Odometry message errors

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Hi!

With examples I found online I made an odometry publisher that generates odometry data (for gmapping) from encoder ticks. But When the robot is going straight forward, the odometry data gives the same translation in x en y direction. Where are my mistakes? Here is the full publisher:

#include "ros/ros.h"
#include <geometry_msgs/Vector3.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>
#include "mastering_ros_demo_pkg/encoder_msg.h"

long _PreviousLeftEncoderCounts = 0;
long _PreviousRightEncoderCounts = 0;
ros::Time current_time_encoder, last_time_encoder;
//double DistancePerCount = (3.14159265 * 0.1524) / 200;
double DistancePerCount=1;
double x;
double y;
double th=0;
double W;
double V;
double vx;
double vy;
double vth;
double deltaLeft;
double deltaRight;


double d=50; //afstand tussen de wielen

int ticks_links;
int ticks_rechts;


//void WheelCallback(const geometry_msgs::Vector3::ConstPtr& ticks)

void WheelCallback(const mastering_ros_demo_pkg::encoder_msg::ConstPtr& msg)
{
  //tijd van de meting 
  current_time_encoder = ros::Time::now();

 //verschil in ticks tov vorige berekening 
  deltaLeft = msg->ticks_links - _PreviousLeftEncoderCounts;
  deltaRight = msg->ticks_rechts - _PreviousRightEncoderCounts;

 //snelheid links (x) en rechts (y)
  vx = deltaLeft * DistancePerCount / (current_time_encoder - last_time_encoder).toSec();
  vy = deltaRight * DistancePerCount / (current_time_encoder - last_time_encoder).toSec();

 //als snelheid links = snelheid rechts -> snelheid = snelheid links en hoeksnelheid = 0
if (vx == vy)
{
    V = vx;
    W = 0;
}


else
{
 // Assuming the robot is rotating about point A   
 // W = vel_left/r = vel_right/(r + d), see the image below for r and d
        double r = (vx * d) / (vy - vx); // Anti Clockwise is positive
        W = vx/r; // Rotational velocity of the robot
        V = W * (r + d/2); // Translation velocity of the robot
}

 //vth = hoeksnelheid
vth= W;

 //aantal ticks onthouden 
  _PreviousLeftEncoderCounts = msg->ticks_links;
  _PreviousRightEncoderCounts = msg->ticks_rechts;

 //tijd van de laatste meting onthouden
  last_time_encoder = current_time_encoder;
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "odometry_publisher");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("tick_topic", 100, WheelCallback);
//ros::Subscriber sub1 = n.subscribe("tick_topic", 100, ticks_topic_callback);
  ros::Publisher odom_pub = n.advertise<nav_msgs::Odometry>("odom", 50);   
  tf::TransformBroadcaster odom_broadcaster;


  ros::Time current_time, last_time;
  current_time = ros::Time::now();
  last_time = ros::Time::now();


  ros::Rate r(20.0);
  while(n.ok()){

    current_time = ros::Time::now();

    //compute odometry in a typical way given the velocities of the robot
    double dt = (current_time - last_time).toSec();
    double delta_x = (vx * cos(th) - vy * sin(th)) * dt;
    double delta_y = (vx * sin(th) + vy * cos(th)) * dt;
    double delta_th = vth * dt;

    x += delta_x;
    y += delta_y;
    th += delta_th;

    //since all odometry is 6DOF we'll need a quaternion created from yaw
    geometry_msgs::Quaternion odom_quat = tf::createQuaternionMsgFromYaw(th);

    //first, we'll publish the transform over tf
    geometry_msgs::TransformStamped odom_trans;
    odom_trans.header.stamp = current_time;
    odom_trans.header.frame_id = "odom";
    odom_trans.child_frame_id = "base_link";

    odom_trans.transform.translation.x = x;
    odom_trans.transform.translation.y = y;
    odom_trans.transform.translation.z = 0.0;
    odom_trans.transform.rotation = odom_quat;

    //send the transform
    odom_broadcaster.sendTransform(odom_trans);

    //next, we'll publish the odometry message over ROS
    nav_msgs::Odometry odom;
    odom.header.stamp = current_time;
    odom.header.frame_id = "odom";

    //set the position
    odom.pose.pose.position.x = x;
    odom.pose.pose.position.y = y;
    odom.pose.pose.position.z = 0.0;
    odom.pose.pose.orientation = odom_quat;

    //set the velocity
    odom.child_frame_id = "base_link";
    odom.twist.twist.linear ...