# Does fixed joint have a bug in sdf ?

I use gazebo7 with ubuntu16.04 to simulate a four wheel robot. If my sdf model has a fixed joint (not for the wheel but for a cemera) like below, the wheel rotation velocity will oscillate, see the picture below the code.

<!-- the zed joint -->
<joint type="fixed" name="zed_hinge">
<pose>0 0 0 0 0 0</pose>
<child>stereocamera</child>
</joint>


If I change the fixed joint to a revolute joint with all other things the same, the result will be much better shown below.

<!-- the zed joint -->
<joint type="revolute" name="zed_hinge">
<pose>0 0 0 0 0 0</pose>
<child>stereocamera</child>