Just to clarify: systems with different ratios front and rear are using separate front and rear motors.
In some cases the difference is more accidental than intentional, and is not important.
Even when the gearing difference intentionally places the ideal motor speed ranges at different road speed ranges, the "out of range" motor is usually not physically disconnected - it is just used at lower power or electrically idled. The out-of-range motor is still in a physically safe speed range - it's just not the right speed for efficiency, or perhaps too fast for the available battery voltage to effectively drive the motor.
Mechanical disconnection is rare, although Tesla is doing it in one axle of the Semi, and Rivian is doing it at the rear of the R1T. When mechanical disconnection is used, the mechanical hardware is readily available and well-proven in non-electric powertrains.
If a mechanical disconnection device fails in the engaged mode (won't disengage), the vehicle still drives (perhaps with a limited top speed). If a mechanical disconnection device fails in the disengaged mode (won't engage), the vehicle still drives (with only the other axle driving). Only if the mechanical disconnection device fails in a "broken bits thrashing around in the case" mode would the failure disable the vehicle. I don't think we know if that worst case failure has happened to any Rivian R1T or Tesla Semi, or any of the many ICE vehicles with these components.
Disconnecting one axle for optimal efficiency is beyond the level of sophistication of control in any DIY EV that I've seen. Generally they don't even have traction control.
In some cases the difference is more accidental than intentional, and is not important.
Even when the gearing difference intentionally places the ideal motor speed ranges at different road speed ranges, the "out of range" motor is usually not physically disconnected - it is just used at lower power or electrically idled. The out-of-range motor is still in a physically safe speed range - it's just not the right speed for efficiency, or perhaps too fast for the available battery voltage to effectively drive the motor.
Mechanical disconnection is rare, although Tesla is doing it in one axle of the Semi, and Rivian is doing it at the rear of the R1T. When mechanical disconnection is used, the mechanical hardware is readily available and well-proven in non-electric powertrains.
If a mechanical disconnection device fails in the engaged mode (won't disengage), the vehicle still drives (perhaps with a limited top speed). If a mechanical disconnection device fails in the disengaged mode (won't engage), the vehicle still drives (with only the other axle driving). Only if the mechanical disconnection device fails in a "broken bits thrashing around in the case" mode would the failure disable the vehicle. I don't think we know if that worst case failure has happened to any Rivian R1T or Tesla Semi, or any of the many ICE vehicles with these components.
Disconnecting one axle for optimal efficiency is beyond the level of sophistication of control in any DIY EV that I've seen. Generally they don't even have traction control.
