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This article says a bit about the differences: https://www.caranddriver.com/features/a27630736/awd-vs-4wd/ https://www.caranddriver.com/features/a27630736/awd-

This article says a bit about the differences: https://www.caranddriver.com/features/a27630736/awd-vs-4wd/ https://www.caranddriver.com/features/a27630736/awd-vs-4wd/ The primary reason there's a 4WD vs AWD distinction is the ability to lock the differentials, and that determines the mechanisms that the wheels are driven by. And that's why they use those terms in a prescriptive law context. to simplify: AWD -> power gets to all wheels (driven wheels) but wheels are usually allowed to spin at different speeds to facilitate pavement driving and turning. 4WD -> power gets to all wheels, but there is a locking mechanism that locks all wheels so they all turn at the same speed - if you know the physics of turning objects, it means turning while underway is either impossible or damaging to the drivetrain. This is however, useful in mud or rock, or challenging terrain. When on mud or rock or whatnot, you turn, move a bit, stop, check, turn a bit, move, stop, check, turn a bit, movie, stop, etc. So the fact that all wheels turn at the same time is an advantage. The reason some 4WD don't have a "differential lock" is that it's fused by bare metal - it doesn't need a lock because it's welded together, so no lock needed! The car and driver article gives an easy test: if the car manual tells you "this is bad for road driving" - that's 4WD. you might think "well 4WD is terribly useless for regular driving" and you would be entirely correct. That's why most 4WD cars incorporate both modes of driving mechanisms so they can switch. You can tell apart a permanent 4WD / full time rock climber by the owner needing to use a trailer to move the vehicle instead of driving it over the highway, usually they use a pickup truck or SUV with a regular drivetrain to haul gear and pull the trailer with the 4WD vehicle to the trailhead, unload, then go into places like Death Valley Back Country or Moab with the 4WD vehicle.

loeg 2y ago
There are many 4WD vehicles that don't have locking or even limited slip diffs, though. > 4WD -> power gets to all wheels, but there is a locking mechanism that locks all wheels so they all turn at the same speed This isn't universally true of 4WD vehicles. E.g., https://www.reddit.com/r/overlanding/comments/61yyam/4x4_without_lockers/ https://www.reddit.com/r/overlanding/comments/61yyam/4x4_wit... or https://www.quora.com/Do-all-4WDs-have-lockers https://www.quora.com/Do-all-4WDs-have-lockers
JumpCrisscross 2y ago
> but wheels are usually allowed to spin at different speeds to facilitate pavement driving and turning To drive home why NPS cares about this, the way the car detects how fast the wheel should spin is by detecting slippage. But every time that happens, you're shredding up the road. That turns AWD vehicles into pothole machines. I drive a Subaru and I drive it off road. I would feel comfortable driving it in most off-road conditions, from a safety perspective, but I won't because I'm fucking up the trail.
acdha 2y ago
I’d add that from the park service’s perspective this is pretty simple. They don’t want to have people getting stuck in the backcountry or damaging trails, and as an agency with limited staffing, millions of visitors, and huge territory to protect it’s understandable that they do not want some complicated argument with every wannabe rules lawyer who thinks they can drive like they saw in the commercial. “Only 4WD” is a policy that doesn’t require training thousands of rangers around the country about which models are considered acceptable.
3np 2y ago
> 4WD -> power gets to all wheels, but there is a locking mechanism that locks all wheels so they all turn at the same speed - if you know the physics of turning objects, it means turning while underway is either impossible or damaging to the drivetrain. Is my understanding of your usage of "underway" right if you're saying that the only recommended and safe way to use the steering wheel on a 4WD is when at standstill? That you can't/shouldn't turn at all when in motion?
- loeg 2y ago
  > you're saying that the only recommended and safe way to use the steering wheel on a 4WD is when at standstill? Or on loose surfaces (the intended use). But yeah, you absolutely should not drive on paved roads with locked diff 4WD engaged.
- barkingcat 2y ago
  On pavement, yes, turning while moving (on pavement) by definition damages a 4WD engaged / locked differential drivetrain and tires, because the system is forced to move all wheels at the same speed, thus either the drivetrain takes the force, or more commonly, the wheels skid (shedding tire material), which leads to unsafe driving conditions on the road (imagine turning the wheel and your whole car starts skidding). The benefit of 4WD is when there are loose surfaces (like on gravel) and you actually want all wheels engaged like if 2 or even 3 wheels are off the ground. during 4WD rock crawling the tires do get ripped to shreds very quickly, but you get the advantages of being able to climb almost vertical rockfaces and drive through rough terrain. Through all this drivetrain talk, I hope you understand why Park Services would want to enforce 4WD - because AWD and 4WD are fundamentally set up to do entirely different things. And it's not right to say "4wd and awd are the same"

loeg 2y ago

There are many 4WD vehicles that don't have locking or even limited slip diffs, though. > 4WD -> power gets to all wheels, but there is a locking mechanism that locks all wheels so they all turn at the same speed This isn't universally true of 4WD vehicles. E.g., https://www.reddit.com/r/overlanding/comments/61yyam/4x4_without_lockers/ https://www.reddit.com/r/overlanding/comments/61yyam/4x4_wit... or https://www.quora.com/Do-all-4WDs-have-lockers https://www.quora.com/Do-all-4WDs-have-lockers

JumpCrisscross 2y ago

> but wheels are usually allowed to spin at different speeds to facilitate pavement driving and turning To drive home why NPS cares about this, the way the car detects how fast the wheel should spin is by detecting slippage. But every time that happens, you're shredding up the road. That turns AWD vehicles into pothole machines. I drive a Subaru and I drive it off road. I would feel comfortable driving it in most off-road conditions, from a safety perspective, but I won't because I'm fucking up the trail.

acdha 2y ago

I’d add that from the park service’s perspective this is pretty simple. They don’t want to have people getting stuck in the backcountry or damaging trails, and as an agency with limited staffing, millions of visitors, and huge territory to protect it’s understandable that they do not want some complicated argument with every wannabe rules lawyer who thinks they can drive like they saw in the commercial. “Only 4WD” is a policy that doesn’t require training thousands of rangers around the country about which models are considered acceptable.

3np 2y ago

> 4WD -> power gets to all wheels, but there is a locking mechanism that locks all wheels so they all turn at the same speed - if you know the physics of turning objects, it means turning while underway is either impossible or damaging to the drivetrain. Is my understanding of your usage of "underway" right if you're saying that the only recommended and safe way to use the steering wheel on a 4WD is when at standstill? That you can't/shouldn't turn at all when in motion?

loeg 2y ago
> you're saying that the only recommended and safe way to use the steering wheel on a 4WD is when at standstill? Or on loose surfaces (the intended use). But yeah, you absolutely should not drive on paved roads with locked diff 4WD engaged.
barkingcat 2y ago
On pavement, yes, turning while moving (on pavement) by definition damages a 4WD engaged / locked differential drivetrain and tires, because the system is forced to move all wheels at the same speed, thus either the drivetrain takes the force, or more commonly, the wheels skid (shedding tire material), which leads to unsafe driving conditions on the road (imagine turning the wheel and your whole car starts skidding). The benefit of 4WD is when there are loose surfaces (like on gravel) and you actually want all wheels engaged like if 2 or even 3 wheels are off the ground. during 4WD rock crawling the tires do get ripped to shreds very quickly, but you get the advantages of being able to climb almost vertical rockfaces and drive through rough terrain. Through all this drivetrain talk, I hope you understand why Park Services would want to enforce 4WD - because AWD and 4WD are fundamentally set up to do entirely different things. And it's not right to say "4wd and awd are the same"