yeah, i used to run a mylar disc at the track long befor the j-disc that i built myself. it weighs almost nothing and would be alot easier to build these days for a road bike with the deeper section rims that are available whereas for the track you can glue the mylar right up to the edge of the tire as you don't need to worry about the braking surface. why someone would build them out of coroplast i don't know