I suppose someone who wants to use an extra wheel for instrumentation could calculate how much force it needs to contact the floor with, to prevent it slipping under maximum robot acceleration conditions (it would slip if it takes more torque to overcome it's own inertia, than will be transfered to it by the frictional force of it's contact with the arena). I expect it won't take (much) more than it's own weight. I haven't done the calculations....I'll leave that for the students
