When I started slotracing I wondered why others tires performed much better than the ones I got. I made this little tool for the first time on my own PLA 3D printer. It worked like a charm! 
Put a drop of your favorite tire juice on the tires, and let the magic happen! 

Made out of a laser engraved acryl base, 3DP SLS parts. The package also includes 4 bearings and motor fixing screws. Just find and old motor laying around, two axles, a pinion and a spur gear!