viscous coupling

There is a center differential. The engine drives it and one output goes to the front and the other to the rear. The VC is inside this center diff.

Definitely. The front and rear are driven at all times through the center differential. As long as the fronts and rears are turning the same speed, there's a 50/50 torque split and the VC is not doing a thing.

When there is a speed difference, the fluid in the VC heats up. That forces the plates inside the VC closer together and causes the front and rear drive to be locked together, "short circuiting" the center dif.

No. The VC will lock any time there is a large

*difference* between the front and rear. It doesn't matter which end is spinning and which is standing still.

I don't think that you could feel the drag on a turn if the VC was working right. It takes some finite time for the VC to heat up and lock.

My wife claims she can smell the VC locking when she drives in the snow.

That is an excellent response. FWIW, I thought that when the VC heat up, the fluid thickens which gives it its "lock-up' characteristics. I don't believe there are any plates in there.

Regards,

Without looking it up (I'm not absolutely sure) I thought that the front and rear diffs were open on the 01 auto.

Rob, yeah there's no friction plates but rather a series of interleaved slotted plates (vanes). Half are coupled to the front while the remaining half are coupled to the rear. They don't contact each other, but they're spaced very close to each other. -Danny

Thanks. You know, we really need a VC/AWD FAQ somewhere. There's info scattered on the web, but nothing that's real specific to Subaru. I spent about 20 minutes going through the service manual so I could back up what I said, but it's really no help.

Rob

There def>

I didn't mean to imply that she smelled anything coming from the inside of the housing. I'm sure she could smell a generalized "hot oil" smell if the case was heating up though.

She can smell WD-40 on me hours later if I've used it for a cutting fluid so I don't doubt her veracity.

Got the pics posted anywhere? :-)

I read all the responses, and I still don't understand how there can be any torque transfer to the rear wheels on dry pavement driving in a straight line, or indeed any torque transfer at all unless the front wheels are turning faster than the rear wheels. I think the front differential is hard-coupled to the engine (thru the clutch) and is not driven thru the viscous coupling; the rear wheels are driven thru the coupling. It is impossible for the front wheels to turn at a speed different from what is determined by the engine speed and gearing. But the rear wheels can turn slower (or faster, in a turn) than the speed determined by the engine and gears. If the front wheels are turning faster than the rear wheels (due to slippage), then the torque gets divided 50-50 front and rear, but if everything is turning the same speed, then the rear wheels are turned by the pavement going by under them as the front wheels pull the car forward, there is no relative motion in between the disks in the viscous coupling, and no torque is transmitted.

If this is wrong, please explain how. I'm not insisting that I am right,but I just don't understand. To say there is a "center differential" is not really correct in the sense of a standard differential with spider gears etc. If there were a differential like the one on the front or rear axle, then the front and rear wheels could turn at any speed whatsoever relative to each other, and all torque would transfer to the slipping wheel as happens left/right in a normal

2-wheel differential (not limited-slip). Clearly this is undesirable for traction. The viscous coupling, on the other hand, transfers torque to the rear wheels that are not slipping (yet), but ONLY if a front wheel is slipping.

You are wrong. Simple as that. The center dif input is hard-coupled to the engine.

the rear wheels are driven thru the

Yes it is. It's a dif just like on a rear-wheel drive car.

If there were a differential like

Let's go back to first principles.

For the sake of discussion, think of a rear wheel drive car. You have a dif with power in and 2 wheels out. Spyder gears and all. The car is going straight, what's the torque split between the wheels.

50/50 right?

Now take a front wheel drive car. Put power into the dif and you get 50/50 again, right?

Ok, feed the front dif and the rear dif with the outputs of a center dif. 50/50 front and rear,

25% to each wheel. We haven't broken any rules and we haven't said a word about the VC. That's the way your Subie goes down a straight and dry road. Exactly like that. You don't have to make it any more complicated and I have *not* over- simplified it.

Now, assume one or both wheels on the same dif start to spin. The VC in the center dif heats up and starts to hard couple the front dif drive to the rear dif drive. Since now they both

*have* to turn at the same speed, most of the torque goes to the unspinning end of the car.

That's all there is to it.

A minor correction to my earlier post. After doing some research, it appears that a properly operating VC does not ever 'lock', as in creating a solid linkage between the two output shafts of the center dir. It appears to be more like a slipping clutch, causing torque to be transmitted along with some slippage. This does not affect my explaination in any significant way.

Ok... that makes good sense vis-a-vis front/rear slip.... What about right/left? Is there a lateral diff. on the car so that all that new traction is evenly split or sent specifically to the wheel that needs it the most?

I'm not sure I understand the question.

Front and rear difs are not limited slip. Spin a front wheel and a rear wheel at the same time and you're outa luck. Doesn't seem to be a big problem in practice.

The center diff in manual Subarus is in essence exactly the same as and open diff as found in most cars *except* that the two drive shafts are connected by a viscous coupling. A viscous compling on its own would serve no purpose what so ever. it needs to be combined with the open diff to work.

An open diff applies exactly the same amount of torque to each wheel, If the front wheels slip and can only recieve 1Nm of torque this is how much the rear wheels will get regardless of how much traction they can take. In this scenario the two drive shafts have a significantly different rotational velocity. Now imagine that the diff suddenly becomes a VC Center diff (essentially an open diff with the two drive shafts connected by a VC) What the VC does is try to make the two shafts spin at close to the same speed. The front wheels' drive shaft is spinning faster than the rear, this causes the vanes or plates to stir the thick fluid in the VC and it begins to flow round in the housing, this flow then drags the the vanes or plates for the other drive shaft round with it thus applying more torque to the rear wheels. One way to imagine how the VC works is to stir a cake mixture in a bowl. you need to hold the bowl to stop it turning with the spoon. this is a viscous coupling in a very simpe sense.

An explanation on how they work can be found here...

Thanks Ed. I guess what I meant to say is that there are no plates that get forced together when the fluid heats up.

-rm

Actually, this is not true. A viscous coupling w/o a center diff will lock the front and rear together whenever there is slip. This is basically what the clutch pack does in the auto box except the technology is different. I'd prefer to have the VC w/ center diff, however, many *AWD* vehicles nowadays do not use a center diff.

-rm

Ok, I accept that. You are saying that what we basically have in the transfer case is a limited-slip center differential, similar to what one might find in a limited slip rear axle. In the absence of the VC in the mix, a car with a center diff. would never experience any front-rear windup or binding, because the front and rear axles are free to turn at whatever independent speeds they want, just as right and left wheels on an axle with a differential are free to turn. However, without the VC, torque would be limited by the axle with the least traction, while with the VC, torque is limited (more or less) by the axle with the most traction.

And, with the VC, any difference in rotation between the front and rear outputs is "abnormal" in the sense that it will cause relative rotation in the VC, and drag and heating and so on. Very slow and/or brief relative rotation, as in cornering, causes little drag and little heating. Rapid rotation (as in wheelspin) or long-term rotation (as in mismatched tires) will cause drag and heating etc., and in the case of tire mismatch, possibly long-term heating and damage to the VC.

If the VC lost some of its ability to transfer torque (wear, damage, whatever), the front and rear wheels would still both be driven but you would lose some of the advantage of transferring torque to the axle with traction.

And according to some other posts, some other cars have the VC without a center differential; this was the impression I had about the Subaru.

Now do I have it right?

If this is the case then you'd have a FWD car until you had slip this is not the case on modern manual Subarus. The rear wheels would be rolling stock. So on a loose surface with all 4 corners having equal grip if you dumped the clutch only the front wheels would spin getting 100% of the torque until the VC has enough difference in rotational velocity to transfer torque to rear and even then the front wheels would need to contunue to spin faster than the rears to maintain torque to the rears (More difference = more torque transfer), a very messy situation which would produce insane amounts of understeer. Subaru AWD is symetrical and this is not what happens.

I know from fact that my STi Type UK will spin all 4 wheels instantly and at the same rate on a hard launch from loose surface suggesting power being supplied to the transmission via an open diff with a VC (No DCCD fitted).

If you drove the housing of the VC from the engine then your VC would last about an hour and your fuel economy would be even worse than it already is!!!

Ross

You got it.

Actually, there is a VC in both the automatic and manual versions of Subies. However, they use the VC in completely different ways in each case. In the case of manual tranny Subies, the VC acts as a slip-limiter for the centre diff. In the case of auto Subies, the VC activates a progressive clutch pack that locks the front and rears together: there is no centre diff in an auto Subie.

In the case of a manual Subie, the VC starts causing drag when the front and rear axels have a big enough difference. In the case of an auto Subie, the front gets the majority of the power and the rear axel follows along like in a standard FWD car, until it looks like the front axel starts spinning much faster than the rear axel, then that causes the VC to start causing drag which locks up the front and rear axels together. In a manual, the front and rear are getting 50:50 power at all times, even during slippage. In an auto, the front gets 90% of the power when there's no slippage, and when there is slippage, they get locked together at 50:50.

Yousuf Khan