[nuttle]

This is why I used the word directly -- I agree that weight makes a difference indirectly, and that this needs to be modeled if you want to really understand what is going on and design for optimal performance (which has to be defined in the context of the requirements). Losses through interaction with the carpet can be considered a form of friction, again with a dependency on weight...

I often write too much, but I guess I wrote too little before!