I think it is very difficult to get a wheel to spin just on a wet surface if you start from 50-50. Remember, each wheel only needs ~1/2 the traction it requires with FWD or BWD, now. If you have the rear LSD, you are even better off due to the dynamic weight distribution.
So in this case, the AT tries hard to be about as good as the manual.
This example I give to you. But then I can come up with plenty other examples of real world driving, where it is better to be 50-50 from the outset during driving (not from a dead stop).
Also, once you are a bit up to speed in the AT, it will basically turn into FWD until it senses or comes into the next problem situation. Then the rear kicks in, and if you happen to be in a turn at that time, you will probably say "Whoa! Where did this come from?".
When it is ~ continuosly slippery, the fluid in the MT will be thick all the time, so it is pretty much locked, no sudden changes there.
Actually, most of the above post are wrong. I'm describing the basic system AT subaru have. VTD and VDC are different things. Subaru really have an full-time AWD. The belief that the AWD on MT is better is erroneous. And like other posters said, CRV AWD cannot be compared to the subaru.
The basic system can only make distribution from 95/5 to 60/40. VTD allows to do distribution from 95/5 to 5/95. When there is no slippage VTD give a distribution of 40/60 to be more like a sport car.
VDC on the other hand use the ABS system instead of torsens to distribute torque lateraly.
Wow... I have an '02 Outback VDC H6... and I can't relate to this situation. I feel always firmly in control. Never have I felt the need to say "Whoa! Where did this come from?". Of course, maybe it's just me, or just my car; however I doubt it. In any case, my car is not 'basically FWD until it senses a problem'. It's intelligent AWD that constantly puts the torque where it's needed... no muss, no fuss. Regards, John
The AT tranny is not dangerous. Many factors go into a spin. I had a '98 GT sedan with the AT that is refferred to here and it was VERY controllable. Under accel it would send power to the rear and create a SLIGHT oversteer, helping to point the nose where you want it. If you don't want it, don't hammer the gas. On dry pavement, you could do some pretty awesome power slides on highway interchanges. Once you hit the apex, punch it and the rear would hang out a little and go where you told it. In the snow it would allow for some control of direction when the front tires were sliding. The only thing i wished i had on the '98 was a rear LSD. I lit up an inside rear tire a few times starting from a stop under hard accelleration and turning sharply. VCD may provide for more traction, but I would prefer the vehicle control. VCD aims to remove oversteer. I prefer oversteer to understeer any day of the week. I've lost control of various cars suffering from both and once the under steer kicks in, there's nothing you can do.
Nice research, of course some of us have been frustrated for years by this jumble of true tidbits and useless marketing speech.
The only thing I would add to the above is that 50:50 for a locked center only applies in the case of same traction front and back. If the traction is different, the torque applied is different since the axle with the most grip will do most of the work.
You are absolutely right. You have the better AWD of the two AT AWDs. We were talking about the cheapo AWD in the lower cost AT Subarus (that are not VTD/VDC). I should have made that clearer.
I originally thought that it wasn't possible to transfer more than 50% to the rear, with an MPT automatic. I now see that if the front is slipping and the transfer clutch is locked, 100% will go to the rear, so anything in between is possible, though perhaps hard on the transfer clutch.
I think the misleading description of the MPT automatic is most harmful to buyers of Foresters. There are no Foresters with VTD transmissions. If I were buying a Forester for true AWD and then found out that it's really only a FWD with (fancy) rear backup, I'd be kind of peaved. (There are manual Foresters, with true AWD).
A few corrections :
I mentioned that the system didn't have any torque sensors, only wheel speed sensors (and only front and back, no left and right). I believe it's possible to deduce torque :differences: from the speed differences, though I didn't manage to figure out the formula after a bit of trying. I'm too rusty, I guess. Figuring out the actual torque would have to take into account the power output of the engine, not just the state of the throttle.
I also said that I wanted balanced AWD during engine braking to prevent a wheel from locking. After further review this doesn't really happen. It may feel like a wheel is locked but it's probably only turning less fast than the other wheels, (still sliding so little traction). The wheel may stop moving at some point but that would only be a particular case. In fact (if I understand things correctly) the slipping wheel may end up going backwards, like the other three wheels are trying to get it to do.
Finally, I said that there wasn't a balancing function with a transfer clutch, compared to a differential. I now believe that there is, as long as the clutch is slipping. Not the same kind of balancing, but probably effective. With the MPT the front is connected directly to the engine so the front will always get the same or more torque as the rear (unless the front is slipping) but I saw somewhere that the front never turns less than the rear anyway (not sure about this last bit, I haven't worked it out).
Thanks again TransFixed for your help in my understanding of things. It's all quite mind-boggling.
Hi, Some Jeep(SUV) drivers are real dumbass. In the middle of summer, a woman rolled a Jeep making a turn in the intersection downtown here knocking down mom and kid waiting at the sidewalk to cross the street. Does not have to be winter. Go figure. Tony
Actually, in the case of a viscous-coupled differential or just a plain-old viscous coupling (and I'm not clear myself which Subaru uses to distribute power between the front and rear axles in its manual transmission cars -- it sounds like they might use both depending on the application), there are absolutely no electronics involved at all. The lockup is accomplished entirely by differences in clutch plate speed heating up the fluid in the clutch pack, which changes its viscosity.
FWIW, Volkswagen's old "Synchro" system (circa 1986 or so) uses a non-differential viscous coupling. It has only two shafts going into it -- one from the front differential and one from the actual drivetrain. As long as the front wheels are turning the same speed as the rear wheels, the unit is "open" and no torque is transferred to the idling axle. When the viscous coupling unit experiences a difference in speeds between the axles, it "locks up", transferring torque to the idle axle. A friend who is a devoted fan of old VW Synchro Vanagons tells me that lockup of this unit takes less than a second in that application.
A viscous-coupled differential employs the same concept but between the two output shafts of the differential -- some wheel speed difference is tolerated, but too much and the unit locks up. I'm pretty sure that this is what Subaru employs in the rear differentials that are advertised as "limited slip" such as on my new Forester XT or on the WRX. But as I stated earlier, I'm not certain whether manual-transmission Scoobies use a center differential with viscous-coupled limited slip or a plain old viscous torque transfer unit, like in the old VW Synchro application. The salespeople to whom I've talked about it know way less about the functioning of the various AWD systems currently on the market than I do, and can't tell me any more about Subaru's systems than what's printed in the brochures. They seem to neither know nor care how their cars' AWD systems work. All they know is "the wheels that slip to the wheels that grip" -- a catchy line but not terribly informative.
To Subaru's credit, they've been in the AWD business since long before it became sheik. They share this proud heritage with just a few other companies -- most notably, Audi and Volkswagen. These companies were AWD when AWD wasn't cool, to paraphrase a particularly annoying song. But when it comes right down to it, I still think that Audi's Quattro is the best AWD system available. Simple, cheap, and light as viscous coupling units are, they just can't hold a candle to the Torsen differential that has become Audi's trademark. Torsens are instantaneous where viscous units don't engage until *after* wheel spin is occurring. And Torsens can handle driving with different output shaft speeds all day long, where viscous units will lock up and cause potentially serious handling and tire wear problems if both output shafts are not turning at exactly the same speed (as with slightly different diameter tires on the front and rear axles).
Been a lot said on this topic by now... I say who cares! AWD on my 02 OBW with an automatic tranny works just fine when it counts. While I rarely take my car to it's limits, when I have it hasn't let me down. If I punch it from a stop light while turning 90 degree's it has yet to slip (even in wet conditions). Studded tires are Hell on the roads up here (Alaska) and when it rains there is an extremely bad hydroplaning problem, but not with my Subie. When I bought my car (in FWB Florida) I test drove it on the beach, I couldn't get it stuck in the sand...
60/40 or whatever, my tires seem to grip to what's underneath them period.
Tony was referring to the automatic version. You're right that the manual has no electronics involved. It's a viscous-coupled differential for the center diff.
I think someone already posted this, but it's really worth the read if you're interested in AWD systems:
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Remember that you can't assume that Quattro = Torsen anymore. Torsens are used in Audi Quattro's (and VW 4Motions) with longitudinally mounted engines. Haldex's are used in Quattro's and 4motions with transverse engines.
Andy (perfectly happy with the AWD in my manual WRX...)
Point taken. I'd actually completely forgotten about the Haldex system, which I perceive as being used by VAG to implement AWD in platforms originally designed to be FWD only. The specific arrangement to which I was referring was the use of a Torsen center diff, as is used in all of Audi's 4 and 6 models, and was used in my old '89 200 sedan, as well as the original Quattro, and most every other Quattro car that Audi released before about
1998 or so. (One notable exception is the old V8 Quattro Automatic, which used a computer-locked center diff.)
And while I'm planning to purchase a Subaru that AFAIK doesn't have a Torsen anywhere in it, I still think that the best AWD system ever commercially produced was that in the Audi V8 Quattro 5-speed, which had Torsen diffs in both the center and rear axle locations. The perfect AWD system would have
*three* of 'em. (Note that some of the more powerful FWD cars are now offering Torsens in front to limit the torque steer that results from violent front-wheel spin in a FWD car. I've never driven one of these cars, and the torque steer-limiting effect of a Torsen seems counter-intuitive to me. I'm trusting Car and Driver magazine's editors that the application really does work.) I guess I'm just a big fan of the Torsen -- much more so than of viscous coupled differentials.
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