5spd AWD

[CNY_Dave]

OK, I don't have a 5 speed, but I wonder about this. Have not seen a clear answer.

 

Does the 5spd AWD mechanism have a front/rear differential that is bridged by the silicone fluid viscous coupler, or is there just a front 'driveshaft' coupled to the rear driveshaft by the viscous coupler?

 

It's the 2nd thing, right?

 

And only the uber-fancy VDC automatics have the front/rear differential, yes?

 

 

 

Dave

[EVOthis]

I believe the answer to your question is yes...the 5 spds have a viscous fluid coupler as the "center differential" ...Its kind of hard to explain....um...take a look here...this might answer some of your questions... http://endwrench.com/images/pdfs/VarNov06EW.pdf

[CNY_Dave]

From that:

"Continuous All-Wheel Drive

In Subaru models equipped with a five-speed manual

transmission, a viscous-coupling locking center differential

built into the transmission case distributes engine

power 50:50 front-to-rear. Slippage at the front or rear

wheels causes more power to transfer to the opposite set

of wheels.

The viscous coupling contains a series of opposing discs

attached to the front and rear drive shafts, surrounded in a

type of silicone fluid. Slippage at the front or rear wheels

causes a rotational difference between the front and rear

discs in the viscous unit, which in turn shears the fluid.The

shearing action heats the fluid, causing it to thicken. As the

fluid thickens, power transfers from the plates rotating

faster (the slipping wheels) to those rotating more slowly

(the wheels with the best traction). When the slippage

ceases, all the discs turn at the same speed, restoring the

50:50 power split."

 

I would take "viscous-coupling locking center differential" to mean there is a differential that can be locked by the viscous assembly, not that the viscous assem is the only front/rear connection.

 

Dave

[nickb21]

That's my understanding too Dave. There is a front diff for the front wheels and at the back of the transmission there is another (probably smaller) diff that is coupled to the viscous unit. This would allow 50/50 front/back split normally and then when a speed difference occurs the viscous unit would 'lock up'.

 

There is a great generic diagram/explanation out there somewhere.. but I can't find the link to it off-hand.

 

Not exactly what I was looking for, but pretty neat: http://www.awd.ee/cars.html

Edited June 17, 2009 by nickb21

[ferret]

Reading for you from the FSM ( too bad I can't attach the pdf file, There are drawings to support the words )

 

The center differential consists of a set of bevel gears and a viscous coupling.

The center differential has the following two functions: distributing the engine torque to the front and

rear wheel drive shafts and absorbing the difference in rotating speed between the front and rear

wheels.

The engine torque enters the center differential case from the transmission’s driven shaft. The engine

torque is then distributed through the bevel gear set directly to the drive pinion shaft and via

the transfer drive and driven gears to the rear drive shaft.

The viscous coupling limits the bevel gear set’s differential action when either front or rear wheels

spin so that adequate torques are transmitted to the front and rear wheels and proper traction is

obtained.

C: FUNCTION

When there is no speed difference between the front and rear wheels, the center differential delivers

the engine torque to the front and rear wheels at a ratio of 50:50.

When a rotating speed difference occurs between the front and rear wheels, the center differential

operates to absorb it in a controlled way by the function of the viscous coupling.

1. DURING NORMAL DRIVING

During straight-line driving on a flat road at a constant speed, all the four wheels rotate at the same

speed. The center differential delivers engine torque evenly to the front and rear wheels. The viscous

coupling does not generate shear torque because there is no relative movements between the

inner and outer plates.

2. DURING TURNS AT LOW SPEEDS

During turns at low speeds, rotating speed difference occurs between the front and rear wheels, as

well as between the left and right wheels. More particularly, the front wheels rotate faster than the

rear wheels. The center differential then acts to absorb the speed difference to enable smooth driving.

Although the speed difference is small under this condition, operation of the viscous coupling causes

more torque to be transmitted to the rear than to the front.

3. DRIVING ON ROUGH OR SLIPPERY ROADS

When front wheels are on a slippery surface

When the front wheels begin to spin, the resulting speed difference between the front and rear drive

shafts causes the viscous coupling to generate significant amount of shear torque. As a result, the

torque distributed to the rear wheels becomes much larger than that distributed to the spinning front

wheels. The traction and driving stability are thus ensured on a rough or slippery road.

When rear wheels are on a slippery surface

When the vehicle is accelerated quickly from a standing start with the rear wheels on a slippery surface,

the distribution of the vehicle weight on the front and rear wheels changes and the rear wheels

start spinning. Due to the resulting speed difference between the front and rear drive shafts, the viscous

coupling generates a significant amount of shear torque, now in the direction opposite to that

generated when the front wheels are on a slippery surface. As a result, the torque distributed to the

front wheels becomes much larger than that distributed to the rear wheels.

[CNY_Dave]

Excellent- thanks.

 

So there's a LSD in the 5spd trans, just like in the rear axle (if LSD equipped).

 

So you could (if the mechanical bits permit) remove the viscous coupling and you'd still have drive to the front and rear, as long as no wheels slipped. It'd be like old-timey 'full time 4wd', that is, two wheel drive.

 

 

Dave