4 ms·

I still dont understand the difference between 4wd and awd, but the article spends more or less the entire time talking about locking and limited slip different

by olliej 2y ago

I still dont understand the difference between 4wd and awd, but the article spends more or less the entire time talking about locking and limited slip differentials and how AWD generally don’t have them which is “why they’re not allowed”, while also acknowledging that just being 4wd doesn’t mean there’s any kind of diff lock either. If the actual reason ia the difference in how the diff is locked, then access should be restricted on the basis of that, not on the mechanism the wheels are driven. I assume that the actual reason for the 4wd only rule is because there is a real difference in how they work?

barkingcat 2y ago
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
I think you're correct -- there's not any functional benefit to 4WD (vs AWD) without locked diffs. The signage should probably demand locked diffs.
- chung8123 2y ago
  Plus AWD can have locking differentials.
- alamortsubite 2y ago
  "4WD" implies transfer case, rather than center diff.
  - loeg 2y ago
    This isn't a significant functional benefit by itself.
    - alamortsubite 2y ago
      Yes, it is for part-time transfer cases (again, as opposed to center differentials).
hnav 2y ago
Serious offroad vehicles traditionally don't have a center diff and instead let you lock the axles together by putting the transfer case into 4hi or 4lo (for extra torque multiplication) which means that you need to lift at least a front and a rear wheel off the ground before getting stuck. Front/rear locking diffs further reduce the odds of getting stuck. Crossovers typically have a FWD drivetrain with an appendage that sends power to a rear axle that contains an electromechanically actuated or viscous coupling which is very similar. The coupling is actually in a way similar to a transfer case but takes wheelspin to kick in which sucks offroad. Then some trucks and sporty cars use an electromechanically actuated transfer case that sends power forward. The Mercedes G-class and Humvee have a center diff so are technically full time AWD but then they also have a reduction gear as well. Basically it's hard to classify vehicles' offroad capability so the copout is using 4WD to mean real offroad and AWD to mean "get you unstuck from a couple inches of snow".
- dboreham 2y ago
  Quick note from a place with feet of snow to say that AWD vehicles work well here. People either drive trucks or Subarus and I don't see more of one kind stuck in snow than the other. Obviously winter tires required.
  - Hamuko 2y ago
    Yeah, I have pretty good experiences with AWD vehicles like xDrive BMWs or a Nissan Qashqai in snow. The Qashqai even performs pretty well in deep snow if you use the differential lock.
SkyPuncher 2y ago
4WD drive means all tires spin at the exact same speed, no matter what. AWD drive means each tire can rotate at a different speed. Tires with less grip will spin faster. Notably, if a tire on an AWD car is in the air, only that tire will spin. You won’t move anywhere because all of the force is expelled through the free wheeling tire. 4WD solves this issue by essentially locking everything together. Great for unsticking yourself, but terrible for every day driving. Things like turning into a parking spot can cause binding since wheels on the outside of the turn need to travel a longer distance than those on the inside of a turn
- olliej 2y ago
  Ah, so AWD vs 4WD is _definitionally_ the presence or lack of some form of locking differential? e.g if I were to take an AWD and weld the diff it would be 4WD (and not be too fun for daily driving)?
  - SkyPuncher 2y ago
    Practically speaking, yes. The whole conversation is a bit tough because there are a LOT of blurred lines. Some 4WD systems can act like AWD systems. Some AWD systems can act like 4WD systems.
- aranchelk 2y ago
  > 4WD drive means all tires spin at the exact same speed, no matter what. Make sure you only drive in a straight line.
  - SkyPuncher 2y ago
    Yes! You end up with binding when you turn. At best, the wheels either slip or "crow hop". At worse, you can destroy the transmission.
- dingaling 2y ago
  > Notably, if a tire on an AWD car is in the air, only that tire will spin On the contrary, here's a demonstration of the opposite. Subaru AWD, three wheels without grip, power goes to the wheel with grip. https://m.youtube.com/watch?v=mryGnENTsoI https://m.youtube.com/watch?v=mryGnENTsoI

by olliej 2y ago

barkingcat 2y ago

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

I think you're correct -- there's not any functional benefit to 4WD (vs AWD) without locked diffs. The signage should probably demand locked diffs.

chung8123 2y ago
Plus AWD can have locking differentials.
alamortsubite 2y ago
"4WD" implies transfer case, rather than center diff.
- loeg 2y ago
  This isn't a significant functional benefit by itself.
  - alamortsubite 2y ago
    Yes, it is for part-time transfer cases (again, as opposed to center differentials).

hnav 2y ago

Serious offroad vehicles traditionally don't have a center diff and instead let you lock the axles together by putting the transfer case into 4hi or 4lo (for extra torque multiplication) which means that you need to lift at least a front and a rear wheel off the ground before getting stuck. Front/rear locking diffs further reduce the odds of getting stuck. Crossovers typically have a FWD drivetrain with an appendage that sends power to a rear axle that contains an electromechanically actuated or viscous coupling which is very similar. The coupling is actually in a way similar to a transfer case but takes wheelspin to kick in which sucks offroad. Then some trucks and sporty cars use an electromechanically actuated transfer case that sends power forward. The Mercedes G-class and Humvee have a center diff so are technically full time AWD but then they also have a reduction gear as well. Basically it's hard to classify vehicles' offroad capability so the copout is using 4WD to mean real offroad and AWD to mean "get you unstuck from a couple inches of snow".

dboreham 2y ago
Quick note from a place with feet of snow to say that AWD vehicles work well here. People either drive trucks or Subarus and I don't see more of one kind stuck in snow than the other. Obviously winter tires required.
- Hamuko 2y ago
  Yeah, I have pretty good experiences with AWD vehicles like xDrive BMWs or a Nissan Qashqai in snow. The Qashqai even performs pretty well in deep snow if you use the differential lock.

SkyPuncher 2y ago

4WD drive means all tires spin at the exact same speed, no matter what. AWD drive means each tire can rotate at a different speed. Tires with less grip will spin faster. Notably, if a tire on an AWD car is in the air, only that tire will spin. You won’t move anywhere because all of the force is expelled through the free wheeling tire. 4WD solves this issue by essentially locking everything together. Great for unsticking yourself, but terrible for every day driving. Things like turning into a parking spot can cause binding since wheels on the outside of the turn need to travel a longer distance than those on the inside of a turn

olliej 2y ago
Ah, so AWD vs 4WD is _definitionally_ the presence or lack of some form of locking differential? e.g if I were to take an AWD and weld the diff it would be 4WD (and not be too fun for daily driving)?
- SkyPuncher 2y ago
  Practically speaking, yes. The whole conversation is a bit tough because there are a LOT of blurred lines. Some 4WD systems can act like AWD systems. Some AWD systems can act like 4WD systems.
aranchelk 2y ago
> 4WD drive means all tires spin at the exact same speed, no matter what. Make sure you only drive in a straight line.
- SkyPuncher 2y ago
  Yes! You end up with binding when you turn. At best, the wheels either slip or "crow hop". At worse, you can destroy the transmission.
dingaling 2y ago
> Notably, if a tire on an AWD car is in the air, only that tire will spin On the contrary, here's a demonstration of the opposite. Subaru AWD, three wheels without grip, power goes to the wheel with grip. https://m.youtube.com/watch?v=mryGnENTsoI https://m.youtube.com/watch?v=mryGnENTsoI