) has only the fixed option, which seems silly to me.  It would make more sense to have the option of a diff lock, rather than be stuck (pun intended) with a fixed axle.

Coppula eam se non posit acceptera jocularum.

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