Question: Importance of CFM for heavy mass

[ShuffleWire]

I need some advice on how to understand CFM :
Considering this project : force.zip

The goal is to lift the inner box (the other one is just to get a reference point).
Note that the most inner box is 1000kg.
Max LinearMotor force is 10000 N.
Also note that the CFM is 1e-8.

As it, it works well, the box go up at the speed of 0.033 m/s, as requested by the controller.
Also the feedback force is shortly at a steady 9810 N, which is great !

Now if use CFM = 1e-5, which is the default value.
Question 1 : I need to reload (not reset) to apply the new value, is that normal ?

Running the simulation, and the box can't get up, and the feedback force is unstable, with low values...
I've been initially screw up because I've left to default value, until I realized it was that.
Question 2 : How is it important ? Objects are very heavy, so it should make a difference for sure, but I'm not sure to understand why. I've read about CFM in https://cyberbotics.com/doc/reference/worldinfo. It specified that CFM should be reduced to avoid objects sinking or penetrating each other. But that's not the use-case presented here. It's not about collision, but about a Slider and a Motor.
Question 3 : If it's the expected behaviour, should we document it, in a way that the next person with heavy robot don't get trap as well ?

Now, my next issue with this solution is on another project :
It fix the issue on one part of my robot (a really heavy arm), but that it cause trouble on others part (wheel on the ground)
Question 4 : Is there a way to specify "local" CFM, other than using ContactProperties ? Because I've not actual contact simulated between my parts (or I least I don't care), and I don't know how to apply a specific CFM on arbitrary link like Hinge or Slider.

Question 5 : Does reducing "that much" the CFM could cause a performance cost ?

Sorry for this confused message, I hope it's still understandable.

[ShuffleWire]

Thanks @omichel for moving it where it belong :)

[omichel]

Question 1: I need to reload (not reset) to apply the new value, is that normal?

No, that's probably a bug, as the parameter change should take effect immediately, not even after reset.

Question 2 and 3

You should read the documentation about CFM in the ODE documentation here.

Question 4: Is there a way to specify "local" CFM, other than using ContactProperties?

No.

Question 5: Does reducing "that much" the CFM could cause a performance cost?

No, it should not.

[ShuffleWire]

1. Ok, it seems indeed to work, I don't know what happend
2. Ok, I was worried it could increase the number of speed to solve the step, but it work fine.
   Thanks