Fixed joint unstable compared to revolute joint

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Hello,

I'm building a robot. Some of the joints are "fixed" However I notive that these fixed joint are really unstable. Replacing it with revolute joint with 0 limit works better.

This happens when the ratio between two links is very high: in the following example the cube is 3000kg while the cylinder is only 0.1kg. This is done to illustrate.

Here is a GIF of what happens when trying to apply forces on the cylinder link (on the left the fixed joint vs the revolute joint on the right)

C:\fakepath\Peek 2020-03-20 16-09.gif

Note: this is easily reproducible by running gazebo and creating the model directly from the model editor.

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