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Discussion Starter · #1 ·
GM set up a prototype Hummer EV and a Hummer EV chassis (vehicle without the body), and let the creator of the Engineering Explained videos (Jason Fenske) take a tour. His comments don't add much, except in the battery configuration description, the gearing, and one protective feature, but it's the most detailed look at the Hummer EV that I have seen yet.

Hummer EV First Look! All The Details Of GMC's All-Electric Supertruck

Some interesting bits, specific to the EV aspects:

Motors
  • separate left and right rear motors, driving through fixed 10.5:1 reduction ratio
  • single front motor, driving through fixed 13.3:1 reduction ratio and electronically controlled differential
  • all motors 250 kW and about 335 lb-ft (peak) output each
  • motor torque limited to protect CV joints when wheels are not steering straight
  • virtual (and generally pointless) rear differential lock function available by dash switch
  • inverters mounted directly on top of motors
Battery
  • 2 layers of 12 modules each
  • all modules with pouch cells in vertical plane
  • 24 cells per module
  • ~100 Ah per cell
  • wireless communication between BMS master and slaves
Operating configuration:
  • 96S or 400 volts (maximum)
  • 2P [ 12S { 8S 3P } ]
where
{ } is the module​
[ ] is a string of modules​

Charging configuration:
  • 96S operating configuration, or
  • 2S [ 12S { 8S 3P } ], for an overall 192S or 800 volts (maximum) when connected to 800 V charger
  • Yes, they have include contactors to reconfigure the two strings as parallel for operation and most charging, but in series for charging from a high-power 800 volt DC charger.
My Comments
  • The module configuration and arrangement in the pack are generally similar to the Bolt, but with
    • fewer cells in series per module and so more module, and
    • two complete full-voltage strings
  • This size of module will not be useful for many high-voltage DIY conversions, because a set to produce 360 V (nominal) will be large (over 100 kWh); it may be useful for low-voltage conversion, with a 32S system requiring only four modules.
  • The 500 kW dual motor rear drive unit could be interesting for a high-performance DIY car.
  • Fenske attributes the single front motor (instead of dual independent motors like at the rear) to the lack of traction to use more power. That's sort of valid, but not the real reason...
    • GM has three sizes of motor available, and could have used two smaller motors instead of one of the largest size;
    • however, 2 smaller inverters plus 2 smaller motors plus 2 reduction gearboxes is more expensive than one larger inverter plus one larger motor plus one reduction gearbox plus a differential, and
    • also, it is generally undesirable to apply unequal torque to the front wheels, so at the front it is better in most cases to just back off the power and let a simple differential keep the wheel torque equal.
 

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I saw the same video, was pretty solid.

- They make a pretty big deal out of "Crab Mode" where both front and rear tires steer the same way, allowing you to move forward at 30 degrees without actually turning.

- 18 cameras, including lots of undercarriage cameras to see if you're hung up on anything.

- "Watts to Freedom" mode, a ripoff of Cleetus McFarland, just a drag race mode.

- Removeable ceiling (leaving the B-pillar and a T back, but nothing between A and B pillars). Stores in frunk.

- Headlights that visually indicate charging level via segmented lighting.

- Adjustable high-clearance mode.
 

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Discussion Starter · #3 ·
Yeah, I generally skipped the features that were not EV-specific, although I missed the goofy headlight bar graphs, and didn't mention the obvious "showoff" mode.

The rear tires don't steer that much - the four wheel steering (4WS) section starts at 4:38, and he says in the crab mode part that the rears can only steer 10 degrees or so, which is typical of 4WS systems. Crab mode could be handy, but few of many 4WS road vehicles made so far bother to include it. GM themselves did 4WS on a previous truck - the Quadrasteer option for full-size pickups of 2002 to 2005 - and even though the 4WS system was entirely electronically controlled (no mechanical connection to the front steering), they didn't provide any manual tweaking or mode selection and just set rear steer angle based on front steer angle and road speed. Quadrasteer was capable of up to 15 degrees steering angle - a lot more than the Hummer EV allows.
 

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The rear tires don't steer that much - the four wheel steering (4WS) section starts at 4:38, and he says in the crab mode part that the rears can only steer 10 degrees or so, which is typical of 4WS systems. Crab mode could be handy, but few of many 4WS road vehicles made so far bother to include it. GM themselves did 4WS on a previous truck - the Quadrasteer option for full-size pickups of 2002 to 2005 - and even though the 4WS system was entirely electronically controlled (no mechanical connection to the front steering), they didn't provide any manual tweaking or mode selection and just set rear steer angle based on front steer angle and road speed. Quadrasteer was capable of up to 15 degrees steering angle - a lot more than the Hummer EV allows.
I've not researched on trucks as much as I have cars, so actually the 10 degrees was quite a surprise for me.

Most of the cars with 4WS today have the rear steer about 3-5 degrees, while newer ones are starting to get more angle (e.g. 2021 Mercedes Benz S-Class).

Well, now I know even 15 degrees was done 15 years ago. I learn something new everyday!
 

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Discussion Starter · #5 ·
Most of the cars with 4WS today have the rear steer about 3-5 degrees, while newer ones are starting to get more angle (e.g. 2021 Mercedes Benz S-Class).
True, I should have checked my numbers - 10 degrees is actually high compared to most on-road systems. :)

4WS was popular briefly starting in the late 1980's, then vanished, and has come back more recently.
 

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The rear tires don't steer that much - the four wheel steering (4WS) section starts at 4:38, and he says in the crab mode part that the rears can only steer 10 degrees or so, which is typical of 4WS systems. Crab mode could be handy, but few of many 4WS road vehicles made so far bother to include it. GM themselves did 4WS on a previous truck - the Quadrasteer option for full-size pickups of 2002 to 2005 - and even though the 4WS system was entirely electronically controlled (no mechanical connection to the front steering), they didn't provide any manual tweaking or mode selection and just set rear steer angle based on front steer angle and road speed.
My 02 has 3 modes, selectable on the fly (though the rear and front wheels have to be centered for it to switch):

a) 2 wheel steer
b) 4 wheel steer which is speed dependent
c) 4 wheel trailering mode

The truck crabs during lane changes at highway speed in trailer mode, which is interesting when you intentionally try to "crack the whip", in ice skating parlance. The truck's tail does not fishtail out, the trailer merely follows the hitch ball. Was messing with abrupt lane changes, pulling a 3,000 lb dump trailer, and it was like the trailer wasn't even there. Hopefully. they bring Qsteer back with the eSilverado and its GMC twin.

Low speed crab would be possible on the Quadrasteer with a control module reflash...how to get the source code, lol?
 

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Discussion Starter · #8 ·
If the truck has 35 inch tall tires, and a top speed of (as an example guess) 150 km/h, then
maximum wheel speed = 41.7 m/s / pi*0.89 m = 14.9 rev/s = ~900 RPM​
maximum rear motor speed = 900 RPM * 10.5 = 9,450 RPM​
maximum front motor speed = 900 RPM * 13.3 = 11,970 RPM​

Those are reasonable motor speeds, but they're only approximations for a guess at a top speed.

We could also speculate on the reasons for the different ratios front and rear (despite these being identical motors and both truck-specific drive units), or whether we should actually believe the ratio information (which is through Fenske, not direct from GM).
 

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Discussion Starter · #9 ·
Now that I think about it, it might crab at low speed in trailering mode. I'll get back to you on that one.
I haven't looked it up yet, but behaviour might be different in forward and reverse, although that would be problematic when switching between them with the front wheels not straight ahead. Also, desired behaviour may be different between towing with a hitch at the bumper (conventional trailer) or over the axle (fifth-wheel/gooseneck).
 

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Gooseneck towing is prohibited on Quadrasteer iirc. It's only a half ton truck, though there are some very very rare 3/4 ton HD ones out there. You also aren't supposed to put oversize rubber on it or lift it.

The steering motor can't handle it. It has a monster spring in the mechanism to recenter on power loss, so it's fighting both the spring and rolling resistance (as well as any friction in the kingpins) when stopped.

It won't switch modes unless the wheels are straight ahead, which can be a real pain at times.
 

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Discussion Starter · #12 ·
That model and brand of tires are speed rating Q (99 MPH), so your numbers are pretty darned close.

GM's lawyers have speed limited pickups to 95MPH in the past, so you might even have nailed it.
Interesting... I just picked that speed as all that is needed on North American highways, and consistent with other EVs. Good to know that it's probably a reasonable guess. :)
 

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I'm still scratching my head on the differing f/b drive ratios. May be a motor efficiency cheat to where there are two drive cycle speeds they are targeting?
My guess is the rear drive ratio is tuned for max launch torque (without overspeeding at max vehicle speed), while the front is tuned for efficiency at highway speeds.

Somewhat similar to the Tesla dual motor system I guess? I can't find the article from way back in 2014 when they first released the 85D on the model S, but I recalled they said something like this.
 

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Discussion Starter · #15 ·
The ratio difference could be for motor use optimization; however, I suspect that like the Tesla Performance models, it's just accidental...

At Tesla, the difference is the result of using a small front drive unit (designed for use with the small rear drive unit having the same reduction ratio) with the large rear drive unit (with a different motor and ratio, borrowed from the RWD variant).

In the Hummer EV, perhaps the front gearbox is shared with other coming non-truck models that are geared for a higher top speed.
 

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At Tesla, the difference is the result of using a small front drive unit (designed for use with the small rear drive unit having the same reduction ratio) with the large rear drive unit (with a different motor and ratio, borrowed from the RWD variant).

In the Hummer EV, perhaps the front gearbox is shared with other coming non-truck models that are geared for a higher top speed.
Yes this might be a fair reason as well, since the front gearbox is different (i.e. includes a differential) compared to the rear gearbox, and GM wouldn't want to spend millions making a specific gearbox with the same ratio as the rear just for 1 EV model
 
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