John Rhodes recommends urethane for friction-drive wheels, and
suggests freezing it in liquid nitrogen before machining or grinding
it.  "Where," I hear everyone ask, "am I going to get liquid nitrogen?"

You get it from a welding supply company, but you must have a
container.  The vessel of choice is a one- or two-liter stainless-steel
Thermos.  It's possible to use a glass Thermos, but you risk fracturing
the glass with the first impact of the cold liquid unless you carefully
chill the bottle first with a few drops, rather than just pouring it
all in at once.

Peter Neilson