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Lets have a discussion about AWD and Transmissions

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23 replies to this topic

#1
OFFLINE   2point5RS_Dan

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On another message board this morning there was discussion about AWD systems, and the different ones Subaru uses in their cars. This pots from LegacyGT.com was brought up:

http://www.legacygt....ead.php?t=48112

There is a lot of confusion about the differences between the various Subaru AWD systems:


  • ]Continuous (for 5MT)
  • Active (for 4EAT)
  • VTD (for 5EAT)
  • availability or non-availability of Rear LSD
So heres an attempt to explain the differences....

1. Continuous AWD System:

The manual transmission’s all-wheel drive is referred to as a continuous all-wheel drive system. It uses a center differential located inside the transmission case that is controlled by a viscous coupling device. In effect, the center differential is a limited-slip differential.

In normal operation, power is distributed equally to the front and rear wheels. Plates are alternately attached to the front and rear output shafts inside the viscous coupling. When a rotational difference occurs between the front and back wheels, the plates inside the viscous housing shear inside the contained fluid (a type of silicone) heating it and causing the fluid to thicken. The thickened fluid causes the plates to transfer torque from those that rotate faster (the slipping wheels) to the plates that rotate slower (the wheels with the best traction).

This no-maintenance system is simple, compact and virtually invisible in its operation. The system can distribute torque from a 50:50 torque split for maximum traction to mostly front or rear wheel drive.

(Source: http://www.autoworld.com/news/Subaru..._All-Wheel.htm
)

A. Continuous AWD with Rear LSD: Available with 5MT on WRX, Legacy GT, Outback 2.5i, Outback XT and with 6MT on Spec.B

B. Continuous AWD without rear LSD: Available with 5MT on Impreza 2.5i, Outback Sport and Legacy 2.5i


2. Active AWD System:

Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission (4EAT) from other "reactive" all-wheel drive systems on the market today. What makes this all-wheel drive system so special is its ability to anticipate traction needs and act before a wheel slips.

The mechanism that transfers torque fore and aft is contained within the transmission’s tailshaft. To the casual observer it looks just like a typical hydraulic clutch found in any automatic. The key difference in this clutch pack is its operation. It’s designed to slip according to how much all-wheel drive is needed. When an automatic’s clutch slips, it is due to a malfunction and will eventually burn up. But the multi-plate transfer (MPT) clutch uses a special friction material that easily withstands the friction loads generated during torque transfer. (Also referred to as VTC = Variable Transfer Clutch)
The MPT’s operation is controlled by the Transmission Control Unit (or TCU) and constantly changes dependent on how the vehicle is being driven. To get more all-wheel drive, the TCU increases the hydraulic pressure to the clutch for less slippage. Less all-wheel drive calls for more slip and the TCU reduces the hydraulic pressure to the clutch.
Under normal, dry pavement operation torque split is about 90% front and 10% rear. This distribution helps to compensate for the car’s weight distribution and resultant smaller effective rolling diameter of the front tires. As weight transfers to the rear of the vehicle, (i.e., under acceleration), the TCU shifts the torque split more toward the rear wheels. Under hard braking, torque is directed forward. Torque distribution is changed based upon how the vehicle is being driven. Throttle position, gearshift lever position, current gear and other factors combine to influence the TCU and it, in turn, selects a software map that determines how aggressively torque split will be adjusted.
Two speed sensors are used by the TCU to detect wheel slippage. One sensor monitors the front axle set, the other the rear axle set. Pre-programmed variables help the TCU differentiate between slipping wheels and normal wheel speed differentials as what occurs when cornering. A speed differential (front-to-rear) of up to 20% signals the TCU that the vehicle is cornering and torque is distributed to the front wheels to help increase traction during the turn. Anything above 20%, however, indicates to the TCU that wheel slippage is occurring and torque is then distributed to the rear wheels.
Another feature of the all-wheel drive system is its interaction with the anti-lock brake system. When ABS is engaged, the transmission selects third gear, reducing the unpredictability of engine braking and, thus, reducing the possibility of wheel lock-up. But all four wheels are still connected to the engine through the AWD system and are brought back up to overall vehicle speed quicker and can, therefore, be controlled again sooner. In a two-wheel drive system if the locking wheel isn’t a drive wheel, it can only be brought back up to overall wheel speed by whatever traction exists between it and the road. The quicker a wheel is controlled the better the stopping performance

(Source: http://www.autoworld.com/news/Subaru..._All-Wheel.htm
)

A. Active AWD with Rear LSD: Available with Outback 2.5i

B. Active AWD without Rear LSD: Available with 4EAT on Impreza 2.5i, Legacy 2.5i, Legacy 2.5i Ltd


3. Variable Torque Distribution (VTD):

It has the MPT (multi plate transfer) clutch aka VTC as found in the Active AWD system but it also has a planetary-type center differential and a Rear LSD. The center differential provides the ability to have a default torque split of 45/55 front/rear (as against the 90/10 split in Active AWD). In every other aspect it is similar to the Active AWD in that it anticipates wheel slippage instead of reacting to it as in the case of a MT. We can say the the VTD is an advanced Active AWD system

Available with 4EAT on Impreza WRX
Available with 5EAT on Outback XT, Outback 3.0R, Legacy GT


4. Driver Controlled Center Differential (DCCD):

Uses an electronically managed multiplate transfer clutch and a mechanical limited-slip differential in conjunction with a planetary gear-type center differential to control power distribution between the front and rear wheels. Normally, DCCD splits power 41% front and 59% rear. Sensors monitor parameters such as wheel slippage, steering angle, throttle position and braking to help determine torque distribution and direct it to the wheels with optimum traction. DCCD also features a helical-type limited-slip front differential and a Torsen� limited-slip rear differential.

Available with 6MT on Impreza WRX Sti


Which is Better?

Now that brings us to the next point of debate as to which one is better?

So here's an excerpt from an article I found:

(Source: http://www.cars.com/carsapp/cars/?sr...all_wheel.tmpl)

"Simpler AWD systems “bias” the power to the front or the rear in this way and react to slippage when it occurs. Subaru's “continuous” AWD system is this type. More advanced systems are designed to be proactive rather than reactive. For example, Subaru’s “active” AWD is claimed to anticipate and prevent slippage for a seamless driving experience. Audi's quattro and related Volkswagen 4MOTION AWD systems route power based on vehicle dynamics: Rear tires have greater grip during acceleration, so during that action the rear wheels receive more of the engine’s power. In turns, the outside front wheel has the most grip and thus gets the most power, followed by both rear wheels.

With advanced AWD like this available on cars such as the Porsche 911 Carrera 4, it’s clear that four driven wheels are no longer the province of high-ridin’ mud buggies. For surefooted handling during acceleration — even on bone-dry roads — AWD is tough to beat."

As per the author of this article the Continuous AWD system found on Subarus with 5MT and 6MT is a simpler system in that it is more reactive than proactive. Whereas the Active AWD systems (with Variable Transfer Clutch) found on 4EAT Subarus and the VTD (which is an advanced Active AWD system) found on 5EAT Subarus are proactive in predicting wheel slippage conditions and hence react faster. But obviously the Continuous AWD systems have the advantages of being less complex and hence less prone to failures and low repair costs.


Edited by 2point5RS_Dan, 18 January 2011 - 07:18 PM.




#2
OFFLINE   2point5RS_Dan

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Does my Subaru have a Rear LSD?



Another point of confusion is which Subarus have Rear Limited Slip Differentials (Rear LSD) and which ones do not. For this you can refer to the list of Subaru AWDs above where I have also listed the Subaru models which have that type of AWD. But to make things even clearer heres a list of Subarus with and without Rear LSD:

Subarus without Rear LSD:


Impreza 2.5i (5MT and 4EAT)
Outback Sport (5MT and 4EAT)
Legacy 2.5i (5MT and 4EAT)
Legacy 2.5i Ltd. (4EAT)

Subarus with Rear LSD:

Impreza WRX (5MT and 4EAT)
Outback 2.5i (5MT and 4EAT)
Outback XT (5MT and 5EAT)
Outback 3.0R (5EAT)

Legacy GT (5MT and 5EAT)
Spec.B (6MT)

WRX Sti (also has Front LSD)



As we all know that the center differential can split the torque in between the front and the rear axles depending on the front/rear wheel slippage. But its the Front and the Rear LSDs which can split the power between left and right wheels. So it is very advantageous to have at least the Rear LSD if not the Front LSD. If the left rear wheel slips the Rear LSD can transfer the power to the right rear wheel.

Subaru does not offer Rear LSDs in its lower models except in Outback 2.5i!! which is understandable since it is designed for off-road conditions. (This might explain the mpg difference in Outback 2.5i and Legacy 2.5i)






This is contuned from the article. The actual LSD use is wildcard. The RS's only had them in 00 and 01, so generally you have to know what model and diff you're looking at.

Add the Forester X premium 5mt winter package to the Rear LSD list MY06-08 and the MY01-02 Forester S Premium 5MT with winter package all of which have the viscous center and rear diffs with 50/50 torque split.


07 Spec B has a Torsen limited slip rear.


Edited by 2point5RS_Dan, 18 January 2011 - 07:20 PM.


#3
OFFLINE   2point5RS_Dan

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Active, Continuous, 4EAT, Part-Time, Full-Time, ramblings, and more.

(if this bores you, skip to the next post for pictures and video)

A lot of info there, especially highlighting the differences between the auto 4EAT (active), and the 5 speeds (continuous).

I was curious about the 4EAT and its active system, which brings me back to an article on RS25 about how a guy made a lock out switch for his 4EAT to make it 50/50. This post builds upon the ideas discussed in the last post about the different ways of applying the power to the road.

http://www.rs25.com/...emada-lite.html

If on the other hand, you have the pre 02, 4-speed automatic (4EAT), you're not so lucky. Your torque split can vary from 50/50 to a 90/10 front-bias.
The Transmission Control Module (TCM) on the 4EAT tries to intelligently divide the torque based on conditions, and for most situations it does just fine. However, it's practically worthless for precise, low traction driving. It has a 90/10 (F / R) torque split in most forward gears, calls for shifts at all the wrong times, and the overall gearing is poorly suited to manual shifting. Don't even get me started on the stupid shift gate that prevents hassle free 2-1 downshifting. If you ever rallycross your 4EAT, or go for a brisk drive on snowy, muddy, or gravel roads, you'll quickly learn that the time it takes for the TCM to react is precious time.
Since the default bias is 90/10 you're effectively driving a FWD car. When you lose traction, the car understeers (pushes) to the outside of the corner where bad things like curbs/ditches/mailboxes/guardrails, live. Eventually the TCM reacts and transfers some power to the rear wheels, but by then, you've either gotten past the issue, or it's entirely too late.

Here's where my mod comes in - with the flick of a switch, you can lock your car into a full-time 50/50 split - no more waiting for the TCM to react. AWD all the time!
The feature can be turned on and off at will - no need to stop the car (although if you can, it's probably not a bad idea).
It should be noted that this isn't a new mod by any means, and has been around for years. However, I have yet to find a true "How-To" that lays everything out - so I made one. Not satisfied, I had to come up with a little twist. An extra bonus for us lowly 4EAT drivers is that we can also set the system up to allow the Holy Grail of rally driving - the handbrake turn! Finally - the 4EAT can do something the 5MT cant.
Ok - let's get started.

I should note here that I'll be specifically referring to the 4EAT that came on 95-01 USDM Subaru's - it may even work on older 93/94 models, but I can't verify. As near as I can tell, the basic technology was shared across the lineup, so the Legacy and Forester can use this mod as well.
The only "gotcha" is that the OEM turbo models MAY use a different wire, along with different power reqirements.

The following work was done on a 1999 Impreza Outback Sport with a bone stock transmission.


I haven't finished reading the entire thread, but another guy on a different forum is interested in the project and had this to say about it:

I've read through the entire thread and I think I'm going to take a shot at it once I can get some parts together.

The most interesting thing about that thread is that the creator of the lock switch also tested the solenoid voltage to see what the torque split was during normal driving and it sits around 60-40 during dry pavement cruising, which is different than most documentation I've found online which states 90-10 F/R. He found that it only went 90/10 while stopped or parked. He also found that the TCU is quite slow to move the power to the front on braking. It can take up to 1.5 seconds to transition. All other times during driving the power was moving back and forth constantly depending on his driving.

Too bad he lost his job and had to stop work. He was working on a gauge to show the current torque split and a variable control that would let you adjust the torque split with a knob.



The thread is a good read and he talks about a lot of practical stuff in there, such as the wear components of the transmission and how the possibility of torque bind is present in his setup.

The RS25 addresses this:

Q: Holy Crap I have to try this!

Why should you feel binding on asphalt? Would this be safe to use for autocross or is it for offroad only? How long has this mod been done to your car. Is there any chance you could pre fab the resistor board for me and include the wiring and switch so all I would have to do is solder the wires to the car.?


A: You'll feel binding on dry pavement because the Multi Plate Transfer Clutch (MPT) is physically locked. The 4EAT doesn't have a "true" geared center differential like the 5MT, so when you activate the lockup, the transmission has no way to make up for the difference in rotational speed of the front & rear axles as you take a tight turn.
Subaru is obviously comfortable enough with the strength of their drivetrain components to allow the "fail" position of the solenoid to fully engage the AWD. If you look up "torque bind" on most Subaru sites, you'll see that it's most commonly traced to a DutyC Solenoid failure - which is exactly what we're mimicking with this mod.
Under normal driving circumstances on dry pavement, you're very unlikely to break anything unless you try really, really hard, but the wear & tear will certainly be accelerated.

As for autocross, I'd be very, very careful - especially if you're running in one of the "super sticky tire" classes. Crappy all-season tires will just skid and dissipate the load, but a nice set of R compounds will probably just transfer it on to your axles, cv joints, and the transmission itself. this mod is probably best left to situations where the wheels can slip just a little .
I also don't know how the handling would be at the limits with a locked differential on tarmac. I'd assume you'd pick up a pretty nasty push in the tighter stuff, but that's just me - and I'm a Rallycross guy.


To me this guys setup of his 4EAT vs the regular 5 speeds reminds me of the Jeep transfer-cases where you have the part-time command-trac vs the full time selec-trac.

The idea behind "part time" use of awd or 4wd is that the front and rear are locked together, all the tires rotate at the same speed, as we found out in my differential thread from a while back (link in the video section), when taking turns, especially on pavement, the outside tires rotate a different amount then the inside tires. There is also the different rotational speed of the front tires and rear tires in 4wd or awd systems. This can cause stress on the components as they bind up by not being allowed to slip in a locked system. In snow or in dirt the tires are more inclined to slip on the loose surfaces, where as stick R-comps on pavement does not.

"full-time" on the other hand allows the components to slip somewhere along the line, using differentials. The exact nature of this I do not really understand because at this point I've mixed in Jeep/Chrysler talk into the Subaru mix of my ramblings.


The overall idea here is that you have a vehicle applying power through four tires, whether the setup is locked, open, or uses clutches or what not. If you have spinning tires, and all of a sudden one of them catches traction, that power and spinning momentum is now transferred to your driveline as your tire has caught traction or possibly even stopped. All of your drivetrain takes this force, and it is either strong enough to stand up to it, or the weakest component fails (similar to how a fuse works in wiring). This is where you see guys with locked differentials in offroad vehicles breaking things like differentials, axles, etc. How this compares to the various Subaru setups is something else entirely, as I do not know the exact pieces of the driveline for each model and what is most likely to fail. That is for another day.

Edited by 2point5RS_Dan, 18 January 2011 - 09:26 PM.


#4
OFFLINE   2point5RS_Dan

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Videos:

General knowledge:

How a differential works:


How clutches work:


How Manual Transmissions work:



Subaru:

Subaru AWD vs competitors:


AWD in motion, offroad test:


Subaru 5speed gear turning:
http://www.youtube.com/watch?v=Nm7jqi056M0

Subaru 5speed, roasted 2nd gear:
http://www.youtube.com/watch?v=lmlLL2ZANa4

Foreign video showing the Subaru symmetry. Linked for the diagrams:
http://www.youtube.com/watch?v=UlaI1gtv1fI

Subaru transmission mounts, stock vs sti:
http://www.youtube.com/watch?v=r6XQByQO8IY




Posted Image

Here we see different drivetrain layouts. On the far right is your traditional SUV setup, the middle is generally in the crossovers, and the left Subaru. The Subaru video comparing the AWD directly mentions the flaws of the middle diagram.

Edited by 2point5RS_Dan, 06 January 2010 - 04:26 PM.


#5
OFFLINE   2point5RS_Dan

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reserved!

#6
OFFLINE   Stein

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So I assume you're done reserving now? :D
Quick question for you or anyone else who knows: What exactly is a limited slip differential and why is it better than the rest? How does it work?

#7
OFFLINE   2point5RS_Dan

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So I assume you're done reserving now? :D
Quick question for you or anyone else who knows: What exactly is a limited slip differential and why is it better than the rest? How does it work?


Done reserving.

The wikipedia page sums up limited slips very well. It is a bit of a read though.

http://en.wikipedia....ip_differential

The main advantage of a limited slip differential is shown by considering the case of a standard (or "open") differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate freely—the torque transmitted will be equal at both wheels, but will not exceed the threshold of torque needed to move the vehicle, and thus the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use, such as driving in mud, off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the angular velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some traction available on at least one of the wheels.


The article goes on to explain torque vs speed sensing.

#8
ONLINE   i-wagon

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that's some good info. sticky?

#9
OFFLINE   RedForester08

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Add the Forester X premium 5mt winter package to the Rear LSD list MY06-08 and the MY01-02 Forester S Premium 5MT with winter package all of which have the viscous center and rear diffs with 50/50 torque split.

#10
OFFLINE   Mighty Subie

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good write up on it. I know a lot of people generally have no better idea of what's going on other than ''it's AWD" so hopefully this will help people better understand what's happening under their car to keep them happy in slippery going.
--Matt--

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#11
OFFLINE   krazykarpenter

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I think that my IQ just went up!

Great find dude. I knew a fair amount about AWD but that definitely spelled everything out clearly and concisely.

#12
OFFLINE   2point5RS_Dan

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Add the Forester X premium 5mt winter package to the Rear LSD list MY06-08 and the MY01-02 Forester S Premium 5MT with winter package all of which have the viscous center and rear diffs with 50/50 torque split.


Added to OP.

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OFFLINE   MAINIAC XV

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07 Spec B has a Torsen limited slip rear.

It ain`t over till the fat lady sings ( for those too young it`s a Yogi saying )


#14
OFFLINE   bigsxy

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wow that was a read and a half but good info

#15
OFFLINE   rallyimp

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when i was in school my teachers said that in some awd systems like in ford or chevys there front differentials (sp?) have a differnt gear ratio then there rear they also claimed subarus used it to. which i thought the front and rear diffs were the same in the subaru system or else it would cause the center diff to displace power un equally was i wrong in thinking this or was i right?

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OFFLINE   IBratmanI

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You were right, Subaru has ALWAYS had the same gear ratio front and rear. I honestly don't know about the others
AdidaSubarus

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OFFLINE   jimiii51

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I have never seen different gear ratios in front and rear diffs in production vehicles. I have seen it on modified vehicles with varying tire sizes.

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OFFLINE   krazykarpenter

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Found thes, this morning, and they are very basic but gives a visual aid to some of what is mentioned above:




Edited by krazykarpenter, 11 January 2010 - 11:20 AM.


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OFFLINE   Nate

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stuck
I used to be a big deal...

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OFFLINE   krazykarpenter

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??





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