Center diff vs transfer case

[Diamondpilot]

I have an 03 chevy that has the "AUTO" as well. I agree, I am spart enough to decide.

The big thing I dont like about it, it has to see a difference in wheel speed from front to rear (slip) to engage. By that time, It may be too late.

Last winter while plowing, I decided to us it in my drive just to see what it was like. While it does detect wheel slip rather quickly, it might not be fast enough in a real spin-out situation on the road. And the other thing i dont like about it, once the rears are spinning and it decides to engage, everything is moving and under load. Engaging the fronts with the backs already spinning and under load cant be good on components. So I dont use it at all anymore.

This is the samething that happened to me. I was nearly in the ditch but had it corrected then the system decided to do its thing and it only made matters worse at that point. I now only buy full time AWD or just regular old 4X4

Chris

 

[Wis Bang]

I had a co-worker w/ an Olds Bravado w/ a full time AWD w/ traction control. The first fall he backed down the river bank to pull his dock. One wheel at a time spun until all four were buried in the sand. A neighbor w/ a Tahoe had to pull him out. AWD isn't always the answer.

Going back to my '62 Willys w/ Warn locking hubs, I use 4x2 until I get in trouble then I shift to 4x4 Snow covered roads needed 4x4 until the roads dried up, no shift on the fly in the old beast. That old CJ5 went sideways a few times on black ice...short wheelbases do that.

I'm glad my '05 Silverado is a 'work truck' and still has the floor shift 4x4 and posi traction instead of the auto setup.

 

[timswi]

I drive alot of country roads, interstates an ciity streets in my commute. I can honesly say that I am now a big fan of the auto 4wd given that I can go from full snow covered to dry and back again very quickly. The auto system works flawlessly. For off road use, deep snow and stuff like that, I do run it it full 4wd.

Also, the auto works good on wet tar and chip hills where that truck just spins in 2wd when trying to move from a dead stop and that's with a locking diff. PA loves to turn the local roads into skating rinks.

As for the Bravada example...Let's face it that was a supposed to be nothing more than a luxury vehicle and not an off roader.

As far as the center diff...I think it used to be called a viscus coupling. This is a memory from my import dealer days.

 

[dstig1]

A differential is essentially the same thing no matter where it appears on a vehicle: It allows the 2 output shafts to turn at DIFFERENT speeds.

The rear diff on all RWD vehicles allow the L&R wheel to turn at different speeds, such as around a corner.

A center diff allows the front and rear output shafts to turn different speed for corners, differences in tire size, whatever...

All vehicles will have a diff on any driven axles (FWD is at the front, RWD is at the rear and AWD/4wd will have one on both F & R axles) as you can't even really turn without one. You can "get away" without a center diff on 4wd vehicles as you are letting the road surface act as the "differential". You get tire slip from a loose surface to allow the tires to make up any difference F to R. Typically the difference in speeds F - R are not as dramatic as side to side. But this is not good to use for any period on a high traction surface like dry pavement as you get little slip. Then things break. True 4wd systems are generally considered off road only, or for other low traction surfaces (snowy roads) as they have no center differential.

And then you get into all manner of specialty diffs like viscous coupling, limited slip, lockers... These are common for center differentials on AWD vehicles.

 

[Ryan03]

AWD vehicles typicaly have a diffrential or fluid coupling in the transfer case. In the case of a diffrential in the transfer case, it can be a open type, like a power divider in a large class 8 tandem truck, or a clutch type diff. AWD vehicles typicaly can not equaly split torque between the front and rear drive axles, it usualy winds up being 70/30 or 60/40 etc. The exception to this rule is when the AWD transfer case has an inter axle diffrential lock mode, which couples the front and rear transfer case output shafts together to allow for a 50/50 torque split between the front and rear drive axles.

IMO, the AWD functions better for slick winter weather conditions, especialy when negotiating sharp corners, such as pulling into or out of a driveway or parking space. This is due to the fact that all wheel drive allows some varience between front and rear driveshaft speeds, which greatly reduces understeer and the kicking and bucking that you get with 4 wheel drive. On the flip side, in deep snow, mud, or pulling something out of a ditch etc. 4 wheel drive has the obvious advantage due to its ability to equaly tranfer torque to both axles under high load conditions.

 

[Diamondpilot]

These last two post were very informative and well written. Lots of good info guys. Thanks.

Chris

 

[4runner]

I agree with Chris, very helpful information. Thanks.

I understand the 50-50 torque split that occurs in a part time 4WD with only a transfer case. This same occurs when the center diff is locked on vehicles with a center diff.

In this way it seems like a 4wd with a lockable center diff is the best of both worlds.

Or is there a down side? Is there some reason/advantage to eliminating a lockable center diff? Is it just cost? Or does a center diff has limited ability to transfer torque? In 2002 the 4Runner had the lockable center diff. The current model 4Runner has a part time system on the SR5 version. And the Tundra only has a part time system.

Bob

 

[Diamondpilot]

To me the center diff thing with a locker is good for light vehicles and for casual, and I mean very casual use. Such as in something like a cross over SUV of small SUV for driving in adverse weather conditions, not off roading.

For pulling stumps, heavy trailers though the muck and mud, pulling stuck equipment from the ditch, ect its tough to beat a 2 range transfer case with good axles and real lockers. Owning three 4x4 trucks currently the one with lockers is hands down twice the vehicle that the other 2 are without in these type of condition's. When it also comes to 4 wheel drive a independent front suspension versus the solid front axle like that found in my 2 Ford Diesels is far superior. Its not as strong so its not good for all applications. Yes, the solid front axle is super strong and great for carrying a load like a snow plow but it has its downside also.

Once again, there is no perfect setup. You need to chose the right tool for the job and sometimes you have to sacrifice and get one that does 90% of what you want.

Chris

 

[Redneck in training]

I rented a GM SUV last winter. Forgot what it was but it had AWD system with a traction control that used ABS to prevent slippage of single wheel. I assume that it simply applied break to the wheel that had tendency to slip.
When disengaged I could do doughnuts on a snowy parking lot at will. When engaged it was near impossible to do so. Also climbing very steep hill on snow when disengaged I would get stuck not too far from the bottom. When engaged the car climbed up only with some hardly detectable slippage. In other words I was impressed how well it worked. The great thing about system using electronics is speed of response I suppose. I also read somewhere that it allows for an "active steering". When steering wheel is turned the system applies certain amount of breaking to the wheels inside of the turn.

 

[FlaDon]

I rented a GM SUV last winter. Forgot what it was but it had AWD system with a traction control that used ABS to prevent slippage of single wheel. I assume that it simply applied break to the wheel that had tendency to slip.
When disengaged I could do doughnuts on a snowy parking lot at will. When engaged it was near impossible to do so. Also climbing very steep hill on snow when disengaged I would get stuck not too far from the bottom. When engaged the car climbed up only with some hardly detectable slippage. In other words I was impressed how well it worked. The great thing about system using electronics is speed of response I suppose. I also read somewhere that it allows for an "active steering". When steering wheel is turned the system applies certain amount of breaking to the wheels inside of the turn.

Mom and dad bought a Pontiac Vibe AWD for those reasons. Last winter they were the only car to get out of the hotel parking lot after the massive storm in Dec. AWD does have its advantages if done right. The transfer case in it decides which way to send the power with 95/5 splits to 60/40 splits possible.