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Driveshaft disconnect against high RPM? (4wd split dual motor setup?)

897 Views 7 Replies 3 Participants Last post by  Swing
I am wondering how feasible it would be to have a 4wd setup in which you gear the front axle different from the rear axle. Much like a Tesla passenger car does, or now the Semi.
Ideally you would do that in a way that you stay in the RPM range that the motor is allowed to mechanically make.
The Tesla Semi dual motor axle doesn't even stay in the range of the high RPM motor (goes even higher) and disconnects the motors.

That could be followed for a 4x4 perspective. Most preferably a short geared motor on the rear axle and a long geared motor on the front axle.
Than the rear axle might disconnect once it is doing a higher speed than lets say a Hyper9 is allowed (6k rpm).

What you can find are rear driveshaft disconnects (with levers). Not only are they manually operated, it won't work to disconnect and connect while driving.
Are there any other creative options?
Preferably something automatic.

I thought about using front axle with automatic free locking hubs, but I believe that is not how they work. They will stay engaged until you reverse I think?
Or they might actually release earlier than reversing, but not early enough.

I guess an electronically controlled or automatic disengage of the rear drive shaft would be the best, but it has to be capable of doing that during driving.
So it should be some kind of clutch based system? Or even planetary wheels?

I couldn't find anything on the forum but may have used the wrong search terms.
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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.
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