VBC is an acronym for Variable Bolt Circle. The beauty of VBC rings is they allow you to set up various tooth configurations to get the gear ratio you desire. The outer ring is designed with a slotted section and the inner five bolt ring is self centering allowing more options when compared to traditional crank designs with set bolt circle requirements. We recommend at least a ten tooth difference between the outer gear and the inner gear and a 20 tooth on the high end but the maximum is 24 teeth. The VBC rings can be configured with an off road ratio and mated to your mountain bike uses the ENO crank arms, or, if you have a road frame, use the VBC road crank arms and configure a suitable ratio for road riding. It is Compatible with 9 10 and 11 speed.