AWD/Full-Time/Centre-Diff vs Part-Time 4WD in greasy snow?

[carbon60]

I currently drive an 80.

I'm wondering if there is any real performance difference at highway speed, in snow or on icy patches, between a vehicle with a centre diff (like my 80 or my previous Impreza or a 4Runner) and a vehicle with part-time 4WD (like most American pickups and the Tacoma). Basically, I'm looking to remove that factor from my personal vehicle choice grid, if it is unimportant.

Thanks,

A.

 

[AxleIke]

My personal experience has been that my 4runners (all part time 4wd with a standard transfercase) are more squirrely in the snow than an AWD Subaru.

However, that isn't totally fair, since the 4runners have been lifted, with larger tires, whereas the Subaru was a 96 Outback with All season tires. Totally different dynamics.

Friends with 80 series seem to handle better than my 4runner, but again, its a much heavier truck.

I simply drive slower. I always take a new vehicle out to an empty parking lot when the snow comes, and figure out how it slides, and when it slides. With the 4 runner, I drive it slow enough that I don't have an issue.

 

[Aquafbody]

I've driven both and I honestly can't say I have noticed a significant. For example my 78 Cherokee (center diff AWD) and my 01 S-10 (transfer case 4WD) are both very good in snow, what I have noticed is that the Cherokee with open diff's and pizza cutter tires has a little more issues in the deep stuff than the S-10 and it's LSD.

 

[libarata]

How fast can you drive in a locked in 4x4? As far as I remember, my Frontier could not go past 45-50ish without risking damage. Full-time AWD wont require that you anticipate when you need to move power around, or activate certain switches and knobs.

 

[Franco]

carbon60, you mentioned highways speeds, so are you more concerned with a vehicle that is safe to stop or control in the snow or about getting traction to get going in the snow?

I haven't had too much experience with traction in really deep snow, although a good set of tires and possibly chains seem to be paramount.

If you're talking about a vehicle stopping and turning safely at higher speeds, then don't get too caught up in the drivetrain. Stopping will be the same since all vehicles have brakes at all four corners, what you really need is a good set of snow tires. It's really the same deal with turning, though a lower, lighter vehicle will be easier to control. A good example being the aforementioned Subaru.

 

[DaveInDenver]

I've had a few part time 4WD trucks, currently transitioning from my trusty 1991 to a newer 2008 Tacoma. The main difference between these two is that the '91 has manual hubs and manual shift t-case and the '08 the automatic front end and electric t-case mechanism. Once in 4WD they are for all intents and purposed the same thing. We had a 2005 4Runner V6 with the selectable transfer, 2WD/Auto/High/Low 4WD. I think the mode you're asking about is the automatic 4WD or in the 80 series the unlocked center diff state.

I found on consistent snow the selectable 4WD and auto did about the same as a part time case in 4WD. If did think regular modes (e.g. locked center diff) was more consistent, but that's maybe a personal bias. The center diff on slippery snow seemed to me to be constantly trying to find it's happy place. This to my mind was because being a fluid coupled arrangement it's going to try to compensate based on how much torque is being input, so it's coupling will change with throttle. The locked center diff or regular 4WD high feels to me like the center of mass is right in the middle of the truck rather than one end pushing or pulling. The sensation of a rear end pushing was still there in my mind in auto 4WD.

However on mixed conditions, like plowed pavement, the automatic mode was better. The Tacoma has electric shift and to me this is the worst of all options. This is because the shift in and out is not super quick, so I tend to turn it on sooner and leave it in 4WD longer than with my 1991. With the hubs locked on the '91 I can just flick the transfer case in and out and get almost the same effect as the automatic t-case in the 4Runner gave me. It was of course completely done on its own and probably a bit better as it gave a front/back bias other than 50/50. But with decades of 4WD pickup in snow experience I am, if I may be so bold, not too bad. If I didn't I could see liking the auto mode a lot. You just drive and it never really feels like it's going to fishtail.

librata, Toyota in my book recommended 50 MPH as the top end to engage 4WD, but there's nothing to prevent at any speed. With newer trucks like the Tacoma I think the computer does use vehicle speed and won't allow the shift. But my old truck is cable speedometer and the computer has no control over the drivetrain. Now there is a mechanical limit when spinning all the front end stuff, the truck just doesn't like trying to go >50MPH with the hubs locked and I've never tried shifting between 2WD and 4WD at these speeds. I figure if conditions have improved the point that I can go >50MPH then I will unlock the hubs and go.

 

[libarata]

The worry about going to fast, is uneven tire wear, and overheating/binding your transfer cases. The fastest I ever too my Frontier, was 35. My Forester though, I have gone as fast as 75 in deeper snow

 

[DaveInDenver]

I could see binding as possible with a part time system, but I would have a hard time knowing since mine tends to wind up pretty bad. After a while of even in town snow driving I have to kick the transfer case by dumping the clutch in reverse to get the stick back into 2WD. I've never really even drive in 2WD beyond about 60MPH with the hubs locked. Things tend to vibrate and howl.

 

[libarata]

And that will be your largest difference between Fulltime, and part-time. Part time is meant more for slower speeds, and is certainly not highway safe.

 

[AxleIke]

I have driven both my old 4runner (manual hubs, gear drive tcase) and my new one (ADD with a chain drive case) at 65 in 4wd many, many times. The old truck would drive at 65- 70 with the hubs locked, but 2wd on the dry highway, as I would usually just keep the hubs locked in the winter time, whether it was snowy or not.

You will only overheat or bind the case if you drive in 4wd on a high traction surface and go around bends or turns (dry pavement, etc..). On a dirt road in the desert, you can drive as fast as you care to and not have any issues with the tcase.

 

[p nut]

At highway speed, no difference. I've had a 3rd Gen 4Runner and LC (both AWD/FT4WD--whatever you want to call them), another 3rd gen and Tacomas with PT 4WD.

By the way, the Push Button 4WD found on some 3rd Gen 4Runners indicated a max speed of 60MPH (I think), where as the J-Shift Tacomas were 40MPH.

 

[Herbie]

A vehicle you might want to look at just from a data-collection standpoint is the Astro/Safari van.

Depending on the year of manufacture, the AWD models were equipped with either a viscous-coupled (mechanical) center diff (early vans), or an electronic actuated center diff (later vans) that used a shift motor to basically lock and unlock the diff in response to inputs from the ABS sensors, etc. Additionally, there is a statistically-significant population of Astro/Safaris running around with true locked transfer cases (most of us having done so for the purposes of gaining low-range, etc.)

Generally, the consensus amongst the community is this:

The viscous-coupled transfer case offers the best performance on snow/ice and other low-friction surfaces. The supposition is that because the VC is biasing the center diff along a continuum of torque, there's generally a smoother engagement and smoother behavior which translates to better road manners. Anyone with a Subaru can attest to this behavior because generally Subaru achieves the same effect (although through a variety of methods - either a viscous coupler or an electronically-controlled clutch pack that pulses with high frequency) of biasing the torque front to back along a continuum.

The GM electronic transfer cases do not contain a clutch mechanism, rather they simply engage to 100% locked (50%/50% torque distribution) and back to unlocked (100% rear torque) on and off. This happens very quickly, but does not happen as fast as Subaru's clutch pack, so you don't get as smooth an engagement. That said, this system is still quite manageable in slippery conditions. However, owners who've had both in the right conditions do indicate the viscous behavior is better.

Finally, there's those of us who've modified to a fully locked TC, and that seems to be a mixed bag. On the one hand, it's always engaged (within the span of a few seconds or minutes, at least) so there's no "change" in behavior mid-corner. On the other, this setup seems to be somewhat more susceptible to other influences, like the behavior of the rear differential. Those running a locked 4wd TC, along with an "automatic" locking rear diff, like a Detroit Locker, etc., report borderline scary behavior whereby the van will "push" mid-corner if power is not applied smoothly by the driver. This is a more common scenario than I'd like, since there's no selectable locker available for the Astro/Safari's 7.625" rear. Those with limited-slip (Eaton clutch or G80/Gov-Lock types) report more "tame" behavior under the same conditions.

In Summary: AWD and full-time center diffs DO seem to have a behavior advantage on the slippery stuff. Behavior of a fully-locked part-time transfer case may be tolerable, or scary, depending on a variety of other factors.

 

[Martinjmpr]

At highway speed, no difference. I've had a 3rd Gen 4Runner and LC (both AWD/FT4WD--whatever you want to call them), another 3rd gen and Tacomas with PT 4WD.

By the way, the Push Button 4WD found on some 3rd Gen 4Runners indicated a max speed of 60MPH (I think), where as the J-Shift Tacomas were 40MPH.

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That's the maximum speed at which the t-case should be engaged/disengaged. There is no maximum speed once it's engaged.
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Not sure if that's what you meant but I wanted to clarify because I know there are people on the T4R.org message board who seem to think you can't go faster than 55mph if you're in 4wd and of course that's not true.
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I really like the multi-mode 4wd on my 4runner. It's nice because when there are intermittent patches of ice on an otherwise dry road I don't have to choose between trying to drive in 2wd and risking loss of traction, or drive in 4wd and risk driveline binding. If it looks "iffy" I just go ahead and put it in 4wd, center diff unlocked and it's basically an AWD car.
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To be fair, when I had my 3rd gen with conventional 4wd, I got pretty adept at shifting in and out of 4wd while driving. Of course, that had a by-God lever and not an electronic gizmo.
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I do notice the traction control from time to time. If I take a curve on a slippery surface sometimes it will beep and I'll slide a bit. If it gets too annoying (like when driving in deep snow) I'll just lock the center diff and then I don't have to worry about it.

 

[zidaro]

we own (my wife) a '10 limited runner with the Torsen full time center diff. I also live in E. Sierra snow country, we get nasty deep snow and also icy roads. My wife has to be reminded how to put her runner into 4wd. I am saying that the full time center AWD mode is so good that she NEVER uses 4wd on a daily drive around town basis. traction on ice, around corners, or up our driveway with 3' of snow is exceptional.

I also have a '10 Tundra. I must use 4wd to drive up my driveway anytime there is snow on it. I am constantly putting it into and taking it out of 4x mode when the roads are "somewhat" covered. Otherwise it will bind- not good. Now- i JUST added a rear Auburn LSD and am excited cause this last storm was a different story, i know have much better traction in 2wd.

edit: to comment on above. I have had no issue driving in 4w4 on fully snow covered highway at highway speeds, regularly. But- be aware just cuz you can haul *** does not mean you can stop. I spend my days at work scraping people off the road that do not know their limits or their vehicles. Don't be one of THEM
By necessity, we drive our ambulances in 4x4 on the highway all winter when warranted

 

[AxleIke]

Must be system/vehicle/wheelbase dependent. My 4runner will only "bind" or "crab" when making a hard (full lock) turn into my garage. I shift out of 4wd for that. I've run it for days in 4wd and never felt it binding up like you guys are talking about.