Both driveshafts and half-shafts are components in a car responsible for transmitting power. The driveshaft transmits power to the differential in the drive axle and is mostly a hollow shaft. The half-shafts transmit power to the left and right drive wheels; since each wheel occupies half of the drive, they are called half-shafts, and the vast majority of car half-shafts are solid.
We all know that a solid shaft is stronger given the same diameter and material. So why is the driveshaft designed as a hollow structure? The main reason is its length.
If the driveshaft were solid, its diameter wouldn't need to be very large to meet strength requirements. However, the driveshaft runs from the front to the rear of the car, and over such a long distance, it would naturally bend slightly due to gravity. Since the driveshaft rotates at high speeds while the car is moving, even a slight bend would cause it to vibrate violently.
Designing the driveshaft hollow solves this problem. Because the closer to the center of the shaft when transmitting torque, the less torsional force it bears; the center doesn't bear any torsional force at all. In other words, the outer part of the shaft bears most of the torsional force when transmitting torque. Therefore, even if the middle is hollowed out, the outer part is sufficient to meet the transmission requirements if the shaft is thick enough.
Therefore, driveshafts are designed as hollow structures. This allows for a thicker driveshaft without changing the material weight, meeting the torque transmission requirements while preventing bending due to excessive length.
Half-shafts, on the other hand, are short and don't have this bending issue. Furthermore, half-shafts need to connect to the wheels, and their installation position intersects with the steering and suspension systems, so a solid design is more convenient for layout. Additionally, many passenger cars use semi-floating half-shafts, meaning they not only transmit torsional force but also bear the shear force transmitted from the car body. In this respect, solid shafts are much stronger than hollow shafts. Therefore, almost all automotive half-shafts use a hollow design.
