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. 0 0 0 0 0 0stereocamerabase_link  If I change the fixed joint to a revolute joint with all other things the same, the result will be much better shown below. 0 0 0 0 0 0stereocamerabase_link0 0 1  I want to know, does fixed joint works properly?