carmudgeons.com  

Go Back   carmudgeons.com > Automotive Forums > Car Talk

Reply
 
Thread Tools Display Modes
Old 11-28-2019, 03:36 PM   #1
rumatt
Mugwump
 
rumatt's Avatar
 
Join Date: Oct 2003
Carmudgeonly Ride: E46 330i, Chevy Colorado, Tesla Model 3
Location: NY
Posts: 17,475
Death to Diffs! Separate motor for each wheel

Given my PTSD from spending most of the last 15 years driving an open diff, and now driving a Colorado with one... I'm really intrigued by the idea of doing away with diffs forever and having one motor per rear wheel.

The Tesla model S "Plaid" has two rear motors. So does the big Cybertruck. I assume this means there's no diff. I believe the Rivian also has one motor per wheel.

It does put you at a disadvantage compared to locking diffs which can send all the power to a single wheel. Now you're at 1/N (N = number of motors) per wheel.

But it means there's no need for an lsd and the tradeoffs there. I suspect for street driving it'll be just awesome.

Last edited by rumatt; 12-06-2019 at 11:42 AM. Reason: Title
rumatt is offline   Reply With Quote
Old 12-06-2019, 11:08 AM   #2
rumatt
Mugwump
 
rumatt's Avatar
 
Join Date: Oct 2003
Carmudgeonly Ride: E46 330i, Chevy Colorado, Tesla Model 3
Location: NY
Posts: 17,475
I'm still intrigued by this.

Pros of separate motors for each wheel
- No LSD required to put power to wheels with grip
- No diff needed at all!
- Traction control simpler: No need to brake the spinning wheel to transfer torque to the other. Just cut power to spinning wheels
- Torque vectoring algorithms can now be developed in software and rolled out as they improve. They could develop user-selectable drive modes.

Cons of one motor per wheel
- Max torque per wheel = 1/NUM_MOTORS. I'm guessing this is not an issue unless you're doing extreme rock crawling. Massive torque to a single wheel is what breaks things anyway
- If one motor dies things might get weird. But I assume the software could disable the other motor on the same axle and the failure mode is no different than one motor per axle.
- Even the slightest difference in torque will be felt in the vehicle under acceleration / deceleration. How difficult is it to ensure identical torque to each wheel? Sending equal power won't cut it if there's any efficiency variation in the motors. Is some kind of adaptive loop needed to compensate for this over time?


Assuming the last con is a non-issue (or easily solveable), overall this seems like a huge win. I'm not sure why more people aren't excited about this. This is a big step forward in drivetrain function and capability.
rumatt is offline   Reply With Quote
Old 12-06-2019, 11:12 AM   #3
rumatt
Mugwump
 
rumatt's Avatar
 
Join Date: Oct 2003
Carmudgeonly Ride: E46 330i, Chevy Colorado, Tesla Model 3
Location: NY
Posts: 17,475
Also - when I first heard about the tri-motor Model S Plaid, I thought they might hook them both up to the existing rear axle, but geared differently. One geared lower for low end torque, and the other higher so it pulls harder at high speed. This way you can avoid needing a 2-speed gearbox like the Taycan.

However, I'm thinking this is not worth it because
- It's not worth losing out on the other advantages (torque vectoring, diffs suck ass, etc)-
- With two motors you have so damn much torque you can gear them both higher (no point in spinning wheels) so the usable torque may be shifted high enough that a gearbox is truly not needed.
rumatt is offline   Reply With Quote
Old 12-06-2019, 11:14 AM   #4
Nick M3
Relic
 
Nick M3's Avatar
 
Join Date: Oct 2003
Location: Bethesda, MD
Posts: 12,438
Quote:
Originally Posted by rumatt View Post
Also - when I first heard about the tri-motor Model S Plaid, I thought they might hook them both up to the existing rear axle, but geared differently. One geared lower for low end torque, and the other higher so it pulls harder at high speed. This way you can avoid needing a 2-speed gearbox like the Taycan.

However, I'm thinking this is not worth it because
- It's not worth losing out on torque vectoring and the other advantages
- With two motors you can gear them both higher (no point in spinning wheels) so the usable torque may be shifted high enough that a gearbox is not needed for sane speeds.
That seems more complicated than a gearbox.
__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo


Nick M3 is offline   Reply With Quote
Old 12-06-2019, 11:14 AM   #5
Nick M3
Relic
 
Nick M3's Avatar
 
Join Date: Oct 2003
Location: Bethesda, MD
Posts: 12,438
You’re overthinking this. You’d never want more torque to the wheel - the electric motors already exceed the traction limit. Differentials never provide equal torque, so why would micro-management of torque be necessary?

Also, the big issue with a dead motor is not disabling the opposite motor (I strongly suspect that you would not want to do that). It’s the drag caused by the dead one.
__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo


Nick M3 is offline   Reply With Quote
Old 12-06-2019, 11:25 AM   #6
rumatt
Mugwump
 
rumatt's Avatar
 
Join Date: Oct 2003
Carmudgeonly Ride: E46 330i, Chevy Colorado, Tesla Model 3
Location: NY
Posts: 17,475
Quote:
Originally Posted by Nick M3 View Post
You’d never want more torque to the wheel - the electric motors already exceed the traction limit.
Huh? The car has a gearbox. It's a 9:1 ratio. The had to pick a number to balance low vs high end torque.

Quote:
Differentials never provide equal torque, so why would micro-management of torque be necessary?
After putting the LSD on my E46 if the tires are even slightly different in diameter the car is miserable to drive on a highway. There's obviously some range where you wouldn't feel it, and maybe the motor tolerances are well within this so it's a non-issue.

The neat thing about an LSD thought is that it's putting more torque to the larger diameter tire, so it will wear faster and over time they will equalize. It's a self-stabilizing system. Will the opposite happen with one-motor per wheel (since the smaller wheel will put more torque to the ground)? If so this would create an unstable system - ie if you tires aren't perfectly equal they will continue to get worse over time.

Quote:
Also, the big issue with a dead motor is not disabling the opposite motor (I strongly suspect that you would not want to do that). It’s the drag caused by the dead one.
Hmmm. If the dead one died because of an electrical malfunction and the car switches it off, I'd assume it's the same as being manually switched off, no? But if there's some kind of physical malfunction, then yeah you're fcuked.

Quote:
That seems more complicated than a gearbox.
Having two motors input into a fixed (single gear) transmission is complicated? It would mean they always have to be powered together and turn together (although at different speeds). Maybe this is infeasible for some reason?

Shifting gears while in motion sucks. It's either a) slow and jerky, or b) requires heroics to prevent it from being slow and jerky. No thanks, do not want.

Last edited by rumatt; 12-06-2019 at 11:38 AM.
rumatt is offline   Reply With Quote
Old 12-06-2019, 12:28 PM   #7
kognito
older fart than ZBB
 
kognito's Avatar
 
Join Date: Oct 2003
Location: On the road again
Posts: 8,887
I wonder if you were at a cruising speed, what could go wrong if you tried to just "power" the rear motors, and let the front wheels regen to the batteries?
__________________
2017 GMC Sierra 1500 SLE
2020 Fusion Titanium
kognito is offline   Reply With Quote
Old 12-06-2019, 12:39 PM   #8
Nick M3
Relic
 
Nick M3's Avatar
 
Join Date: Oct 2003
Location: Bethesda, MD
Posts: 12,438
The reason why different size tires are a problem for an LSD is because the wheels are linked together. This is obviated by decoupling the wheels.
__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo


Nick M3 is offline   Reply With Quote
Old 12-06-2019, 12:57 PM   #9
rumatt
Mugwump
 
rumatt's Avatar
 
Join Date: Oct 2003
Carmudgeonly Ride: E46 330i, Chevy Colorado, Tesla Model 3
Location: NY
Posts: 17,475
Quote:
Originally Posted by Nick M3 View Post
Furthermore, I don’t understand how two motors would be less complex or expensive than a gearbox?
Ah, I was operating under the assumption that you already decided to have two motors, for some reason. For example, because you already have them and can just stick them in. Rather than design a whole new motor, which might be an awkward shape to fit in. Which is, I assume, why they are doing this.

Quote:
you’d have two motors attached to the axle at all times.
Exactly. That doesn't seem complicated at all. There's already a transmission and gears in there already. Just modify it to be 2 sources in, 1 out.

Quote:
The reason why different size tires are a problem for an LSD is because the wheels are linked together. This is obviated by decoupling the wheels.
The problem is that the torque to the ground is uneven. Torque vectoring works independently of why the uneven torque is occurring. In the LSD case it's because they are linked together, but that's not the only way it can happen. If the torque is uneven enough, for any reason, the car will want to not drive straight.

Torque vectoring is one of the reasons people are excited about Plaid - so it can obviously occur without an LSD.
rumatt is offline   Reply With Quote
Old 12-06-2019, 12:41 PM   #10
Nick M3
Relic
 
Nick M3's Avatar
 
Join Date: Oct 2003
Location: Bethesda, MD
Posts: 12,438
And yes, the mechanism for switching between the two motors would either resemble a gearbox, OR you’d have two motors attached to the axle at all times.

Furthermore, I don’t understand how two motors would be less complex or expensive than a gearbox?
__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo


Nick M3 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -4. The time now is 07:22 AM.


Powered by vBulletin® Version 3.7.1
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Forums © 2003-2008, 'Mudgeon Enterprises - Site hosting by AYN & Associates, LLC