There are really only a few reasons that constrain hub motors.
1. The most important is the inability to reduce the unsprung mass (unsprung mass). Underspring mass refers to the moving mass of the wheels, tires, and the series of parts that move with the wheels. Car companies are forever trying to reduce unsprung mass, otherwise comfort is poor and crossing a pothole is fatal. The mass of the motor itself is impossible to reduce, the density of copper, permanent magnets, iron density, are all there, so the application of wheel motors will significantly reduce the performance of the vehicle chassis.
2. In fact, the hub motor efficiency is not high. Because the wheel motor size is limited, the motor will be forced to make ultra-high-speed, low-torque design, rather than the low-speed, high-torque design needed for the wheel. Therefore, it must be matched with a super-high reduction ratio planetary gear reducer, which is actually not very efficient. If you add a two-speed transmission, the system will be even more complex.
3. Multi-motor coupling is also difficult, but can be relatively easy to solve, just not necessary.
4. Power distribution problem. Don’t look at the hub motor this design is very elegant, but there are actually disadvantages. For example, my car is designed with 100 kW of power and front drive. For a vehicle with a single motor output and power distribution through a differential, only 100 kW of power is needed in total. But in order to achieve the same acceleration targets, the hub motor must deliver 70 kW left and right, which means a total of 140 kW, or 40% more than the 100 kW of the single front axle motor. So the hub motor needs more back-up power. If it is four-wheel drive, this problem will be even more exaggerated. So a wheel motor that is already too heavy will become even heavier.
5. Actually it does not save the engine compartment. The pure electric car platform represented by Tesla all put the battery under the passenger compartment, and the front and rear compartments only need to put the motor and controller. Under the reasonable design, the engine compartment can save half of the space, which is actually very good. If it is replaced by a hub motor, the size of the battery and controller will not be saved. However, the wheel side structure of the hub motor will significantly increase the occupied volume, so from the lateral side, it will also eat the size of the engine compartment.
The advantage of hub motors is actually one, direct power response. But a well-designed electric drive axle, there will not be a significant gap