# Robot rolls and rotates with different parameters

I am currently trying to build a robot with ackermann steering. As an example I found https://github.com/trainman419/ackermann_vehicle-1 for this robot. It works pretty well with the set parameters and with a small python script that converts the joy commands from my xbox 360 controller to the ackermann message I can move it in Gazebo.

The problems start when I try to edit parameters though. I would like to change the robot and want to use smaller wheels for example:

<xacro:property name="hub_dia" value="0.055"/>
<xacro:property name="tire_dia" value="0.066"/>


When I do that the robot starts moving by itself and I can't really work out why it does that. At the start it just moves forward, when I drive it myself a bit with the controller it also starts to rotate on the spot, which means I can't drive straight lines any more.

I am using ROS Kinetic on Ubuntu 16.04 with the newest version of gazebo that is available on that platform.

Why or how is it possible for the car to move on its own when I just decrease the radius of the wheels. And how can I fix this? Is it maybe depending on the PID Values in the YAML files or do I need friction parameters at some point?

• Welcome to Robotics Cpt_Frey, but I'm afraid that it is not clear what you are asking. We prefer practical, answerable questions based on actual problems that you face, so it's a good idea to include details of what you want to achieve, what you tried, what you saw & what you expected to see. Please take a look at How to Ask & tour for more information on how stack exchange works and work through the Robotics question checklist to edit your question to make it clearer. – Mark Booth Apr 23 '19 at 10:27
• I hope i am more specific now with the "my question part" – Cpt_Frey Apr 23 '19 at 11:12
• Thanks @Cpt_Frey that does help. I don't know ros. but does recalculating axle_vel_limit in em_3905.urdf.xacro change the behaviour? – Mark Booth Apr 23 '19 at 13:52
• what were the wheel values before you changed them? – jsotola Apr 24 '19 at 1:58
• I used the original wheel values before. Meaning: "hub_dia" value="0.09652" "tire_dia" value="0.14605" Changing axle_vel_limit only changes the physical maximum speed of the axle as far as i know. So it shouldnt have any effect – Cpt_Frey Apr 24 '19 at 8:54