Android orientation as steering - roll corrupting pitch?

I am working on a project which includes an Android application which is used for controlling/steering.

• Speed: When you tilt the phone forward/backwards (pitch) it simulates giving gas and breaking.
• Direction: When you tilt the phone left/right (roll) it simulates steering to the left and right.

I have already written some code which seemed to work fine. But when I took a closer look, I found that some values are acting weird.

When I tilt the phone forward/backward to handle the speed it works perfect I get the expected speed and direction values. But when I tilt the phone to the left/right to handle the direction it seems to corrupt some values. When it is tilting to the left/right that doesn't only change the direction value (roll) but it also affects the speed value (pitch).

For extra information:

• Programming for Android 2.2
• Device is an Google Nexus One
• Holding the device in portrait

The most relevant code I use to read the sensor values is as follows:

public void onSensorChanged(SensorEvent sensorEvent)
{        
    synchronized (this)
    {
        if (sensorEvent.sensor.getType() == Sensor.TYPE_ORIENTATION)
        {
            float azimuth = sensorEvent.values[0]; // azimuth rotation around the z-axis
            float pitch = sensorEvent.values[1];   // pitch rotation around the x-axis
            float roll = sensorEvent.values[2];    // roll rotation around the y-axis

            System.out.println("pitch: " + pitch);
            System.out.println("roll: " + roll);
            System.out.println("--------------------");

            // Convert the sensor values to the actual speed and direction values
            float speed = (pitch * 2.222f) + 200;
            float direction = roll * -2.222f;    

So when I run the code, and I look at the printed values. When tilting the device left/right, it seems to affect the pitch value as well. How come? And how can I get the pure pitch value, when 'roll'-ing? So that tilting the phone to the left/right doesn't affect/corrupt the pitch value.

You could read up on Gimbal lock. That's bitten me before.