How to convert quaternion rotation to steering wheel motion?

Question

I'm creating a game where you use rotational remote(it knows only its rotation) to steer a car. I have a problem with converting Quaternion(this is the output of the controller) to steering wheel rotation.This is what is the closest to working form all the things that I have tried(transform.localRotation is a rotation of the steering wheel):

void Update() {

    transform.localRotation = GvrController.Orientation;

    transform.localRotation = new Quaternion(0.0f, 0.0f, transform.localRotation.z, transform.localRotation.w);

}

This obviously isn't a good solution and works not so well. There is a very simple visualization of what I'm trying to do: Default orientation of the controller is facing forward as on the picture.

Basically the whole problem is how to skip all the axis other than the one responsible for rotating(axis 1 on the picture) and apply it to the steering wheel. Do you know how can I do this?

EDIT: Because it's hard to explain the problem without proper visualization i took photos of my controller and drew axis on them.

This is the default orientation of the controller:

And this is how it's held with axis marked on it:

Answer 1

Use Quaternion.Euler and Quaternion.eulerAngles. Note the order for the Euler function is z, x then y, unlike Vector3. EulerAngles returns an angle in degrees (whereas rotation.x returns the quaternion amount for that axis). The Euler function expects an angle so you want something like:

Quaternion rot = GvrController.Orientation;
transform.localRotation = Quaternion.Euler(rot.eulerAngles.z, 0, 0);

Depending on how the axes of your controller are set up you may need to experiment with other orientations if it's not y-up z-forward e.g.

transform.localRotation = Quaternion.Euler(rot.eulerAngles.x, 0, 0);

or

transform.localRotation = Quaternion.Euler(0, 0, rot.eulerAngles.z);

and so on. It should soon become clear which system it uses.

Also, if the steering wheel is not parented to anything use rotation rather than localRotation.