[WillNess]

Thank you Ether, that helps tons.

Originally Posted by Ether

Look at the WPILib code for mecanumDrive_Cartesian():

    public void mecanumDrive_Cartesian(double x, double y, double rotation, double gyroAngle) {
        if(!kMecanumCartesian_Reported) {
            UsageReporting.report(tResourceType.kResourceType_RobotDrive, getNumMotors(), tInstances.kRobotDrive_MecanumCartesian);
            kMecanumCartesian_Reported = true;
        }
        double xIn = x;
        double yIn = y;
        // Negate y for the joystick.
        yIn = -yIn;
        // Compenstate for gyro angle.
        double rotated[] = rotateVector(xIn, yIn, gyroAngle);

Now look at the WPILib code for rotateVector():

    /**
     * Rotate a vector in Cartesian space.
     */
    protected static double[] rotateVector(double x, double y, double angle) {
        double cosA = Math.cos(angle * (3.14159 / 180.0));
        double sinA = Math.sin(angle * (3.14159 / 180.0));
        double out[] = new double[2];
        out[0] = x * cosA - y * sinA;
        out[1] = x * sinA + y * cosA;
        return out;
    }

The cosine of zero is 1. The sign of zero is 0. So passing zero as the fourth parameter to mecanumDrive_Cartesian() will result in no rotation of the vector, which is what you said you want.