[MrFlippant]

Thanks, guys. And Kev, you're absolutely right that can happen. The lower the grit, the more likely it is to happen. I ordered some 35 grit, thinking it would be great for silicone tires. It might still be, but it was a disaster for rubber tires for that reason. The 80 grit is a lot better, of course. This is why I designed it so the entire sanding spindle can be easily replaced by removing only two screws. A user can have two or more whole spindles with their various grits to cut fast, and then cut slow, and then polish.

That said, that's a level of perfectionism that, while I'm trying to accommodate, I'm not really aiming at. Since getting both wheels perfectly parallel is a challenge on any machine, most of all this one, and even through I've added a feature to help adjust it, this isn't for people seeking that level of perfection. For the casual home racer, round and concentric is good enough. :)

All THAT said, so long as the wheels/inserts don't cause pain, one can lightly press either side of the axle to move it over a little, if a prominent line starts to show itself. :) A mechanism to do that automatically isn't on the cards, but if I get a flash of inspiration, you never know. ;)