Back Packers Gear步行者;背包客
back-gear shaft后齿轮轴
back lash gear反撞齿轮
back gear shaft背齿轮轴
encased back gear封闭式背齿轮
encase back gear封闭式跨轮
back gear ratio倒挡转速比;后齿轮比
back draft change gear后牵伸变换齿轮
frictional back gear摩擦背轮