DRC Vehicle is not moving after giving velocity to wheel joint

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Hello,

I'm currently working on developing algorithms for path planning on DRC Vehicle. For that I used the existing plugin and did some changes in it. I have been able to control the steering via command SetAngle on steering joint but when I give velocity via SetVelocity to rear wheel joints, the vehicle is stationary.

I have added the following in the existing plugin in Load part

if (!ros::isInitialized())
{
int argc = 0;
char** argv = NULL;
ros::init(argc, argv, "hello");
 }
 rosnode_ = new ros::NodeHandle( node_namespace_ )
 cmd_vel_sub_ = rosnode_->subscribe("/cmd_vel", 1, &DRCVehiclePlugin::OnCmdVel, this );

And the following I added after the Updates

 void DRCVehiclePlugin::OnCmdVel( const geometry_msgs::TwistConstPtr &msg)
 {
 this->lastTime = this->world->GetSimTime();
 double vr, va;
 vr = msg->linear.x;
 va = msg->angular.z;


this->brWheelJoint->SetVelocity(0, 10);
this->handWheelJoint->SetAngle(0, 2*atan (va*1.88/(sqrt(vr*vr -va*va*.85*.85))));
}

The handwheel joint is working but the brwheeljoint is not moving. I don't know why this thing is not working, If anyone have solved this sort of problem kindly help me. A little help would be appreciated.

Thank You