# Hummer EV video tour



## MattsAwesomeStuff (Aug 10, 2017)

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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## brian_ (Feb 7, 2017)

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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## ishiwgao (May 5, 2011)

brian_ said:


> 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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## brian_ (Feb 7, 2017)

ishiwgao said:


> 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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## remy_martian (Feb 4, 2019)

brian_ said:


> 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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## remy_martian (Feb 4, 2019)

Now that I think about it, it might crab at low speed in trailering mode. I'll get back to you on that one.


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## brian_ (Feb 7, 2017)

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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## brian_ (Feb 7, 2017)

remy_martian said:


> 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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## remy_martian (Feb 4, 2019)

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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## remy_martian (Feb 4, 2019)

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.


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## brian_ (Feb 7, 2017)

remy_martian said:


> 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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## remy_martian (Feb 4, 2019)

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?


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## ishiwgao (May 5, 2011)

remy_martian said:


> 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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## brian_ (Feb 7, 2017)

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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## ishiwgao (May 5, 2011)

brian_ said:


> 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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