[Zak]

Most AWD systems do not have the ability to vector torque like that. They're usually based on mechanical limited-slip differentials that require some amount of slip before they partially lock, and sometimes the limited slip is only between front and rear, not left and right. There are different types, with some requiring a lot of slip before they lock up and others requiring little.

Limited slip differentials cost more than open differentials. Limited-slip differentials that lock up quickly cost more than those that allow a lot of wheelspin. Electronically-controlled torque-vectoring differentials cost yet more.

The system you describe seems to meet the NPS definition of 4WD someone linked elsewhere: "a means to mechanically power both front and rear wheels at the same time", though I wonder if there might be some more technical regulation with specific requirements. I agree that sort of thing could work well for off-road use.

[jofer]

NPS defines 4WD specifically as part time four wheel drive with a transfer case and low range, FWIW. Low range is a big part of it. And yes, their definition technically excludes higher end "full time 4WD" systems in some cases, though I suspect everyone would look the other way at those.

[Zak]

A quick web search didn't find a formal definition, only information pages for specific parks with descriptions. I agree it's likely park rangers would use common sense in practice such that a vehicle with an extremely low first couple of gears, lockable differentials, appropriate clearance, and suitable tires wouldn't get cited as "not 4WD" because it doesn't have a selectable 2WD mode.

Incidentally, I once owned a Subaru from the 1980s which had a lockable center differential and separate high/low gearshift, which was synchronized and could be shifted in motion. It was not designed for serious off-road use, illustrating the folly of relying on criteria like these.

[jofer]

https://www.nps.gov/cany/learn/management/compendium.htm

> High Clearance Four-Wheel-Drive (4WD) Vehicles

> A Jeep, sport utility vehicle (SUV), or truck type with at least 15-inch tire rims and at least eight inches of clearance from the lowest point of the frame, body, suspension, or differential to the ground. Four wheel drive vehicles have a driveshaft that can directly power each wheel at the same time and a transfer case that can shift between powering two wheel or four wheels in low or high gear. All wheel drive (AWD) vehicles do not meet this definition.

I completely agree they'll use their discretion, but either way, that definition is specifically a part time 4WD system with low range.

[Hamuko]

>NPS defines 4WD specifically as part time four wheel drive with a transfer case and low range, FWIW.

Does the Porsche Cayenne qualify as 4WD? Because as far as I know, at least the early models have both, even though I think Porsche calls it AWD.

[jofer]

Not by that definition, no.