The distance between the motor and the axle are fixed. Carrera states that the total amount of teeth in any tuning motor / rear axle combination must equal 60. Only Z10 motors go with Z50 axles because that combination totals 60 teeth. Other combinations: Z12 motors / Z48 rear axles, Z14 motors and Z46 rear axles.
Please note that the letter Z stands for teeth. Thus, a Z10 motor means the gear on the motor has 10 teeth. A Z50 rear axle means that the gear on the axle has 50 teeth.