Mainely because of rotating mass weight. Fast Traks come with plastic backers and studs that are seemingly lighter than Woodys. Any Fast Trak users out there. I'm going with 153 or close to that using a double and single square backer. They have round and square backers. Anyone know what the differences are. Got 168 1.50's for $270 complete and a drill bit thrown in at Novi Expo.