AWD basic description of operation? - Subaru Forester Owners Forum
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post #1 of 59 (permalink) Old 11-15-2008, 11:21 AM Thread Starter
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AWD basic description of operation?

I am curious about the way the all wheeel drive works. Are all 4 wheels constantly and equally powered except of course for the diif slippage or is the power to front and rear wheels proportional, or what?
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post #2 of 59 (permalink) Old 11-15-2008, 12:35 PM
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I'm not saying it's 100% correct but for the foresters with MT (Manual Transmission) there is 50/50 split front/back in the center diff. Meaning all wheels get around the same amount of power. This is all good until one or more wheels have no traction. That's where you either need to manually try to brake down the wheels with no traction or you got technology or some locking diff installed to take care of that. Or else you got no power to the wheels that have grip but does nothing.

The AT version got more power at the front with ability to transfer power to the back when needed. Besides that it's more or less the same.

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post #3 of 59 (permalink) Old 11-15-2008, 01:27 PM
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Originally Posted by AutoWorld.com
All-Wheel Drive (Automatic Transmission)

Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission 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 transmissionís 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. Itís designed to slip according to how much all-wheel drive is needed. When an automaticís 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.

The MPTís 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 carís 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 isnít 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

All-Wheel Drive (Manual Transmission)

The 5-speed manual transmissionís 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.
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post #4 of 59 (permalink) Old 11-15-2008, 02:52 PM Thread Starter
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Thanks a lot FX4me, this is precisely what I was looking for.
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post #5 of 59 (permalink) Old 11-15-2008, 04:32 PM
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In the auto , when you have the gear position in 1st or 2nd you have an approximately 50/50 split. The Subaru stands almost alone at its ability to shift enough power to the rear wheels to go up a ramp with zero traction at the front wheels.

Be careful what you wish for.
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post #6 of 59 (permalink) Old 11-17-2008, 03:47 AM
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In the auto , when you have the gear position in 1st or 2nd you have an approximately 50/50 split. The Subaru stands almost alone at its ability to shift enough power to the rear wheels to go up a ramp with zero traction at the front wheels.
Apart from every RWD on the planet of course......

Golf 4-motion/Audi TT and A3 quattro's will all do it.

Simon

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post #7 of 59 (permalink) Old 11-17-2008, 04:50 AM
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... Golf 4-motion/Audi TT and A3 quattro's will all do it.
"... the 4MOTION system is... called the Haldex coupling... has the major advantage of being electronically controlled and having a very fast response... The coupling provides an infinitely variable torque transfer between the front and rear axles.... In the normal case... 90 percent of the available torque is transmitted to the front axle and ten percent to the rear axle... In extreme cases up to 100 percent of the available torque can be transmitted to the rear axle..."
VW Passat 4Motion In Depth - worldcarfans
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post #8 of 59 (permalink) Old 11-17-2008, 04:55 AM
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In extreme cases up to 100 percent of the available torque can be transmitted to the rear axle..."
I wonder...If you watch the Subaru on the ramp..since effectively no power is used on the front wheels and the rear wheels drive it up the ramp. It would seem like almost 100% of the power goes to the rear wheels.

Be careful what you wish for.
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post #9 of 59 (permalink) Old 11-17-2008, 11:13 AM
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I wonder...If you watch the Subaru on the ramp..since effectively no power is used on the front wheels and the rear wheels drive it up the ramp. It would seem like almost 100% of the power goes to the rear wheels.
The article said with 4Motion, when the front wheels spin, 100% of the power goes to the rear wheels.
With Subaru, it may seem like almost 100% of the power goes to the rear wheels, but Subaru says it's 50/50.
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post #10 of 59 (permalink) Old 11-17-2008, 12:15 PM
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The article said with 4Motion, when the front wheels spin, 100% of the power goes to the rear wheels.
With Subaru, it may seem like almost 100% of the power goes to the rear wheels, but Subaru says it's 50/50.
Yea, but it would be impossible to dissipate 50% of power on freely spinning wheels. . But I realize you understand that also.

Be careful what you wish for.
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post #11 of 59 (permalink) Old 11-17-2008, 03:21 PM
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Yea, but it would be impossible to dissipate 50% of power on freely spinning wheels. But I realize you understand that also.
I think Subaru AWD splits the power 50/50, like 4WD. I know spinning wheels transmit no power to the ground, but that does not prevent them receiving 50% of the power. They won't spin until the rears spin. Then 100% of the power would be going to freely spinning wheels, but none of it transmitted to the ground.
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post #12 of 59 (permalink) Old 11-18-2008, 01:10 AM
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With the centre clutch plate pack locked, many call this 50/50, in reality its unknown as it depends on the surface and rotational speeds of the axles, if it were 50/50, then with the front on rollers and unable to take any torque, the rear would have the same and the car would roll back (vintage your talking rubbish I'm afraid!), so in this scenario with the centre clutch locked (Subaru or VAG Haldex) its 0 front 100% rear!

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post #13 of 59 (permalink) Old 11-18-2008, 03:16 AM
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if it were 50/50, then with the front on rollers and unable to take any torque, the rear would have the same and the car would roll back
That's what I was getting at.

Be careful what you wish for.
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post #14 of 59 (permalink) Old 11-18-2008, 05:09 AM
 
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As we've found in the past, using traditional 50/50 nomenclature is incorrect.

I could probably write a thesis on how the 5MT is built and works. I'll try to be brief.

I think the best way to describe this, is to describe the center differential.

The output shaft of the transmission has two parts. There is an outer part, and an inner shaft. The outer part is directly linked with the output of the transmission. The inner part is what drives the front differential.

Now the center diff contains very closely spaced plates surrounded by a really thick, gooey, silicone fluid. This goo has a high CTE (Coefficient of Thermal Expansion). The plates alternate attachment points. Half of them are attached to the diff casing itself, the outer part directly linked with the transmission output. The other plates are attached to a shaft that slides over the pinion shaft, effectively linking them to the front differential.

The pinion shaft plates, on the back end, are attached to a set of spider gears (like a rear differential). They spin against a main gear anchored to the back of the diff case. This back gear is splined, and that is what the transfer gear is driven by.

As you can imagine, there is a physical lock between the front and rear wheels by way of the spider gears.

Now, it's best to think of what would happen if there were no fluid to provide any LSD function.

The center differential would, then, operate as a completely open differential. This is where things get hairy.

If this were the case, a lack of traction at the front of the vehicle will unload all power to the front. You can't really say torque, because there must be some force at that end. Torque, for this instance, is the frictional force of the tire on the road times the radius of the wheel.

Given a perfect scenario, the fluidless open center differential will distribute power so long as the torque at the wheels is equal (that is, each wheel has equal grip). We'll bar all "what happens when you punch it" because that involves far more than drivetrain dynamics. To quench interest, the fronts will spin first, but due mostly in part to a change in grip that results from body motion. Once the fronts start to spin, it takes a lot less energy to keep them spinning.

Now, add some goo to the diff. The static distribution is still that equal distribution. But as slip occurs (again, dependent on grip at each wheel), the fluid shears and expands. This fluid expansion puts pressure on the plates in the center differential, effectively locking the pinion shaft to the diff casing (and thus the rear wheels).

So when you hit the gas with a completely open differential (like the Cusco Tarmac 5MT diff, or an uncontrolled DCCD diff, or a non-fluid-filled 5MT diff), the fronts will spin due to a change in grip. Like electricity, drive torque (which is different than wheel torque and is defined by Subaru to be the torque applied to the output shaft of the transmission), finds its way down the path of least resistance.

With goo, as this occurs, the plates lock together in an attempt to get the front and rear output speeds to be the same. NOTE, that this is an attempt to get the front and rear output speeds the same, NOT the wheels side-to-side to be the same. So you could completely stop one wheel and double the speed of the other and the front/rear output speed ratio would be the same.

THAT is all the 5MT center diff does. It tries to maintain a static ratio of front/rear output speeds.

By it's very construction, you can see that it would be highly improbable to completely isolate the front wheels from the rear. The only way to do that would be to completely lock the front wheels. If that were the case, power WOULD transfer entirely to the rear, the diff-goo would try to lock the shafts together, but would soon shear and provide no LSD function.

The center diff is, by design, more than just an LSD. It's a NSD (no slip differential).

On a micro-scale, the pinion shaft will start to twist axially before that rotation actually gets to the differential. With a center diff in higher temperatures (as would happen during aggressive driving, or even on a really warm day), this lag in rotation can actually provide enough of a variation in rotational speed to begin plate-lock-up before any substantial slip occurs. By the time the wheels are slipping, the center diff is already damping the faster spinning shaft.

So, to conclude, the center diff isn't "REALLY" 50/50 when it comes to torque distribution. It IS 1:1 when it comes to output speed ratio, or at least it tries to be.

There it is. Glorious in it's simplicity, it's a design that has ruled the AWD world for 20 years. There are only two systems close in efficiency. One is a torsen style helical differential, which again, on a micro scale pre-empts slip through means of physical locking rather than fluid. The other is a clutch-based system which behaves similarly, though requires the use of computers to mandate clutch modulation. Some will say that the computers cannot predict the coefficient of friction, and thus cannot apply the minimum pressure required to do the job of maintaining shaft-speed-ratios, thus having a finite range of lockup (whereas the 5MT fluid based system provides just enough locking to keep the shafts going the same speed). Also, the electronic controlled diffs are reactive, so wheels actually have to slip before lockup occurs (because they reference ABS sensors or some other speed sensor down-the-line from the "diff"). The former is almost indistinguishable to your average driver (though you can generally "tell" when the car is playing with lockup, vs. the 5MT where it's a seamless transition). The latter can be offset by sensory input like throttle position, load, RPM and vehicle speed. If you suddenly go WOT at low RPM and low speed, it'll lock the center diff up the same time it opens the butterfly on the intake manifold, pre-empting a loss of traction.

09 SWP Outback 2.5XT 5MT 263whp, 337 lb-ft
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post #15 of 59 (permalink) Old 11-18-2008, 12:55 PM
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Seriously excellent read BAC! I really didn't know there was a difference in the AWD system for the auto and manual foresters until recently. Your explanation has clarified my perceptions a lot.

Makes me REAL happy I drive a 5mt Forester. I have bookmarked this thread so I can read that over a few times in the future
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