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Comment by olliej | Hacker News Reader

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?

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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".

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.

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.

This article says a bit about the differences:

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.

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_wit... or https://www.quora.com/Do-all-4WDs-have-lockers

> 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.

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.

> 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?

> 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.

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"

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

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)?

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.

> 4WD drive means all tires spin at the exact same speed, no matter what.

Make sure you only drive in a straight line.

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.

> 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

That's because Subaru does torque vectoring. Most of these use the brakes to fight the natural tendencies of AWD systems.

And yet a Subaru was the car cited in the article as not being compliant with their regulations... it's just clear that the National Park Service doesn't understand how cars work.

I'm not sure there's anything wrong with the rule. It's easy to understand by everyone and doesn't require any additional level of testing of certification.

Realistically, people who regularly drive on these types of trails want a proper 4WD system. Not just for the "locking diff", but for the other capabilities that tend to come with a "proper" 4WD car.

Beyond "hard" capabilities, AWD torque vectoring tears roads up. The torque vectoring essentially requires the tires to spin before it will kick in. That doesn't happen in a "locked" 4wd system.

Please just stop. Where are you getting your information?

Maybe some crappy AWD systems have damaged roads at some point, I will give you the benefit of the doubt, but most do not. I've honestly never even heard of this.

4WD absolutely loses traction on roads if you leave the differential locked, and I'm sure if you amortized the extra weight and clueless drivers, 4WD does more damage then AWD.

I got rid of my 4WD truck for an AWD system specifically because the performance was better. It wasn't even close. Trucks suck compared to a Subaru in traction, and off roading for obvious reasons:

1. Trucks have no weight in the back most of the time.

2. Trucks have poor clearance.

3. Subaru's AWD is pretty danged good.

(I really didn't want to like Subarus, trust me, I looked at all the data though, and test drove everything)

Of course, none these vehicles compare to purpose built and custom off-road vehicles, but that's not what we are talking about here...

And yes, I have seen people fix the deficiencies of a truck, and have a nice cargo vehicle that climb cliffs, it's bad ass, but expensive and rare. Those trucks cost as much as a Ferrari.

And again, advanced AWD systems are more advanced and better than 4WD in every single way, and there is even more room for improvement with AWD systems. Why waste sending power down a shaft that has no traction?

Why wouldn't you want more control over more control surfaces of your vehicle?

I’ve owned multiple AWD and 4WD vehicles.

4WD will bind and hop, but it will not cause the spinning that happens with AWD. The binding and hopping really only occurs at extreme turning angles and is extremely minor.

Torque vectoring is very good, but it’s not perfect. It require slip to detect and react. The problem is you cannot detect slippage when a wheel is completely stopped, so you have to let it spin at least a little bit. This is what causes excessive single wheel rutting.

It can also create unpredictably since you need to wait for a wheel to spin before it can kick in. Notable, if you need a lot of power, this can cause and extremely harsh jerk that does not happen in a 4WD system.

Which vehicles have you taken off road?

I've done Toyota Landcruiser, Toyota Tundra, Subaru Ascent, Subaru Outback, Subaru Crosstrek, Chevy Silverado.

I don't do crazy cliff stuff on them, I take my dirt bikes and quads on gnarly terrain.

The Ascent has impressed the hell out of me. Two wheels off the ground at 30% grade (40% on one corner) getting up a winding switchback in the Smokey Mountains.

If I got a citation from the National Park Service, I'd be pretty danged annoyed, and lose a lot of respect for the sensibility of the organization.

This works in a controlled test. Unfortunately what you’ll find if you own one of these is that with a real obstacle you’d encounter off-road it’s not able to move enough torque to get you unstuck. These systems are good at maintaining some momentum if you have it in my experience. This is where a mechanical LSD or locking differentials make all the difference.

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.

Plus AWD can have locking differentials.

"4WD" implies transfer case, rather than center diff.

This isn't a significant functional benefit by itself.

Yes, it is for part-time transfer cases (again, as opposed to center differentials).

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