Viscous Centre Diff
13 January 2004, 09:48 PM
#1
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My my98 has a viscous centre diff, I'm told. Are there any links showing how the transmission, front and rear driveshafts and the diff connect?
As I understand it, the driveshaft from the gearbox goes in to the diff, and the viscosity within is what turns the rear diff and powers the back wheels. Does this mean that the front wheels have the same deal, power coming from a third shaft out of the diff, or do they get their power straight off the side of the gearbox?
If so, isn't that primarily FWD, with a bit of RWD as needed, and more 55%-45% than 50%-50%? I'm sure I'm wrong, I just don't know why...
Confuseded..
As I understand it, the driveshaft from the gearbox goes in to the diff, and the viscosity within is what turns the rear diff and powers the back wheels. Does this mean that the front wheels have the same deal, power coming from a third shaft out of the diff, or do they get their power straight off the side of the gearbox?
If so, isn't that primarily FWD, with a bit of RWD as needed, and more 55%-45% than 50%-50%? I'm sure I'm wrong, I just don't know why...
Confuseded..
14 January 2004, 12:23 AM
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A picture or two would be worth more than a couple of thousand words in this case, but if you have a general idea of how a differential works, we'll get through...
Erm, no... 
The viscous coupling doesn't transfer power to the rear wheels by itself, all it does is control and curtail the rotational speed difference between the front and rear axles to stop one end or the other spinning all the power off.
The power comes from the gearbox output shaft into the centre diff. The diff splits the power in two, sending half to the rear output, and half to the front, so the inherent power split on the car is 50/50 front and rear.
The viscous coupling then acts on the front and rear outputs. If you've never seen one, it looks like a series of flat plates (or large washers, if they're easier to visualise) with a sticky, gunky fluid around them. The plates alternate, so one is connected to the "front" output, the next to the "rear", and so-on. The sticky fluid (think treacle) makes them bind together, so even though they're not physically connected, they want to turn the same way. This "sticky" action stops one axle spinning away all the power if one or both of its wheels loses traction.
There are three shafts, the input and two outputs, but unless you look closely you'll only see two of them.
The Impreza gearbox and centre diff is a very elegant design. Unlike some 4WD saloons, there are no power take-offs or separate shafts hanging off the side of the gearbox.
The clever bit is that the output shaft from the gearbox into the centre diff is actually hollow, and the shaft that goes back to the front diff actually runs inside it. So, if you look at a cutaway diagram of an Impreza gearbox, it looks at first as though there are only two shafts.
[Edited by greasemonkey - 1/14/2004 12:35:41 AM]
As I understand it, the driveshaft from the gearbox goes in to the diff, and the viscosity within is what turns the rear diff and powers the back wheels.
The viscous coupling doesn't transfer power to the rear wheels by itself, all it does is control and curtail the rotational speed difference between the front and rear axles to stop one end or the other spinning all the power off.The power comes from the gearbox output shaft into the centre diff. The diff splits the power in two, sending half to the rear output, and half to the front, so the inherent power split on the car is 50/50 front and rear.
The viscous coupling then acts on the front and rear outputs. If you've never seen one, it looks like a series of flat plates (or large washers, if they're easier to visualise) with a sticky, gunky fluid around them. The plates alternate, so one is connected to the "front" output, the next to the "rear", and so-on. The sticky fluid (think treacle) makes them bind together, so even though they're not physically connected, they want to turn the same way. This "sticky" action stops one axle spinning away all the power if one or both of its wheels loses traction.
Does this mean that the front wheels have the same deal, power coming from a third shaft out of the diff, or do they get their power straight off the side of the gearbox?
The Impreza gearbox and centre diff is a very elegant design. Unlike some 4WD saloons, there are no power take-offs or separate shafts hanging off the side of the gearbox.
The clever bit is that the output shaft from the gearbox into the centre diff is actually hollow, and the shaft that goes back to the front diff actually runs inside it. So, if you look at a cutaway diagram of an Impreza gearbox, it looks at first as though there are only two shafts.
[Edited by greasemonkey - 1/14/2004 12:35:41 AM]
14 January 2004, 10:54 AM
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Thanks for the explanation! I'm still a bit unclear as to what bits are attached to the gearbox driveshaft to get the power into the diff, though.
Forgive my numptiness, but I found a nice cutaway at http://auto.howstuffworks.com/differential7.htm which sadly has only 2 shafts - powering front and rear, but none from the gear box to provide the power itself... It does nicely show the plates though.
Does the gearbox shaft just have it's own plates, that spin the front and rear plates ?
Forgive my numptiness, but I found a nice cutaway at http://auto.howstuffworks.com/differential7.htm which sadly has only 2 shafts - powering front and rear, but none from the gear box to provide the power itself... It does nicely show the plates though.
Does the gearbox shaft just have it's own plates, that spin the front and rear plates ?
14 January 2004, 11:15 AM
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The gearbox simply sends drive to an "output shaft", which is then connected to the centre diff unit via a "drop gear". From which the output is split to the front and rear shafts.
Incidentally the power is split 60/40 biased to the rear. Only if a diff lock is engaged would you ever get a 50/50 split, otherwise it would make the steering so heavy it would take a weightlifter to turn into a corner!
Thats why they fit a viscous diff unit, otherwise if it was a 50/50 split, you would not need a diff unit would you.
Incidentally the power is split 60/40 biased to the rear. Only if a diff lock is engaged would you ever get a 50/50 split, otherwise it would make the steering so heavy it would take a weightlifter to turn into a corner!
Thats why they fit a viscous diff unit, otherwise if it was a 50/50 split, you would not need a diff unit would you.
14 January 2004, 02:45 PM
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You're usually bang on the money SS, but some fundamental inaccuracies here:
That is incorrect. The gearbox output shaft connects directly to the centre diff. The drop gears are used to connect the centre diff's rear output to the propshaft drive.
That is incorrect. The torque distribution of the viscous coupled Impreza centre diffs is 50/50.
That is incorrect, you don't need a locked axle to get an equal torque split. How do you think the differentials on the front and rear axles work? The static torque distribution of any regular bevel/planet gear differential is 50/50, and the only Subaru diffs that have an inherent torque bias are the DCCD units, which have a 33:66 rear-biased split.
WTH are you talking about? The centre diff can be (virtually) locked on the DCCD cars, and you don't need the arms of Schwarzenegger to turn the steering wheel on those.
Eh? You seem to think that a 50/50 torque split centre differential and a locked centre axle are the same thing. They are not.
The gearbox simply sends drive to an "output shaft", which is then connected to the centre diff unit via a "drop gear".
Incidentally the power is split 60/40 biased to the rear.
Only if a diff lock is engaged would you ever get a 50/50 split,
otherwise it would make the steering so heavy it would take a weightlifter to turn into a corner!
Thats why they fit a viscous diff unit, otherwise if it was a 50/50 split, you would not need a diff unit would you.
14 January 2004, 03:07 PM
#6
you can get any torque split you want with correct centre diff design
you do need the centre diff to be unlocked on car for proper handling on tarmac. steering is more difficult if c.diff is locked, and the locked position is designed for low traction surfaces like gravel. when you are going round a bend the front wheels are travelling less far than rear ones on average. F/R speed difference is taken care of by c.diff
if you lock the c.diff permanently on a DCCD car you will knacker it.
Paul
you do need the centre diff to be unlocked on car for proper handling on tarmac. steering is more difficult if c.diff is locked, and the locked position is designed for low traction surfaces like gravel. when you are going round a bend the front wheels are travelling less far than rear ones on average. F/R speed difference is taken care of by c.diff
if you lock the c.diff permanently on a DCCD car you will knacker it.
Paul
14 January 2004, 03:58 PM
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Wasn't saying that you should drive round on a locked centre diff Paul, merely that the steering wouldn't be as difficult as SS had suggested if you did so.
No problem, let's see if this helps: 
Yes, that cutaway is of the viscous coupling only don't forget that an Impreza centre diff is actually two different components - the viscous unit and the centre differential itself.
The viscous unit in the Impreza c-diff is very much like the cutaway on Howstuffworks. All it does is connect the "front" and "rear" outputs together. Before you get to that step, the power goes from the gearbox to the centre diff itself.
Look at the exploded diagram of a differential here. This is a basic bevel gear "open" differential, very similar to the one in the front of your Impreza. It's called an "open" diff, incidentally, because there's no limited slip, viscous or other system to bind the two outputs together.
When you look at the graphic of this differential working, you can see how the power goes in through the input shaft and is turned through 90 degrees by the input pinion and ring gear. You can then see (in the "turn" demonstration) how the pinion gears inside the ring gear allow the inner and outer outputs to slow down and speed up in relation to each other, while the overall rotation speed of the axle stays the same.
The same principle applies in the Impreza's centre diff, but the layout is a little different. Instead of the output being turned through 90 degrees by the input pinion and ring gear, the centre diff rotates on the same axis as the input shaft. Look at the following graphic:
This is a little rough, and I've changed a couple of details to make it easier to draw (and understand!), but this is the basic layout of an Impreza's gearbox output shaft and centre diff assembly. If you sliced right down the middle of the 'box, it'd look a lot like this.
The first thing to look at is the (pink) gearbox output shaft. This carries the six output gears (five forward plus reverse), which are driven by the input shaft (which I haven't included in this drawing).
The gearbox output shaft is connected to the (blue) centre diff housing, so the housing always rotates at the same speed as the gearbox. Inside the centre diff, you can see the pinion gears (the orange cone shaped things), together with the front (green) output gear, and the rear (red) output gear. The pinion gears, just like in the demonstration on howstuffworks, allow the front and rear outputs to vary their speed in relation to each other while the rotation speed of the diff housing itself remains constant.
As you can see, the green centre diff output gear is connected to the (green) front output shaft, which runs all the way through the hollow gearbox output shaft to the front of the gearbox, where its pinion gear drives the front diff.
Also look though how the front output shaft extends behind the centre diff into the viscous coupling unit.
The (red) rear output gear is again hollow, and extends back into the viscous coupling.
Look inside the viscous coupling and you can see the (light blue) fluid, and the alternating green (front) and red (rear) plates. The purpose of the VC then is to bind the front and rear outputs together, attempting to equalise the rotational speed and torque transfer across the front and rear axles.
Finally, behind the viscous coupling, you can see the (pink and orange) drop-gears, which connect the (red) rear centre diff output to the shaft that leads out of the gearbox unit, to the propshaft and the rear differential.
Make any sense?
[Edited by greasemonkey - 1/14/2004 4:27:33 PM]
Thanks for the explanation! I'm still a bit unclear as to what bits are attached to the gearbox driveshaft to get the power into the diff, though.
Forgive my numptiness, but I found a nice cutaway at http://auto.howstuffworks.com/differential7.htm which sadly has only 2 shafts - powering front and rear, but none from the gear box to provide the power itself... It does nicely show the plates though.
Does the gearbox shaft just have it's own plates, that spin the front and rear plates ?
Look at the exploded diagram of a differential here. This is a basic bevel gear "open" differential, very similar to the one in the front of your Impreza. It's called an "open" diff, incidentally, because there's no limited slip, viscous or other system to bind the two outputs together.
When you look at the graphic of this differential working, you can see how the power goes in through the input shaft and is turned through 90 degrees by the input pinion and ring gear. You can then see (in the "turn" demonstration) how the pinion gears inside the ring gear allow the inner and outer outputs to slow down and speed up in relation to each other, while the overall rotation speed of the axle stays the same.
The same principle applies in the Impreza's centre diff, but the layout is a little different. Instead of the output being turned through 90 degrees by the input pinion and ring gear, the centre diff rotates on the same axis as the input shaft. Look at the following graphic:
This is a little rough, and I've changed a couple of details to make it easier to draw (and understand!), but this is the basic layout of an Impreza's gearbox output shaft and centre diff assembly. If you sliced right down the middle of the 'box, it'd look a lot like this.
The first thing to look at is the (pink) gearbox output shaft. This carries the six output gears (five forward plus reverse), which are driven by the input shaft (which I haven't included in this drawing).
The gearbox output shaft is connected to the (blue) centre diff housing, so the housing always rotates at the same speed as the gearbox. Inside the centre diff, you can see the pinion gears (the orange cone shaped things), together with the front (green) output gear, and the rear (red) output gear. The pinion gears, just like in the demonstration on howstuffworks, allow the front and rear outputs to vary their speed in relation to each other while the rotation speed of the diff housing itself remains constant.
As you can see, the green centre diff output gear is connected to the (green) front output shaft, which runs all the way through the hollow gearbox output shaft to the front of the gearbox, where its pinion gear drives the front diff.
Also look though how the front output shaft extends behind the centre diff into the viscous coupling unit.
The (red) rear output gear is again hollow, and extends back into the viscous coupling.
Look inside the viscous coupling and you can see the (light blue) fluid, and the alternating green (front) and red (rear) plates. The purpose of the VC then is to bind the front and rear outputs together, attempting to equalise the rotational speed and torque transfer across the front and rear axles.
Finally, behind the viscous coupling, you can see the (pink and orange) drop-gears, which connect the (red) rear centre diff output to the shaft that leads out of the gearbox unit, to the propshaft and the rear differential.
Make any sense?
[Edited by greasemonkey - 1/14/2004 4:27:33 PM]
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14 January 2004, 07:59 PM
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No worries Theo.
Suppose while we're here...
This is a top view showing the way the gearbox connects to the centre, front and rear diffs.
Note that the front diff is contained within the gearbox casing, while the rear diff case is bolted separately into the rear subframe and is connected to the gearbox by the propshaft.
Also note that while the front diff is a conventional open one, the rear contains another viscous coupling. This layout applies to the UK/Euro spec turbos/WRXs, and most of the Jap spec WRXes. The STi's (depending on which one you look at) can have plated limited slip rear diffs, the electro-magnetic DCCD centre diff, and various viscous, plated or "Suretrack" mechanical LSD's on the front.
Suppose while we're here...
Are there any links showing how the transmission, front and rear driveshafts and the diff connect?
This is a top view showing the way the gearbox connects to the centre, front and rear diffs.
Note that the front diff is contained within the gearbox casing, while the rear diff case is bolted separately into the rear subframe and is connected to the gearbox by the propshaft.
Also note that while the front diff is a conventional open one, the rear contains another viscous coupling. This layout applies to the UK/Euro spec turbos/WRXs, and most of the Jap spec WRXes. The STi's (depending on which one you look at) can have plated limited slip rear diffs, the electro-magnetic DCCD centre diff, and various viscous, plated or "Suretrack" mechanical LSD's on the front.
15 January 2004, 11:14 AM
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Blimey, Greasemonkey, I think that is the single most useful and detailed post I've ever seen on the net anywhere ever.
Many thanks for putting in all that effort - I now understand, at least basically, what's going on, given the bigger picture, and I think a lot of people will find that very useful.
I most definitely owe you a pint...
Many thanks for putting in all that effort - I now understand, at least basically, what's going on, given the bigger picture, and I think a lot of people will find that very useful.
I most definitely owe you a pint...
15 January 2004, 11:21 AM
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it is good stuff.
iirc, scoobyslut offered me the same explanation last time this came up about 50:50 being impossible as it stops steering being possible. Except last time he wasn't quite so polite.
Moray also pointed out that if 50:50 diffs don't allow steering to work then what happens int he front and rear diffs which have no bias to either side?
iirc, scoobyslut offered me the same explanation last time this came up about 50:50 being impossible as it stops steering being possible. Except last time he wasn't quite so polite.
Moray also pointed out that if 50:50 diffs don't allow steering to work then what happens int he front and rear diffs which have no bias to either side?
15 January 2004, 11:42 AM
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Would I be right in assuming the Legacy GTB twin turbo has the same arrangement (open front diff, viscous centre and rear) as that?
Information is hard to come by it would seem for the GTB!
Rich.
Information is hard to come by it would seem for the GTB!
Rich.
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