1. Vice-variable speed skating gear in the internal teeth wear, under heavy load so that the sliding gears are removed, automatic cutting and jumping gear;
2. The transmission fork wear or deformation, so that the sliding gears to produce axial shift jump gear;
3. The shift lever did not push to the end, so that the sliding gear unleashing jump;
4. Locking groove of steel ball and fork shaft is severely worn, so that the locking spring is weakened and jumps off.