[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.

 

[remy_martian]

Why? You have two motors' worth of torque.

There have been a lot of pics of Semis broken down by the road over the past month or two. Maybe that disconnect is a major reliability hit?

 

[Swing]

It would definitely be more simple to use the motors permanently coupled, simpler and therefore more reliable.
Could be that the semi brakes down on that, or on the many other stuff that is new on that thing.

I think it is interesting for higher rpm motors that deliver much relatively late in the rpm band (torque is available from the start) You could use one to get up to speed and the other for a more reasonable high cruising speed.

I guess there are several alternative options:

• just fixed same gearing, don't try to do anything "smart"
• fixed different gearing but staying in the RPM range of both motors. This way highway cruising speed would probably max out one of the electric motors.

Even if you stay within the max RPM range, you are probably close to peak RPM rate, and not close to the continuous RPM number. So, not good.
- just use one motor, reusing the original transfercase. Giving you the low range option as well. Downside is that if you use a high rpm motor, the 40 year old gearbox is not built for those RPMs, so software limitation is needed, or reduction gearbox.

In case of using two hyper9s, possibly with a 2:1 reduction gearboxes directly connected to the driveshafts it could work.
In case of higher RPM (like that recently discussed Zonic 180, or a Tesla motor dissassembled from the drive unit), then much higher reduction gearboxes are needed (4:1)?

Whatever you build, it will drive, but since these are classic 4x4s, you don't want to go 200kph, and we can try to get the rpm window much closer to the actual usage (0 to 130kph or so.)

 

[Swing]

I read that TorqueTrends will be making a 3:1 reduction gearbox, that would be nice.
However, 2 of those reduction gearboxes with 2 motors will make it expensive.
I guess it would be more affordable to use 1 higher power motor, like the Zonic 180kW and than with that 3:1 reduction gearbox to the existing transfer case.
Which might get completely destroyed over time with double the load on it.

 

[brian_]

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.

 

[Swing]

I am not especially going to do all that for efficiency. Then it is the wrong car to begin with.
Just to get the power down, will make it quicker and better at offroading. While also operating better in the range it is mostly used in.
But anyway, if there is a way to not use this disconnect, that would always be preferable.
For example, 2 Hyper9s with reduction gearboxes such as the upcoming 3:1 would be great, although costly.
Finding a newer/stronger transfer case that can take the power of one strong motor would be great alternative as well, keeping the parttime 4x4 with low gearing reduction.

 

[remy_martian]

Again, you have not explained what you are trying to accomplish.

"Just to get the power down",

"operating better in the range it is mostly used in",

and

"it is interesting for higher rpm motors that deliver much relatively late in the rpm band"

all do not state what you are trying to accomplish with all this complexity.

 

[Swing]

@remy_martian It was more intended as a general question, if people were using these techniques (possibly through existing 4x4 tech), because I wasn't able to find any.

But I will come back later with some worked out calculations.