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# Dynamic index / Yaw axis

## Dynamic index / Yaw axis

(OP)
I'm getting a bit confused with the the yaw axis of the vehicle, which I always assumed to be at the CG. I've recently come across a topic on the inertia match theory which (I think) suggests that there is a different point about which the car yaws, and subsequently the ratio of that to the rear axle distance determines the initial turn in response of the car.

Is this correct or am I missing something here?

Many thanks!

### RE: Dynamic index / Yaw axis

The vehicle does not, in general, rotate around the cg, or any fixed axis at all. There may be a way of establishing that point of rotation by hand.The actual centre of rotation is at the centre of the curve, nowhere near the car at all. So you need to define what you mean.

Cheers

Greg Locock

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### RE: Dynamic index / Yaw axis

(OP)
I understand that the 'turning' centre is far from the car, defined by slip angles and steering angle.

But the theory of 'inertia matching' is referring to the rotation of the car itself around some (yaw?) axis that's well within the car dimensions, so I am not sure what this axis is?

### RE: Dynamic index / Yaw axis

What you are looking for is basically the zero lateral velocity point along the vehicle longitudinal axis.

For initial turn-in, it is the point along vehicle longitudinal axis where lateral acceleration is zero.
The dynamic index can be derived from this condition.

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