Problem with localEulerAngles (180 degrees error?)

Questions & Answers
1

Hello,

I just started learning Unity and I got a problem: I want to rotate a wheel around X axis (simulate movement) and around Y axis (simulate steering), using the following code:

wheel.Rotate(collider.rpm * 6 * Time.deltaTime, 0, 0);  //rotate around X  
wheel.localEulerAngles = new Vector3(wheel.localEulerAngles.x, collider.steerAngle, wheel.localEulerAngles.z); //rotate around Y

The problem is with wheel.localEulerAngles.z : sometimes it returns -1 or -2 and sometimes 180. - and this makes my wheel rotate on Z axis about 180 degrees constantly, something that I don’t want. If I try to modify it’s value (for example add only the values which are lower than 180) it “partially” fixes it but the wheel is not rotating correctly on the X axis.

Any simple solution is greatly appreciated.

*simple = not-very-complicated quaternions or no quaternions at all

4

The conversion quaternion->euler isn’t reliable: there are several different xyz combinations corresponding to any quaternion, and many times the combination returned isn’t the one we expect - specially when more than one axis is != zero.

Fortunately, the conversion euler->quaternion is reliable, thus we can use the following approach: get the initial Euler angles and rotate them mathematically, assigning the result to the actual Euler angles in Update (or other periodic function):

Vector3 euler;

void Start(){
  euler = wheel.localEulerAngles; // save initial rotation
}

void Update(){
  // "rotate" wheel angle around X:
  euler.x = (euler.x + collider.rpm * 6 * Time.deltaTime) % 360;
  // set wheel Y angle:
  euler.y = collider.steerAngle;
  // update actual wheel localEulerAngles:
  wheel.localEulerAngles = euler;
}

NOTE: This works fine because Unity applies the Euler rotations in the order Z-X-Y, thus the angle about X will be set before the angle about Y. This trick would fail if the wheel rotated about Y and steered about X, for instance.

3

As a general rule, never modify localEulerAngles directly. Use quaternions instead.

2

IMO when it comes to coding, Euler angles are more complicated than quaternions, since there are so many friggin’ exceptions you need to consider to avoid a gimbal lock.

Although I cannot give you a simple answer to your problem (there are too many unknows for now). Try putting Debug.Log(wheel.localEulerAngles.ToString()); there and look how the value changes (you need to open a Console window in Unity to see the output).