[ODE] Assembling Bodies

[Wesley Smith]

Hi,
I'm trying to run a simulation where, given certain conditions, bodies
can link up with each other to form larger objects.  Right now, when
the conditions are met, I create a fixed joint between the two bodies.
 This joint is not really documented in the ODE manual (PDF) and I
suspect that's because this is jsut because the joint is so trivial,
but I'd just like to confirm what I think it does.

Does a fixed joint lock two bodies together such that they can't move
relative to each other once it's attached?  This is happening to some
degree in my simulation, but it also looks like the collision
detection calls are causing the joing to malfunction.  Sometimes, the
joint breaks or the objects continually collide and repel.  I'm using
the default CFM and ERP param values if that makes a difference, which
I'm pretty sure it does here.

In any case, how do people usually handle this kind of thing in ODE? 
Is it necessary for me to remove the joined pairs from the collision
callback?

thanks,
wes