"Driving Sports TV" (drivingsports)
10/08/2019 at 12:48 • Filed to: Subaru
2
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"Driving Sports TV" (drivingsports)
10/08/2019 at 12:48 • Filed to: Subaru | 2
| 5 |
If you buy a 2020 Outback, WRX STI, a Crosstrek with a manual transmission or a WRX with a CVT, you get one of four very different all-wheel drive systems, all advertised simply as “Subaru Symmetrical All Wheel Drive. ” In this video we look at the modern history of the systems, and how every version of the system works in current Subaru models. Plus, how do those DCCD options work?
HammerheadFistpunch
> Driving Sports TV
10/08/2019 at 14:04 |
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Im going to copy pasta my response to your video here. BTW, this is good work. Thanks!
Very good content Ryan. As you might guess...I have notes. One thing that’s important to note with the “continuous AWD” system in the manuals is that while they have viscous couplers they aren’t only the coupler, they are viscous over open. So the VC (viscous coupler) unit has a traditional open diff with spider gears and then the VC unit in addition.
(pictured: the VC unit an d transfer case out of the 5MT trans on my old Forester XT)
So in this case it’s a full time open center differential that will transfer torque 1:1, but that also means that limited front traction means that same limit to the rear axle (1:1). With the VC, it adds a limited slip function to the center to be able to change the bias ratio. It’s hard to find specific information on that ratio, I suspect because the VC TECHNICALLY will fully lock in “hump” but that’s just the plates in the VC binding and that’s not good for the unit. In hump mode the continuous system can actually transfer 100% of torque to the rear axle as the center diff is “locked”. Like I said, thats bad for the VC and doesn’t really happen that often but it can. The reason that’s important is because some older implementations of the VC were not coupled with a differential at all and the coupler did all the work. That meant that the rear axle would only get power reactively after there was sufficient speed difference between axles. This was how the older VW synchros worked and it put a ton more stress on the VC and they aren’t as good off-road as a result.
VTD is basically the pricey special version of this system and while it can be true to say that it will only transfer “50:50" that’s not super accurate as the clutch pack can transfer way more than 50% to the rear axle as it has the ability to fully lock the plates like DCCD, only without the ability to control it manually. In a fully locked system (VTD or DCCD) the center differential is fully locked and taken out of the equation where it then acts as a full 4wd system, just like a truck. In that state the system can transfer 100% of torque to either axle.
DCCD is deeply misunderstood as its a torque sensing limited slip center differential with normal torque bias in the gearing. In a fully turned off DCCD system, the system can still transfer more than 50% of the torque to the rear axle as the bias ratio is greater than 1:1 (I think its somewhere like 2:1, maybe as high as 2.6:1). Meaning whatever the front axle is getting up to 2.6 x that limit is what the rear can get. if the front axle can only support, say, 1000 lbs-ft TRG then the rear can get up to 2600 lbs-ft TRG before the center limited slip limit is reached and slip happens. In this instance 72% of torque goes to the rear axle. What DCCD, and to a lesser extend VTD, does is apply addition torque bias by clamping up clutches that act on the center diff. So going from a fully disabled DCCD to say, 50% DCCD setting will increase the bias ratio from the standard 2.6:1 to, lets say, 6:1. That means in the same case as before the torque that could go to the rear axle is now 6x the front meaning that 83% can go rear...all the way up to fully locked which is an 1:0 ratio or infinity:1, in other words fully capable of sending 100% torque to the rear axle (the STI also has limited slips with similar TBR’s front and rear). i.e. LOCKED.
This is specifically why I hate the use of the term “50/50", because it implies percentage, but doesn’t actually have anything to do with torque bias. it CAN mean a nominal torque bias of 1:1, or it CAN mean a locked state, but those are very different things as a nominal 1:1 bias can’t bais any torque, where a limited slip or locking function actually can.
Subaru systems are, indeed, quite robust and aside from the active AWD system in the autos, very different from the on demand AWD systems most crossovers have. Active AWD is pretty much like these PTO systems on most AWD system, but their clutches are strong enough to fully lock and indeed you can modify the signal to the solenoid to do just that - fully lock the clutches.
Hopefully this doesn’t come across as anything but being helpful, your explanations are miles ahead of 95% of all explanations of these systems online.
Side note: VDC is NOT torque biasing, its a 1:1 bias differential but tricks the differential into thinking there is no traction loss at the slipping wheel allowing 50% of the torque coming in at the ring gear to go to the gripping wheel. STI’s and some WRX/XT models have true biasing limited slip differentials in at least the rear axle, though in situations where a tire has no traction, VDC is better than an LSD as 2.6x nothing is still nothing, where half of whatever the rear diff is getting will get you moving.
Wow, posted to kinja its a lot more text than I thought.
dogisbadob
> Driving Sports TV
10/08/2019 at 15:05 |
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Cool video
Subaru needs to start offering FWD cars again
for
better mpg. I know they still sell the FWD Impreza in Japan. And too bad the AWD isn’
t FWD by default and the AWD switching on only when you need it. And no button to disable it, either.
And why do they have all these different
systems when one or two would be sufficient? They’re not the biggest car company out there
Dakotahound
> dogisbadob
10/08/2019 at 15:22 |
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Sorry, this comment got a little long. I guess I don’t like FWD cars:
Unquestionably, FWD and front-biased AWD technology has progressed in recent years. Some performance driving schools now use FWD cars, and there is even an IMSA class reserved for FWD cars. There is one driving situation, however, where RWD and rear-biased AWD cars excel – downhill on slippery roads.
Having driven RWD sports cars most of my life, my first experience with a FWD car was driving my wife’s Honda Civic. One snowy day in upstate New York, driving down a moderate slope, I spotted a car turned sideways on the shoulder just ahead. Letting off the gas slightly caused the Civic to begin a slow 270-degree spin for which there was no recovery. I ended up sideways on the shoulder, about 30 feet from and parallel to a woman in a Prelude who had performed the same unfortunate maneuver just minutes earlier. Any time the Civic encountered a downhill slope in the snow or patchy ice, it had a strong desire to swap ends. On another snowy day it got its way and suddenly left me looking up a steep incline facing oncoming traffic. Please note that the car was always fitted with four good quality snow tires and I am an experienced winter driver.
Several years later, I had some similar experiences with a FWD VW Golf. Driving down snow covered roads leading from mountain gaps in Vermont while looking over 100+ ft. cliffs and fighting the car’s urge to spin out of control was often so harrowing that I needed to pull over and sit in the car for a few minutes when I reached the bottom. Despite the short wheelbase, an Isuzu Amigo proved to be a much better vehicle for the task. The trick was to put it in 4WD going uphill and RWD going downhill. This method also worked with a Jeep Wrangler. Using 4WD downhill in either vehicle proved to be just as difficult to control as the Civic or the Golf.
My favorite vehicle for driving in the snow was a 1994 GMC Safari with a V-6 and AWD. It had a long wheelbase and a 40/60 front-to-rear torque split. The GMC had plenty of power and traction to get up hills, while the rear wheels provided adequate hold to keep it stable on descents. This seems like an ideal combination for inclement weather, especially if it is combined with some of the modern traction control systems.
Highlander-Datsuns are Forever
> dogisbadob
10/08/2019 at 16:20 |
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I am getting close to 30 mph in my AWD outback. They have come a long ways in the last few years figuring out how to get efficiency with AWD.
BoxerFanatic, troublesome iconoclast.
> Driving Sports TV
10/08/2019 at 17:01 |
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Well presented video.
Was hoping for a blurb about the old 5EAT Outbacks, Legacys, Tribeca, and JDM Spec.A (automatic) STI... that had VTD without DCCD. Variable Torque Distribution front to rear, with a rear static bias, but basically permanently set to Auto+ mode for computer controlled management,
VTD without DCCD was also used in the STI-related 6-speed manual in the Legacy GT Spec.B, as well, along with taller highway gearing than the STI’s gear ratios, and an open front differential, with a torsen rear diff.
I wish Subaru had as much focus on their performance offerings as they used to.
WRX and STI are the same formula as they have been for 15 years.
BRZ is not much of a serious consideration, as it is the reason why videos like this have to say “most” instead of “all” subarus have symmetrical AWD.
Everything else is like a clothing rack. The same garment in four or five different sizes, and all pretty much grey or khaki.
Legacy GT is no longer around, the non-WRX turbos are basically an excuse not to have to update Subaru’s 6-cylinder engine, and not much else.
I used to badly wish they would build something interesting, like an SVX revival with an upgraded 3.4L (de-stroked 3.6), maybe with twin turbos, backed by that Spec.B 6-speed.... and only a little bit of styling update, as the original still looks futuristic even today.
But I have no confidence that they’d do it well at all.