Transfer case on AWD models: was it meant to be reliable ?

[Thomby]

If the rears hit ice, then it's still a 50/50 split when the transfer case is engaged. 50% of engine power goes to rear wheels which will slip on the ice, and 50% will go to the front which, hopefully, have traction. If this was an advertisement then we would say "100% of torque goes to the front wheels!" because the rear wheels aren't doing anything. Ads for a limited slip differential will say "the wheel with traction gets 100%" while the other wheel is just along for the ride. Both phrases are correct, it just depends on what, precisely, is being discussed. The usage scenarios for a transfer case are just different than a differential. Kia won't say "100% torque to front wheels!" because the rear cannot be disengaged. But, if the rear wheels are up in the air, and the transfer case clutch is locked, then sure, 100% torque will go to the front. That's not a normal scenario though.

This was a great overall post/explanation. I still don't like calling a fully locked transfer case a "50/50 split" just because they're turning at the same speed, since the only time the split matters (rear wheel slippage) the fronts are seeing up to 100%. I wouldn't describe a locking rear diff as having a left/right split either...torque just goes wherever there is resistance (grip).

If the transfer case can maintain lockup at full engine torque or beyond, that's impressive, and makes me wonder if there's potential in tuning to ramp up the line pressure more aggressively, both to limit heat/wear and to act more like a true locker. I guess with open left/right diffs we'll never do true AWD burnouts (maybe the GTS can do three wheels?), but a hard launch of a highly tuned Stinger should either spin at least one front and rear unless the clutches slip.

 

[oddball]

Eh, the same thing (torque input being distributed to multiple outputs) is happening in a diff, it's just how it's interpreted and described. Diffs aren't progressive - by design they're limited slip, or it's a selectable locker (completely on or completely off). This whole "split" thing is kind of specific to transfer cases. Transferring power to the front is very lossy, so you don't necessarily want to do it unless it'll help. So the ECU can monitor wheel speed and progressively engage the transfer case clutch to push "some" power up to the front. The value of "some" will vary based on conditions (power output, speed, gear, steering angle, variance of wheel speeds, etc etc etc). You'll get ever-so-slightly better gas mileage this way than a on/off transfer case.

*Most* limited slip diff designs also have a min/max torque differential. Like some designs will suddenly go open if one wheel has 0 traction - there must be at least some traction on both wheels for the mechanism to work. And to work in turns there's a maximum at which point it lets go and lets the wheels turn at different speeds. That kind of stuff doesn't happen in transfer cases, at least not mechanically. So there's some differences in the concepts, so the definition of "split" is just different.

I'm certain the pump duty cycle is just a table (well, probably a bunch of tables) in the ECU, so it can be tuned. The problem is finding someone willing to dig into it. The info on the ECU tables has been pretty limited and it's mostly the Korean shops that are going that deep.

 

[rtv900]

good comments in this thread, particularly by oddball
my opinion as follows:

the subaru STI platform calls it a center differential
If you ask me that is what we have. IMO a transfer case is what you have in selectable four wheel drive vehicle (like typical old school suv's or big trucks) where it is either driving both shafts with equal torque or driving one shaft only. No in between.
Those types are just super simple and super tough giant chain drives with a 1:1. No clutches, no nonsense, either engaged or not.

So IMO we have a limited slip center diff, just like the STI, which is controllable just like the STI although in this case the computer does it where the STI has 5 levels of power distribution and also a fully locked option where you can adjust it on the center console. And it works legit too (was my last car)
The WRX by contrast has an open center diff. Which IMO is more or less useless because the car has zero ability to get power to wheels that aren't slipping, making it pretty pointless in snowy or icy conditions which are basically the only time you need that feature.

 

[vipeboy2000]

good comments in this thread, particularly by oddball
my opinion as follows:

the subaru STI platform calls it a center differential
If you ask me that is what we have. IMO a transfer case is what you have in selectable four wheel drive vehicle (like typical old school suv's or big trucks) where it is either driving both shafts with equal torque or driving one shaft only. No in between.
Those types are just super simple and super tough giant chain drives with a 1:1. No clutches, no nonsense, either engaged or not.

So IMO we have a limited slip center diff, just like the STI, which is controllable just like the STI although in this case the computer does it where the STI has 5 levels of power distribution and also a fully locked option where you can adjust it on the center console. And it works legit too (was my last car)
The WRX by contrast has an open center diff. Which IMO is more or less useless because the car has zero ability to get power to wheels that aren't slipping, making it pretty pointless in snowy or icy conditions which are basically the only time you need that feature.

i thought it was an odd part on subaru to offer the planetary 45:55 rear bias split planetary center diff on the cvt wrx but not the manual ones..
but same, in my mind, a transfer case is selectable while most awds are not with only a center diff, then we get hybrids like our hummer where it has a planetary differential in the transfer case for 40:60 split awd all the time, but also has the locking 4 hi 4lo as well..

 

[Thomby]

i thought it was an odd part on subaru to offer the planetary 45:55 rear bias split planetary center diff on the cvt wrx but not the manual ones..

As disappointing as the K5/Sonata offering AWD on the slow 180/190hp models, but only FWD on the 290hp GT/N with dual clutch. Wonder if that's due to a torque limitation of their FWD-based AWD system, or to keep it from stepping on the toes of the 4cyl Stinger.

 

[vipeboy2000]

As disappointing as the K5/Sonata offering AWD on the slow 180/190hp models, but only FWD on the 290hp GT/N with dual clutch. Wonder if that's due to a torque limitation of their FWD-based AWD system, or to keep it from stepping on the toes of the 4cyl Stinger.

I thought about this a lot since the first awd models came out along with the faster models... probably both.. seeing how small the center diff is for the awd models I know it definitely wouldn't live with the hp/torque of the 2.5t.

 

[LordKOTL]

good comments in this thread, particularly by oddball
my opinion as follows:

the subaru STI platform calls it a center differential
If you ask me that is what we have. IMO a transfer case is what you have in selectable four wheel drive vehicle (like typical old school suv's or big trucks) where it is either driving both shafts with equal torque or driving one shaft only. No in between.
Those types are just super simple and super tough giant chain drives with a 1:1. No clutches, no nonsense, either engaged or not.

So IMO we have a limited slip center diff, just like the STI, which is controllable just like the STI although in this case the computer does it where the STI has 5 levels of power distribution and also a fully locked option where you can adjust it on the center console. And it works legit too (was my last car)
The WRX by contrast has an open center diff. Which IMO is more or less useless because the car has zero ability to get power to wheels that aren't slipping, making it pretty pointless in snowy or icy conditions which are basically the only time you need that feature.

I believe we have a transfer case in terms of how it works mechanically.
How does AWD put power to the front wheels?

If I'm gleaning what @dgorbenko 's videos show, this thread, and what the thread above states, it is similar to those big trucks and the BMW X-drive work: The transmission output is always connected to the rear wheels, and the front driveshaft is connected to the rear driveshaft via gears and a clutch pack. The clutch pack allows for some slippage between the front and rear drive shafts. At lower speeds, the amount of force on the clutch pack determines the power split--if the clutch pack is fully engaged, you get closer to a 50/50 split as the clutch will resist slippage between the front and rear driveshaft, while if it's not engaged the front driveshaft will slip easier but the rear wheels will always get engine power. Higher speeds (i believe 60kph), and the clutch pack diengages so it's operated like a RWD--likely for fuel economy. The clutch pack means we don't have to lock the hubs or that it's either RWD or 4WD and nothing in-between.

But, there is no differential in between the transmission output and the rear driveshaft. It's always connected. The clutch pack determines how much the front driveshaft can slip vs the rear drive shaft. But, unlike a limited slip center differential which can allow the rear driveshaft to slip relative to the transmission output and front driveshaft, there is no way to send more power to the front wheels than to the rear wheels. If we had a LSD center differential (like the STI's have if I'm reading how they work correctly), it would be possible to send more power to the front than to the rear because the computer controlling it allows the rear driveshaft to slip more than the front.

 

[Thomby]

But, unlike a limited slip center differential which can allow the rear driveshaft to slip relative to the transmission output and front driveshaft, there is no way to send more power to the front wheels than to the rear wheels. If we had a LSD center differential (like the STI's have if I'm reading how they work correctly), it would be possible to send more power to the front than to the rear because the computer controlling it allows the rear driveshaft to slip more than the front.

There is no way to actively send a higher torque split to the front than rear, but once the transfer case clutches are locked up, you can apply more torque to the front than rear (with the extreme case being zero grip at the rear and 100% torque to the front, since the wheel speed is locked together).

 

[rtv900]

I believe we have a transfer case in terms of how it works mechanically.
How does AWD put power to the front wheels?

yeah I mean the terminology is hazy and there's really no objective definition.
IMO it's more like a differential regardless of the fact that the rears are always connected because it allows differential speed.
So I guess a limited slip transfer case, which is still distinctly different than fixed transfer cases like in big trucks.

 

[D.J.]

Higher speeds (i believe 60kph), and the clutch pack diengages so it's operated like a RWD--likely for fuel economy.

I thought this was true, as evidenced in my post #72 in this thread. But it does not appear to be true in my testing.

My observations from monitoring OBDII transfer case clutch pressure - it is applied at all speeds, and applied much easier and more often in sport mode. (Meaning, clutch pressure is applied with even less acceleration, while in sport mode). Coasting or simply maintaining speech, clutch pressure is very low/idle.


While monitoring clutch pressure, as in post #76 in this thread, even at higher speeds, like 150kph, the clutch pressure is used, during acceleration. I don't have a video of that, but I made observations. Clutch pressure seems to always get applied while accelerating, regardless of speed.

 

[oddball]

I thought this was true, as evidenced in my post #72 in this thread. But it does not appear to be true in my testing.

My observations from monitoring OBDII transfer case clutch pressure - it is applied at all speeds, and applied much easier and more often in sport mode. (Meaning, clutch pressure is applied with even less acceleration, while in sport mode). Coasting or simply maintaining speech, clutch pressure is very low/idle.


While monitoring clutch pressure, as in post #76 in this thread, even at higher speeds, like 150kph, the clutch pressure is used, during acceleration. I don't have a video of that, but I made observations. Clutch pressure seems to always get applied while accelerating, regardless of speed.

A certain PSI won't be enough to engage the clutch. There are return springs (or a perimeter ring made of spring steel) that keep the piston retracted, and a certain pressure is required to push past those. The pump also provide lube, so a certain pressure is just enough to get the fluid circulating and spraying around.
What, exactly, those pressures are? Who knows.

I believe we have a transfer case in terms of how it works mechanically.
How does AWD put power to the front wheels?

Yeah, I probably need to look up what Subaru does to see why they call it a "differential" instead of just a "transfer case". Maybe it can never fully disconnect one of the outputs, making it more like a differential?
Or, maybe it's just advertising.

 

[rtv900]

Yeah, I probably need to look up what Subaru does to see why they call it a "differential" instead of just a "transfer case". Maybe it can never fully disconnect one of the outputs, making it more like a differential?
Or, maybe it's just advertising.

All I know is that in the STI in "normal" mode there was drastically less front wheel drive than the stinger in comfort mode.
When I used to drive on my ramps with the STI I had to go full lock on the center diff otherwise the ramps would just keep scooting forward and pushing rather than the car driving up them.
In the stinger it goes right up them no problem and they never slip.
I have no idea though how the internals acheived the limited slip diff action and also could acheive a full lock.
Also, the "center diff" was an integral part of the manual trans, no stand alone "transfer case" or whatever you want to call it.
So when I changed the trans fluid that was the same fluid used in the center diff, which was 75-90 gear oil.

 

[LordKOTL]

...

Yeah, I probably need to look up what Subaru does to see why they call it a "differential" instead of just a "transfer case". Maybe it can never fully disconnect one of the outputs, making it more like a differential?
Or, maybe it's just advertising.

I may be completely wrong on this, but to my knowledge a center differential is just that--a differential. It can be LSD or not but you have 1 input shaft that drives two output shafts, both of which are usually along the same axis. just like a front or rear differential. It has the same spider gears that turn each output. A transfer case has an input and output directly coupled (or run through a reduction gear for LOW mode), and the other output is coupled via a chain drive or a gear train. So in a transfer case one input and output are on the same axis, but the other output is not.

Which is the case in the Stinger. The rear driveshaft is always coupled to the transmission output, and the front driveshaft is connected to the common input/rear driveshaft via a gear train, and the front/back slippage is done via a wet clutch pack, not a differential with spider gears.

 

[rtv900]

Which is the case in the Stinger. The rear driveshaft is always coupled to the transmission output, and the front driveshaft is connected to the common input/rear driveshaft via a gear train, and the front/back slippage is done via a wet clutch pack, not a differential with spider gears.

differential means exactly that, differential outputs. It doesn't mean spider gears, that's just a certain style.
transfer cases initially were something that can be engaged and disengaged, and had a 1:1 engaged or not.
this can't
IMO it's a hybrid differential transfer case.
Bottom line, doesn't really matter.

 

[D.J.]

And notably from here

Kia Stinger - AWD Control System - 4 Wheel Drive (AWD) System

When a vehicle runs at normal speed higher than 60KPH on regular roads, it is controlled under the 2WD conditions.

Despite the words here saying:

When a vehicle runs at normal speed higher than 60KPH on regular roads, it is controlled under the 2WD conditions.

The transfer case clutch pressure is applied at all speeds. So, I'm not sure what the statement means about higher than 60KPH

Boring video attached.