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Comments
Also, this site focuses on Chrysler products. Maybe they can help.
http://www.allpar.com/
Bob
http://www.youtube.com/watch?v=ooQRxlChvMw
cliff notes
Honda CRV--- basically FWD, 4WD is just badge
system relies on loss of traction to give some power to rear wheels and as you can see it gives very little so you would be better of with FWD vehicle with LSD than this
Toyota Highlander --- no better than CRV, i suspect Rav-4 to have same/similar system
about same process as CRV
Subaru, symmetrical AWD all wheels get same amount of power/torque all the time
VW - as you can see is also really good but not the best
these vehicles were tested because manufacturers claimed full time 4WD or AWD.
Acura SH-AWD is 3.5WD system where front get ~70% of power until slip is detected.
Infiniti ATTESA - all models that have "x" in the end of the model name. "part time AWD" when driving only rear wheels get power, if torque loss at the wheel is detected (no slip yet) and sends power to front also. capable of up to 50:50 distribution. this is a very short version as the system is very complicated and probably the best you can get for street performance. Inifiniti vehicles are not intended to be used off road
Nissan Intelligent AWD, Rogue for example provides 50-50 power split between front and rear wheels when accelerating from a stop. As the vehicle gets to cruising speed, that basis will revert back to full front wheel drive . I-AWD predicts potential problems and automatically transfers to a 70-30 bias when taking moderate to sharp turns. The system still reacts to traction loss and sends power where needed, but it’s improved over the standard brainless AWD systems. Now rather than waiting until a traction loss and adapting as quickly as possible, the vehicle is already prepared by having that power bias already set
Cool vid, though. Even the most basic Forester is able to send enough power to either axle to climb that ramp.
This has been the topic of hundreds of posts. Bottom line, the statement as a generalization is not true, but exactly what happens under the covers can't seem to agreed on. At least reference STI with the VCCD? Probably don't have the acronym correct.
We got lots of snow in New England area. But they put sand/salt pretty fast on highway, so during/after snow, it is wet or with little bit snow or ice. How do we drive a truck on this road? Do we use 2WD, or 4WD H? If 2WD, then what is the advantage of truck. I saw lots of truck on highway that are running fast during/after snow. While small cars with 2WD just follow the traffic slowly.
I sincerely doubt that to be a TRUE statement. No automative design engineer, given a choice, would wish to allow a high level of torque to remain on the front wheels in a turn, tight turn, or even an accelerating turn, certainly not a tight accelerating turn.
The tighter the turn or the harder acceleration being used in a turn the lower the torque delivered to the front should be.
"..I-AWD predicts potential problems and automatically transfers to a 70-30 bias when making moderate to sharp turns..."
I-AWD = Intelligent AWD...??
NOT...!!
Simply PRE-EMPTIVE, NOT inteligent by any means.
An IDEAL, "INTELLIGENT" AWD system would incrementally reduce the engine torque to the front wheels as the turn becomes tighter and/or the acceleration level increases, all the way to 0-100 if need be.
Since that cannot be easily done with ANY base FWD vehicle the best that can be done to help alleviate, reduce, the potential for loss of directional control in these instances is to partially REDUCE the level of engine drive torque to the front wheels.
That's where the advantage goes, DRAMATICALLY so, to the SH-AWD system. The rear "diff'l" is over-driven by 7%(?) so the F/R torque distribution can be as high as 30/70.
To my knowledge, limited as well as it might be, no such system is available. Torque loss at the wheel is ALWAYS the result of wheelspin/slip.
That's why we now have so many PRE-EMPTIVE F/awd systems in the marketplace today. There is as yet no method of predicting that wheelspin/slip will occur as a result of application of too much engine torque for conditions. So manufacturers are choosing the alternative, most especially F/awd vehicle manufacturers.
The alternative is to PRE-EMPTIVELY remove engine drive torque away from the front wheels at the times when wheelspin/slip is most likely to occur. That is: During acceleration from a stop, low speed acceleration, or when turning wherein directional control is often threatened due to wheelspin/slip. In a tight turn or accelerating turn due to the buildup of centrifical forces much of the front tire's roadbed cohesion will needed for lateral stability, directional control.
Today's FWD vehicles, and in the past many F/awd vehicles, compensated for this problem via quickly dethrottling the engine should the VSC's yaw detection and stearing wheel position indicate that the front tires's traction coefficient would/might be exceeded.
But there is a serious short-coming, design flaw, in these new pre-emptive F/awd systems. They engage the rear drive capability, in effect locking the center "diff'l", even on the roadbed surfaces with the highest tractive coefficient possible.
That, of course, except when by pure happenstance the roadbed actually is slippery, results in a dramatically serious level of driveline windup and/or tire scrubbing. Stress and HEATING of the driveline components and early tire tread wear out/off.
Look at the history of these new pre-emptive F/awd systems, the Ford Escape being one of the earlier adopters. "Pre-mature" PTO and rear diff'l drive clutch failures are/were the rule of the day. And the Acura MDX VTM-4 system, transaxle lockup failures as a result.
The rumor is that the new 2011 Ford Explorer's F/awd system (no, that isn't a typo the new Explorer is a base FWD vehicle) uses water cooling for the PTO and the rear diff'l in an effort to abate the premature failures of these components resulting from the coupling of front and rear wheels even on high tractive conditions.
Why not have a simple switch that the driver can use that enables the rear driving torque ONLY in known slippery conditions...?
I thought about you yesterday. Did a U-Turn on a stretch of dead end pavement here in the UP of Michigan where we live now and got the Subaru stuck.
Went a bit too far on the shoulder that's used for a snowmobile trail and the snow was soft and a bit wet. Had to shovel a yard away from behind each wheel (shod with "performance" all season tires btw).
If I'd been in the safer FWD minivan, I would have done a 3 point turn and not had any problems.
Bob
Nothing is near us, but we have made it over to Ironwood.
Do the front wheels show power ever? If the rear wheels were slipping and the front weren't turning in AWD mode, there is a significant problem - abs module or centre diffy is toast would be most likely.
As has already been said, many of these wannabee F/awd systems allow for TC to be turned off, leaving you with a simple ONEWHEEL drive system. That at least allows you to use wheelspin, if that works, to get unstuck or initially in motion.
Your brother-in-law shows good sense, not risking his life in one of these. What did he get to replace..? I'm betting on a R/awd vehicle.
*** Not anywhere near the equal of ANY R/awd system, such as a 4runner, but undoubtedly the best of the best insofar as vehicles with sideways mounted engines are concerned.
http://www.subaru.com/engineering/all-wheel-drive.html
http://www.subaru.com/engineering/safety.html
Ford Escape does terribly in that avoidance manuever.
Results certified by USAC, too.
Is it possible that's what the Subbie driver did?
A rear LSD in a F/awd vehicle, SUBBIE...?? Rear LSDs are pretty rare, VERY rare, in F/awd vehicles.
Oh, I wonder...
A virtual LSD via use of TC differential braking at the REAR could be a possibility. Anyone know for sure..??
So the Subaru doesn't have unequal front drive halfshafts thus "symmetrical" AWD...interesting
http://www.subaru.com/engineering/safety.html
Look at the "set" of the Subbie's suspension at 1:48.
Now notice the missing frames, video "glip" at about 1:45-7.
During the frames you do not see the driver has quickly and very briefly oversteared to cause the rear wheels to "skid" out into the "line" he wishes to enter that upcoming hard left turn.
Just as the Subbie driver is clearing the first set of cones look at the "line" of the Subbie and then at the angle, lack thereof, of the front wheels. The driver has cranked in enough overstear just ahead of the cones that the rear has come about, SWUNG OUT, and now he has counter-steared to correct for the overstearing condition.
I think the difference is that the others have AWD or traction control since they are usually front axle only. That means the rear axle doesn't really have the virtual LSD, since they provide forward propulsion only part of the time anyway.
USAC certifies the results, so presumably they didn't cheat with the handbrake...
Oversteer is consistent with what I used to get with my 98 Forester, in the days before stability/traction control (I had open front/rear diffs). It would wag the tail, swing out the rear, then AWD would shift power to the front axle and it would pull me out of the skid.
Looks like the tail does get loose first, but the stability control catches it quickly, so it completes the turn with pretty good control.
Doesn't seem like a trick to me - in fact that's more or less what I expect it to do. Watch 1:48 to 1:52, the rear view is closer and smoother, so you see it better.
Most stability controls, more modern ones anyway, will not interfere if the driver seems to be intentionally invoking overstear. And note that as soon as the rear wheels are aligned in the direction desirable for entering that first turn the front wheels are straightened, turned straight ahead.
"..Oversteer is consistent..."
Really...?
You don't, typically, encounter overstear, tail "wag", in a FWD or F/awd vehicle unless it is invoked intentionally. And once you get a measure, the measure you wish, of that intentional overstear you had better be well ready to crank in the correct level of counter-steer, "early" stability control.
No obvious deception required, just put highly experienced RWD/R/awd drivers behind the wheel of the competition.
Or use just one driver who has been "coached".
It would predictably break traction on the rear axle first. Weight distribution was 55/45 so the lighter side first makes sense to me.
Your '98 Subbie either has, 1, a SOLID coupling between the transaxle output and the front drive diff'l input, with a VC to serve as a "soft", rubber bandish, coupling to the rear. Or, 2, has a standard "open" center diff'l with a VC mounted, connected across/between the two diff'l outputs. the
Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.
In either case the mostly "flaccid" VC MUST allow a great deal of "give" between the front driveline rotation rate and the rear driveline rotation rate. That's to avoid driveline windup and/or tire scrubbing while driving on reasonably tractive surfaces, especially when turning.
VC fluid formulation, rate of fluid expansion with heat, determine the "attack" rate of "tightening" of the torque coupling to the rear. Most VC's also have a controlled level of inert gas inside the VC container to delay the onset of the VC's torque coupling. No torque is coupled until the gas is compressed to ZILCH..!
Most marques manufacturering F/awd vehicles, including Subaru, have now abandoned the use of VCs for use on F/awd vehicles. Instant TC activation upon detection of wheelspin/slip precludes having any substantive level of disparate F/R wheelspin occur thereby making a VC useless.
Bob
And I thought I had read the VC was discontinued in the latest models.
http://media.subaru.com/index.php?s=43&item=154
Bob
Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.
It's #2, except the VC itself acts in place of the center diff, and yes it's fluid-filled like you described. Rotational differences between the front and rear axle thicken the fluid and tighten the coupling temporarily.
By the way, the VC only applies to manual transmission models, though it's used in several Subaru models. They call it "Continuous AWD" in the brochures.
Models with an automatic use a different system - no VC. Subaru markets those as "Active AWD", which uses an Electronically Controlled Variable Transfer Clutch. On higher-end models, "Variable Torque Distribution" or VTD is used. Finally, the STI uses DCCD.
VDC is their marketing term for their stability control system, though in the past it referred to an Outback model that used the VTD AWD system, further confusing matters.
So really there are 4 separate systems. Symmetrical AWD is a generic term that refers to all 4 of those, though I guess they do have equal length half-shafts in common across the whole lineup, so I guess that's what they're referring to.
But there is no automatic WRX...
Auto-equipped Foresters, Imprezas, and Outback Sports all still use the variable transfer clutch.
Bob
If there is no ACTUAL center diff'l then the VC use must be as stated in #1.
That places the VC in "series" with the rear driveline, just as in the Chrysler T&C, and there is evry little torque to the rear unless the VC has reason to "tighten up". The more recent problem with this type of VC F/awd implementation is that TC activation, INSTANT TC activation, upon even the slightest level of front wheelspin/slip, prevents the VC from EVER tightening.
That's the main reason Chrysler discontinued the F/awd T&C, the adoption of TC (and "VSC") made their VC implementation non-functional, useless.
"...I can guarantee...gets power full-time.."
Even a beginning automotive design engineer could put the fallacy to that statement. Even the new Subbies with the ECU controlled clutch pak only route engine torque to the rear for times, during acceleration, when excessive drive torque is most likely to result in front wheelspin/slip
If that were to be so your tire treads would have an a very short life and early, premature driveline component failures would be the NORM.
Rear axle would break traction first when on power in turns. Less weight on that axle, but it would definitely get power - how else would it break traction and spin the rear wheels first?
I drove it for 9 winters, and that's just how it behaved.
Wife has an 09 automatic now and that feels different - it'll understeer. Night and day difference in feel.
Bob
Manuals use a viscous coupling that is fluid-filled. When the axles spin at different rates the fuild shears and thickens to temporarily bind the axles together. The default split is 50/50.
The CVT uses clutches and defaults to a more FWD-like default, then sends more power to the rear axle as needed.
I've owned Subaru with both, and I prefer the VC system in the manual. It doesn't feel like FWD in day to day use. Both are effective in the snow, particularly when paired up with good tires.
To be honest the limiting factor on a new Outback is probably the approach and departure angles, due to longish overhands. It has plenty of ground clearance.
With the advent, adoption, of TC it is not possible for a VC to be functional absent a TC off capability. Additionally, actual VC functionality, assuming TC off, will always compromise ABS and stability controls.
As a result of the above limitations/restrictions most modern day systems using a VC have had the fluid reformulated such that the VC remains forever flaccid.
Not technically correct.
PRE-EMPTIVELY sends more power to the rear axle, "needed" or no, at times when loss of directional control is most likely to otherwise result. Those times fall into only 2 categories, low speed acceleration, and when turning tightly or turning while accelerating.
Other than those times the system will default into a definite front torque biasing mode. Regardless of the instantaneous mode of operation loss of directional control is such a dire threat for FWD or F/awd vehicles that the INSTANT response to wheelspin/slip, DRIVEN (front) wheelspin/slip, will be activation of TC.
There's an off switch.
http://www.youtube.com/watch?v=q3jiy2nxkCY&feature=player_embedded
And...
Is that a WRX (no hood scoop) and is it the WRX with the "make-do" VC coupling.
The Subie happens to have enough excess traction to compensate for the other.
Sounds as if your's is one of the many F/awd systems out there in the market today that are really "ONE-WHEEL" drive systems. Totally open center differential, "AWD" mode only if all four tires have roughly EQUAL traction.
Most of those have a TC implemented "AWD" system, braking of "slipping" wheels to simulate traction accompanied INSTANTLY by full engine dethrottling...not very helpful in the situation you describe.
Turn off TC = ONE-WHEEL drive.
Bob