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][1].
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).
[1]: http://en.wikipedia.org/wiki/Gimbal_lock
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