This is a larger diameter wheel for the popular "AXA HR Traction power control" bike dynamo. It makes it possible to run it on the rim where it is much quieter and more efficient. the larger diameter also means reduced losses at higher speed (but less light at lower speed - should be fine with modern LED lights though)Get a ~40mm diameter O-ring from the hardware store as "tire" and slip it into the groove on the wheel. Pull the original wheel off and replace with this one, now adjust the position so that it runs on the rim instead on the tire wall. The "skirt" is there to keep dirt from getting into the bearing, hopefully extending its useful life. Not quite as good as a hub generator, but a lot cheaper! And the only option for some bikes like my KMX cobra with non-standard one-sided front hubs...