AWD system: LL Bean vs VDC? - Subaru Outback - Subaru Outback Forums
 
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post #1 of 6 (permalink) Old 05-07-2011, 03:52 PM Thread Starter
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AWD system: LL Bean vs VDC?

I was floating around the web, and I think I read on one site that there are actually two different AWD systems used on the 2000-2004 Outback. What I read was that the LL Bean has the standard Subaru AWD, with a viscous coupling at the center differential. In normal driving, the front wheels receive 90% of the torque, 10% goes to the rear, essentially making this a front drive car that engages the rear in low traction situations.

Apparently, from what I read, the VDC cars have a system that was previously Japan-only, and it entails some sort of electronic control of the center differential. This system, in normal driving, sends 55% of the torque to the rear wheels, and 45% to the front. Supposedly this makes the VDC far superior in adverse conditions, but does effect efficiency.

Any truth to all this?

Thanx!

N

'01 Outback LL Bean, '85 Porsche 928S2 5 speed

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post #2 of 6 (permalink) Old 05-07-2011, 10:21 PM
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Almost accurate.

The beaner and non-VDC autos have the 90/10 split (when nothing is slipping, or not in 1,2,R, or the gas floored), but they use a clutch to put power to the rear wheels.

Standard shift cars have the viscous clutch.

The VDC cars are set up as you describe, and VDC is a more sophisticated system.
I don't know if there's a real efficiency hit, though.

Dave
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post #3 of 6 (permalink) Old 05-20-2011, 04:21 AM
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A late reply, but better late than never.

I'll add:

The AWD system in the VDC models is called Variable Torque Distribution (VTD).

It seems strange to me that Subaru emphasized the stability control system (VDC) when the main reason to buy a VDC model was the VTD drivetrain. But I guess Subaru chose to advertise soccer-mom safety more than performance.

VTD uses a center differential with a planetary gearset that gives a full-time rear-wheel bias. 55% rear/45% front.

Manual transmission Outbacks also have a center differential, albeit a simpler gearset with a 50%/50% front/rear power split.

The other Outbacks (i.e. most Outbacks) have "Active AWD", which has no center differential at all. Instead, they have a transfer clutch that progressively locks the front & rear drivetrains together during acceleration or slippage. You can find lots of forum arguments about the "power split" on these models...because it isn't a consistent split, and it doesn't work like a differential.

If you aren't familiar with the function of a differential, look it up on wikipedia or howstuffworks. Basically a differential allows two driveshafts (left & right or front & rear) to spin at different speeds while still transmitting all power. It is a very different beast from a transfer clutch, which locks the speed of the driveshafts.

ALL Outbacks have a front differential and a rear differential, because the wheels on each side obviously need to spin at different speeds while the car is cornering or turning.

But the only Outbacks that have a center differential are the manual transmission models, and VTD models.

The manual tranny Outbacks also have a viscous coupling that will lock front & rear together when they are spinning at very different speeds.

The VTD Subies have a transfer clutch, just like the "Active AWD" Subies, but this clutch is mostly used for wheel slippage, since the center differential already makes it a full-time AWD vehicle. So this clutch is smaller (fewer plates) than in the Active AWD model.

The VDC Subies also do not need a viscous rear LSD, because they can control wheel slippage via independent wheel braking.

In brief: If you like to drive curves, accelerating out of the apex, get a manual or a VDC/VTD Subie. They are the only ones that have a mechanical center differential.


-Jeff
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post #4 of 6 (permalink) Old 05-20-2011, 09:01 PM Thread Starter
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Quote:
Originally Posted by driveby View Post
A late reply, but better late than never.

I'll add:

The AWD system in the VDC models is called Variable Torque Distribution (VTD).

It seems strange to me that Subaru emphasized the stability control system (VDC) when the main reason to buy a VDC model was the VTD drivetrain. But I guess Subaru chose to advertise soccer-mom safety more than performance.

VTD uses a center differential with a planetary gearset that gives a full-time rear-wheel bias. 55% rear/45% front.

Manual transmission Outbacks also have a center differential, albeit a simpler gearset with a 50%/50% front/rear power split.

The other Outbacks (i.e. most Outbacks) have "Active AWD", which has no center differential at all. Instead, they have a transfer clutch that progressively locks the front & rear drivetrains together during acceleration or slippage. You can find lots of forum arguments about the "power split" on these models...because it isn't a consistent split, and it doesn't work like a differential.

If you aren't familiar with the function of a differential, look it up on wikipedia or howstuffworks. Basically a differential allows two driveshafts (left & right or front & rear) to spin at different speeds while still transmitting all power. It is a very different beast from a transfer clutch, which locks the speed of the driveshafts.

ALL Outbacks have a front differential and a rear differential, because the wheels on each side obviously need to spin at different speeds while the car is cornering or turning.

But the only Outbacks that have a center differential are the manual transmission models, and VTD models.

The manual tranny Outbacks also have a viscous coupling that will lock front & rear together when they are spinning at very different speeds.

The VTD Subies have a transfer clutch, just like the "Active AWD" Subies, but this clutch is mostly used for wheel slippage, since the center differential already makes it a full-time AWD vehicle. So this clutch is smaller (fewer plates) than in the Active AWD model.

The VDC Subies also do not need a viscous rear LSD, because they can control wheel slippage via independent wheel braking.

In brief: If you like to drive curves, accelerating out of the apex, get a manual or a VDC/VTD Subie. They are the only ones that have a mechanical center differential.


-Jeff
Interesting.

My understanding is that my LLBean has a "slipper" clutch at the end of the transmission. This is an actual automatic transmission band-type clutch that continuously varies the amount of torque to the rear driveshaft, based upon the difference between front and rear axle speed. LL Bean has no center differential, so the computer and the limited slip differentials, while all fighting for the same team...aren't in communication with each other. I think I read that he VDC in fact has a center diff with an electronically controlled viscous limited slip, and the same at both axles, and all three work as a team, hence the better ability to put power to the ground.

That is, when the LL Bean is in "D". In 1 and 2, my understanding is that Mr. Bean locks his slipper clutch and we are now in a 50/50 AWD situation.

Can the LL Bean get into a "torque lock" situation?

Thanx!

N

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post #5 of 6 (permalink) Old 05-20-2011, 09:53 PM
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Unless I'm reading it wrong, according to cars101.com (2001 Subaru Outback options, prices, all currrent and older Subarus Outbacks Foresters Imprezas Bajas) the 2001 LLB is a H6 with the 4-speed with active AWD. That's the "Active AWD" driveby described above. The "multi-plate transfer" clutch, or MPT, which connects the rear drive shaft to the transmission output is made up of five pairs of clutch plates that are engaged by a hydraulic piston. There's no "band".

The transfer clutch is always engaged to some degree when the engine is running and the transmission in gear; the amount of engagement varies continuously, responding to a number of different sensor inputs. There is no preset 50/50 "lock" in any particular gear.
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post #6 of 6 (permalink) Old 05-20-2011, 10:05 PM
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Normy - Your LLBeaner has a multi-plate transfer clutch ("MPT" in factory-manual lingo) with 6 plates.

For comparison, the 2.5L H4 AT from that generation has 5 MPT plates (b/c it has less engine power), and the 3.0L H6 VDC/VTD model has just 3 plates (b/c AWD is mostly handled by the center differential).

You also have a viscous limited slip REAR differential. The VDC models do not have this LSD, because they can do it electronically, with individual wheel braking.

Your transfer clutch pressure is increased in 1st gear, or when you accelerate, or when the front & rear drivetrains are moving at significantly different speeds (slippage). The clutch pressure is reduced when the car is turning, to prevent torque bind.

And yes, you can get into a "torque lock" situation. Most folks here call it "torque bind". If the transfer clutch solenoid fails on the MPT drivetrain, the clutch will be full-on by default, so the front & rear drivetrains will be locked together.


-Jeff

EDIT: plain_OM beat me too it with most of this explanation.
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