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Old 12-23-2012, 12:23 AM   #1
motorace
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Default explain imprezas AWD?

from what i know about 4x4 ford trucks, which i can be wrong about, the 2 fronts wheels would be locked together with one of the rear tires, with least resistance rear, having power. thus, 3 wheels have power while in 4x4.



what does an impreza have? with the computer transferring power to slipping wheels, does that means it's similar, as in 3 wheels have power?

(or like other brand awd vehicles, where its just 1 wheel up front and 1 wheel in the back and call it awd?)

Last edited by motorace; 12-23-2012 at 12:41 AM.
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Old 12-23-2012, 07:44 AM   #2
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Old 12-23-2012, 08:07 AM   #3
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Old 12-23-2012, 08:27 AM   #4
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Great videos! I had not seen them before. They pretty much explain why Subarus can deal with bad roads easier than many other vehicles.

John

edit: Friday night when I got off work, the roads in these parts were beyond bad. With the wind and snow, salt and plow trucks could not keep up. Thw vehicle I drive at work is a full size Ford van without posi traction. It is by far one of the worst vehicles I have ever driven in snow, rivaling that of '60's and '70's muscle cars. I can tell you with certainty that the system used with the CVT does as well as the 5 speed manual system that was in my '03 WRX. EVERYBODY around me was struggling to make up hills and I was able to drive easily. No drama. Just as if the road was dry. THAT is why Subaru owners are smiling when driving on wintery roads. ;-))

Last edited by Guzzi 1; 12-23-2012 at 08:36 AM.
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Old 12-23-2012, 08:14 AM   #5
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look at the AWD wiki, it depends upon whether you have a CVT or a 5 Speed which type of AWD the car has.

I don't think every single claim on the wiki document is accurate, but that has been discussed on the forum multiple times. It does give you a general idea of the different systems and how they work.

Most videos including those above show the CVT/Auto version of symmetrical AWD.

In no case is it like the systems you are describing.

http://www.awdwiki.com/en/subaru/
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Old 12-23-2012, 11:18 AM   #6
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Loved the video's! I love my Subies and that is why I'm on my fifth one now.
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Old 12-23-2012, 11:23 AM   #7
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And this one is a lot of fun too, if you have an hour to spare....


It's surprising how many vehicles have trouble on a just pair of front or rear rollers (is that a BMW xDrive that can't get off a single rear roller?) it would be nicer to see some other makes and newer models.
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Old 12-24-2012, 01:20 AM   #8
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Quote:
Originally Posted by grubincan View Post
And this one is a lot of fun too, if you have an hour to spare....

AWD sistems wars (testing) on rollers - YouTube

It's surprising how many vehicles have trouble on a just pair of front or rear rollers (is that a BMW xDrive that can't get off a single rear roller?) it would be nicer to see some other makes and newer models.
watching that video make me doubt a little about the legendary awd system of subaru.
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Old 12-24-2012, 06:30 AM   #9
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Quote:
Originally Posted by ans2k View Post
watching that video make me doubt a little about the legendary awd system of subaru.
i'm not sure rollers is a great indicator of much since no one ever drives on rollers.
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Old 12-24-2012, 10:32 AM   #10
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Quote:
watching that video make me doubt a little about the legendary awd system of subaru.
I think a VDC-equipped Subaru would have behaved differently.
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Old 12-28-2012, 02:26 PM   #11
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Quote:
Originally Posted by ans2k View Post
watching that video make me doubt a little about the legendary awd system of subaru.

does it make a difference if its stick vs auto? i assume it shouldnt since its dependent on the AWD system when traction is lost.

bc the videos above shows the same test but up hill and it climbs fine
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Old 01-23-2013, 01:58 PM   #12
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Quote:
Originally Posted by grubincan View Post
And this one is a lot of fun too, if you have an hour to spare....

AWD sistems wars (testing) on rollers - YouTube

It's surprising how many vehicles have trouble on a just pair of front or rear rollers (is that a BMW xDrive that can't get off a single rear roller?) it would be nicer to see some other makes and newer models.
None of this is too surprising. Most people get caught up in marketing rhetoric, and it's no surprise that people love Subaru's “Legendary” AWD system. Most of the vehicles in the video have open diffs for the front and rear axles (some of the newer vehicles are also backed by e-diffs), thus the lack of moving when front and rear left or front and rear right wheels are on rollers. E-diffs can be defeated when traction is very low. The only Subarus as of 2011 that use mechanical lsds in the front or rear axles are the STI models. The rest all have open front and rear differentials.


The only AWD system that fits the profile of “Legendary” by Subaru is the STI's AWD system and the the VTD AWD systems found on 4EAT GD WRXs and 5EAT 05-09 LGTs and Outback XTs. They all have a planetary gear center diff with a rear lsd (clutch type lsd on the WRX, LGT, XT, and 04-06 STI; Torsen T2 on the 07+ STI). In the STI, the driver has the ability to manually lock its center diff and manually change its degree off lockup. All STIs have a front helical type lsd. These systems, paired with a VDC (essentially e-diffs) are extremely competent systems which direct the most torque to the wheels with the most traction.


Now take the rest of the automatic transmission AWD systems. There's a mutliplate clutch in the center and acts proactively (many here have argued that it's not called a differential—I really couldn't care less about semantics). I have no problem with this system. My fiance owns a 4EAT Impreza. The clutchpack provides ample torque transfer between the front and rear axles and works fine as a daily driver. For the price of the vehicle, it's great, even though it's not the most technologically advanced unit. If it ain't broke, don't fix it. The front and rear axles have open diffs backed by e-diffs. I've never been stuck with this car, but I never had the opportunity to test it in really icy conditions as I don't drive it often. Nonetheless, for the price of the car, I know that it does what it's supposed to do. Is this AWD system legendary? I think it's just rhetoric when comparing it to other systems that I've driven in the ball-park price range. It's basic, but it's put together well. This system will probably fail a roller test whereby the left front and rear wheels are on 0 friction rollers on an incline. The e-diffs will most probably be defeated. I'll even venture on to say that it will fail the same tests in the video, minus the ones where the fronts or rears are on rollers. We know the clutchpack works fine. E-diffs can only do so much.


Now take all the manual tranny AWD systems. Having owned a 5MT 07 Leggy, I can tell you that it's absolutely awful. What I mean by awful is that it's simply the worst AWD system that I've ever driven short of a completely open diff setup. The real joke is that it's pretty much the same as an open diff setup. I remember a thread called “5MT AWD=All wheels disconnected?” by sajohnson and his experience was exactly like mine. The 4kg viscous coupling diff is so awful at regulating any speed differences between the two axles that Subaru may as well have given us open diffs to save weight, as the VCU is useless with the sole purpose of increasing the car's curb weight. I swear to god, you can take any center diff on the market and it will be better than that awful piece of garbage found on manual models. Even the newer models with VDC are terrible, because the braking done by the e-diffs simply is not enough to compensate for the lack of lockup in the center diff.


The only reason I still own a Subaru is because I own a 2010 STI, which has a fantastic, albeit pretty old AWD system. I mean it really stomps face. That's the only high end AWD system provided by Subaru though, and I paid almost as much as a base Audi S4 for it.

Last edited by sti2010rl; 01-24-2013 at 12:54 PM.
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Old 12-23-2012, 01:13 PM   #13
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I like this video the best, expecially because it shows the CVT Crosstrek holding its own with a dedicated 4WD Landrover. You can see how well the Symmetrical AWD (electronic clutch version) and VDC work together to limit wheel slip and transfer power.
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Old 12-23-2012, 01:59 PM   #14
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^^ That's impressive.
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Old 12-24-2012, 10:51 AM   #15
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You don't have to think it, look at the Crosstrek video and notice that when a wheel has no traction it cuts off power to that wheel pretty much instantly. There is almost no wheel slip, and less than the 4WD Landrover!

What subaru was that in the roller video? It may not have had any traction control, only AWD?
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Old 12-24-2012, 11:06 AM   #16
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Quote:
Originally Posted by motorace View Post
from what i know about 4x4 ford trucks, which i can be wrong about, the 2 fronts wheels would be locked together with one of the rear tires, with least resistance rear, having power. thus, 3 wheels have power while in 4x4.



what does an impreza have? with the computer transferring power to slipping wheels, does that means it's similar, as in 3 wheels have power?

(or like other brand awd vehicles, where its just 1 wheel up front and 1 wheel in the back and call it awd?)
No, all Subarus (save the BRZ and foreign kei cars) use full-time awd which is never deactivated. All 4 wheels receive power all the time unless slipping occurs where the system needs to reroute it.

2011+ Impreza 5MT

Rear Differential = Open (VDC applies brake to slow spinning wheel)
Front Differential = Open (VDC applies brake to slow spinning wheel)
Center Differential = 50/50 torque split ALL the time. A viscous coupling center differential has metal plates that are connected to either the front or rear driveshafts and are surrounded by a viscous fluid. During normal driving the metal plates spin at an even 50/50 rate. Upon either axle slipping that shaft's metal plates will begin to spin at a faster rate that the other. The shearing action of the plates and heat from friction causes the fluid to harden. The hardening fluid thus slows the spinning plate and forces all plates to spin at 50/50 again. This is a VERY good system.
Turn off VDC when in deep snow to prevent Engine Power Reduction from stalling the motor.

2011+ Impreza CVT

Rear Differential = Open (VDC applies brake to slow spinning wheel)
Front Differential = Open (VDC applies brake to slow spinning wheel)
Center Differential = Actively changing torque split (60/40 normally per SOJ and SCI). A multiplate clutch system is housed in the center differential which can be compressed based on a number of sensors (throttle position, wheel slip, steering, etc). The computer will determine how much clamping pressure to apply to the center, thus altering the amount of torque flowing between the front and rear driveshaft. A major difference between Subaru and others is that the system is not FWD until slip. The system will alter the torque split if you merely push down the gas pedal. It is not a fwd car that only kicks in the rear tires in bad situations. This system works well for normal snow driving. Not designed for off-road.
Turn off VDC when in deep snow to prevent Engine Power Reduction from stalling the motor.

Last edited by Snow Drift; 12-24-2012 at 11:12 AM.
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Old 12-24-2012, 11:13 AM   #17
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CVT Center Differential = None
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Old 12-26-2012, 03:54 PM   #18
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Quote:
Originally Posted by Commander Keen View Post
CVT Center Differential = None
^This
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Old 12-26-2012, 07:59 PM   #19
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Quote:
Originally Posted by Commander Keen View Post
CVT Center Differential = None
Quote:
Originally Posted by riccnick View Post
^This
Uh, there is a center differential. It is built into the rear of the transmission and contains clutch packs. How do you think power is split between the front and rear axles?
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Old 12-26-2012, 08:02 PM   #20
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Uh, there is a center differential. It is built into the rear of the transmission and contains clutch packs. How do you think power is split between the front and rear axles?
There is not. It is possible to combine a differential with an MPT clutch, just like the 5MT combines a differential with a viscous coupling, but this is not what the CVT has. It has only a clutch pack.
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Old 12-26-2012, 08:46 PM   #21
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Originally Posted by Commander Keen View Post
There is not. It is possible to combine a differential with an MPT clutch, just like the 5MT combines a differential with a viscous coupling, but this is not what the CVT has. It has only a clutch pack.
And the clutch pack is the differential. It allows the front and rear axles to rotate at different rates or compress the packs and come to 50:50. It may not be a mechanical gear but it is a differential. You're playing semantics. If it didn't have a differential you wouldn't have AWD (unless you had two axle motors or hub motors)
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Old 12-26-2012, 08:56 PM   #22
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A clutch is a clutch and a differential is a differential.

A differential allows shafts to rotate at different speeds with very little power loss.

A clutch must either disengage, or dissipate the difference in motion as heat.

The CVT's rear wheels can never turn faster than its front wheels.
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Old 12-26-2012, 09:14 PM   #23
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Quote:
Originally Posted by Commander Keen View Post
A clutch is a clutch and a differential is a differential.

A differential allows shafts to rotate at different speeds with very little power loss.

A clutch must either disengage, or dissipate the difference in motion as heat.

The CVT's rear wheels can never turn faster than its front wheels.
So would you say the Evo 9 and 10 don't have center differentials? Their center is a clutch pack.
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Old 12-26-2012, 09:27 PM   #24
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Like I said before:

Quote:
It is possible to combine a differential with an MPT clutch, just like the 5MT combines a differential with a viscous coupling, but this is not what the CVT has. It has only a clutch pack.
The Evo is an example of a real center differential combined with a clutch pack. Again, this is NOT what the CVT Impreza has.

See: http://www.roadandtrack.com/car-news...ntial-you-5535
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Old 12-26-2012, 10:45 PM   #25
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Evo ACD is just clutch packs.
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