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4WD & AWD systems explained

1460 messages,  Last post on Sep 21, 2012 at 9:29 AM

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Messages Page 145 of 146

#1441 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 14, 2012 (9:06 am)

Replying to: xwesx (Aug 14, 2012 8:53 am)
VDC probably refers to stability control INCLUSIVE of TC, the 2008 timing would be about right for that happenstance.
 
Not knowing anything at all about the specifics of this particular "AWD" system in question the description, only the wheel with traction spinning freely, indicates a fully open center diff'l with the TC turned off.
 
"..were limited slip..."
 
But more likely than otherwise "simulated" limited slip using TC braking.
 
Sounds as if your '96 had some method to automatically "lock" the center diff'l or PTO, VC's were commonly used for that back then. Once TC was implemented those methods because useless, non-functional.

#1442 of 1460 Re: 2005 Outback AWD Failure [wwest] by xwesx

Aug 14, 2012 (10:02 am)

Replying to: wwest (Aug 14, 2012 9:06 am)
But more likely than otherwise "simulated" limited slip using TC braking.
 
No, they weren't, they were viscous LSD. After 2009, however, they did go to open front/rear differentials using the VDC system to compensate. It is a poor compensation, at best.
 
The WRX retains a rear LSD, while the STi has both front and rear LSD.

#1443 of 1460 Re: 2005 Outback AWD Failure [xwesx] by dcm61

Aug 14, 2012 (1:45 pm)

Replying to: xwesx (Aug 14, 2012 8:53 am)
In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that,
 
2000 (w/AWP) and 2001 to 2006 2.5L also had rear LSD. Not sure which years/models of H6 had rear LSD.

#1444 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 14, 2012 (9:32 pm)

Replying to: xwesx (Aug 14, 2012 10:02 am)
I know of a few high end marques that use VC(s) to implement a form of rear diff'l LSD but I suspect in Subbies case you mean center diff'l or PTO drive "locking" is done using a VC.
 
"..The WRX retains a rear LSD,..."
 
That, an actual mechanical LSD, would be a bit of foolishness since the current WRX model has no means for driving the rear diff'l except under TC braking, along with full engine dethrottling, of the front wheels. Probably not even then unless you use an extended period of TC activation.
 
The above assumes the WRX's VC center "locking" is at least minimaly functional, not something I have any faith in at all.
 
"..STi...has both..."
 
I'm willing to bet you good money that if the Sti has a rear LSD it is of a "virtual" nature using "differential" rear wheel braking.
 
And front LSD...??!!
 
NOT.
 
Those are found ONLY on the most robust off-road vehicles (Hummer, etc.)wherein the driver is expected to be fully aware, expectent, experienced, with what a front LSD will do to your fingers, thumbs, etc.
 
I think you will find that the Subbies version of front LSD is via TC braking and of BOTH wheels simultaneously even if only one wheel is slipping. To do otherwise would often result in yanking the stearing wheel right out of the hands of an unwary driver not expecting the resulting hard "TUG".

#1445 of 1460 Re: 2005 Outback AWD Failure [dcm61] by wwest

Aug 14, 2012 (9:34 pm)

Replying to: dcm61 (Aug 14, 2012 1:45 pm)
"Virtual" or "hard" LSD...?

#1446 of 1460 Re: 2005 Outback AWD Failure [wwest] by xwesx

Aug 15, 2012 (9:09 am)

Replying to: wwest (Aug 14, 2012 9:32 pm)
If you had any clue, aside from the informing of your extreme bias, you might actually develop some credibility. As it is, you shot it long ago on this topic.
 
I feel for the original poster. He thought he might actually receive some assistance when he posted on this thread instead of your strongly opinionated blathering.

#1447 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 15, 2012 (10:18 am)

Replying to: xwesx (Aug 15, 2012 9:09 am)
So, what's YOUR answer/solution/explanation for the situation the Op experienced...?

#1448 of 1460 Re: 2005 Outback AWD Failure [xwesx] by wwest

Aug 15, 2012 (10:32 am)

Replying to: xwesx (Aug 14, 2012 8:53 am)
You said....
 
"..Even so, the car should have had traction in the front axle unless the center differential had gone out..."
 
If we take the Op at his word then the alternate explanation, and the most likely one by far, is that his Subbie had a standard open differential, and if TC was available it was turned off.
 
Otherwise, like you say, the "car should have.."
 
Awaiting a better explanation.....

#1449 of 1460 Re: 2005 Outback AWD Failure [wwest] by larryv

Aug 15, 2012 (4:45 pm)

Replying to: wwest (Aug 15, 2012 10:32 am)
So I'm the original poster and have reviewed the various replies. And, yes, sometimes I enjoy snarky responses, but in this case, I'm looking for some serious thoughts and info from people more knowledgeable than me. My original purchase material indicates this '05 2.5L basic OB has "full-time AWD" plus a "limited slip REAR differential," but I see no reference to traction control or other features that some of you mention, nor any buttons or switches for same in the car. From some of the Sub websites, I found these demos and comparisons of 4WD vs variations of AWD: http://www.subaru.com/engineering/all-wheel-drive.html, and http://www.youtube.com/watch?v=ooQRxlChvMw. Plus, a drawing that shows a center differential and a rear one in the typical Sub AWD system. So, perhaps the videos are of newer models that have additional AWD capacities that mine lacks, but who can confirm this? Also, a Sub mechanic told me "off the record" that the solution to my predicament would have been to open the fuse box under the hood and to put a fuse into the vacant slot marked "FWD." He said that doing so would have turned off the "AWD" and made the car only FWD. So, in my situation, with the front wheels firmly on the ground, they would have pulled the car out because no power would have been sent to the rear wheels. Don't really want to try this, but is this mechanic blowing smoke or legit? Thanks. LRV

#1450 of 1460 Re: 2005 Outback AWD Failure [larryv] by wwest

Aug 15, 2012 (6:23 pm)

Replying to: larryv (Aug 15, 2012 4:45 pm)
"full-time AWD" "limited slip REAR differential"
 
That is virtually the same wording that "came" with my '01 R/awd RX300.
 
Technically speaking you and I both actually do have "full-time AWD".....
 
A fully open center diff'l will provide equal drive to both front and rear axles just as long as both are encountering roughly equivalent traction loads. But once a single wheel loses traction it will spin freely thereby limiting the torque level that is available to "other" wheels.
 
For my RX that where TC "steps in", activates and adds artificial traction, via braking, to that slipping wheel. Regretably it also instantly dethrottles the engine.
 
These days not even Porsche normally uses a mechanical LSD, TC braking only.

Messages Page 145 of 146

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