Torque Split & description of the MT/AT AWD system (Merged)

Dedman said:
The center differential in a first gen manual forester is essentially 'open' when there is only a low speed difference between the front and rear wheels. That is why you can jack up one wheel and let the the clutch out with the engine at idle in 1st gear low without the car going anywhere. Its only when you accelerate a little that the centre diff start to change the 50:50 torque split to a ratio with higher torque sent to the end of the car with both wheels on the ground that forward movement is actually obtained.
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I'll have to check this but you may have a shot centre diff.

NOTE: to all those who didnt realise with an AWD that uses a vLSD centre, its critical to have the same size & type tyres front & rear, even having different brands can bugger the centre diff. Also, using the handbrake while moving will also destroy the centre diff. Handbrake turns are fun but very costly in a Subaru! :(

MiddleAgeSubie said:
I am no engineer and I lack any actual knowledge of how 4wd and AWD systems operate. However, from what I read I have to conclude that the only way to get what you want is a true locker.
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There are 4 types of front & rear diff:
*open: useless offroad
*vLSD: better than open but very limited (pardon the pun :raz: )
*pLSD: plated diffs incl Subaru OEM from WRX STi, not sure about other STi's, eg Forester STi or Liberty STi. Also aftermarket like KAAZ & Cusco. Subaru pLSD is very useful at keeping forward movement with a lifted wheel, I can vouch for this. Cusco & KAAZ are even better. Slip between the axles is be varied depending on type & setup
*Locker: there are many types of locker but they all "lock" one axle to the other 100%...ie, no slip
 
Dedman said:
The center differential in a first gen manual forester is essentially 'open' when there is only a low speed difference between the front and rear wheels. That is why you can jack up one wheel and let the the clutch out with the engine at idle in 1st gear low without the car going anywhere. Its only when you accelerate a little that the centre diff start to change the 50:50 torque split to a ratio with higher torque sent to the end of the car with both wheels on the ground that forward movement is actually obtained.
Click to expand...

NachaLuva said:
I'll have to check this but you may have a shot centre diff.
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What Dedman describes is totally normal for a viscous LSD. ie Little or no resistance when the difference in output speeds is small; increasing lockup as the speed difference increases.
 
Good posts above. Learned a couple of things.

Now, does anyone know how exactly does the 5EAT planetary type differential work? I understand that torque constantly moves around, but what is the maximum % that can go to the rear wheels and what is the maximum that can go to the front wheels?
 
NachaLuva said:
I'll have to check this but you may have a shot centre diff.
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Nah pretty sure that how its meant to be. It does try to move the car forwards but if the wheels are choked a small amount it doesn't go anywhere. If you you left it idling like that for a while the diff would probably heat up and engine would stall/car start to move but I have never tried this because im sure its not wonderful for the system.

About the only time I really notice the centre diff actively transferring torque is on the extremely rare occasions when going up very very steep (inclines approaching 45 degrees) hills that are non especially rutted and have loose gravel. The front wheel seem to be slipping quite a lot which you can feel by the front to the car bouncing all over the place where as the rear wheels seem to be fine. This slip is required to get front-rear speed difference that the centre diff needs to send enough torque (close to 100%) to the rear wheels to push you up the hill. I find it quite an odd driving experience in these situations, kind of like driving a front wheel drive car but that being said it still climbs like a mountain goat :ebiggrin: In most offroad driving you do not notice the centre diff working because slip when going up hill is usually associated with diagonal lift so is less noticeable or the slip is only minimal because the traction required to climb the hill is less than twice (as governed by the 50:50 torque split) the traction of the axle with the least grip.
 
MiddleAgeSubie said:
Now, does anyone know how exactly does the 5EAT planetary type differential work? I understand that torque constantly moves around, but what is the maximum % that can go to the rear wheels and what is the maximum that can go to the front wheels?
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The 5EAT planetary setup (VTD) is an open diff, but geared such that the default torque split is biased towards the rear a bit (40:60 or something similar, from memory). It also has a plated coupling controlled by the transmission's computer that can lock-up the diff. Accordingly depending on the conditions you can end up with torque split ranging from 100:0 to 0:100, say if front or rear wheels were on ice. There is probably a limit to how much torque that coupling can handle though, so even when fully engaged it may allow some slip - it's not like a diff lock that positively engages.
 
Simon, great many thanks, I had not been able to find an actual number.

The default split is 45:55 F/R.

Do you know how much can the system on the current CVT models transfer? Unlike the older 2.5 models and like the later 2005-9 generation cars it splits torque 60:40 by default. Apparently, this is also the system that will replace the VTD on the H6 Subarus. Is that capable of transferring the vast majority of the power to the back?
 
MiddleAgeSubie said:
Simon, great many thanks, I had not been able to find an actual number.

The default split is 45:55 F/R.
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No worries :)

Thanks for the confirmation of the default. I think the rear bias is intended to quell understeer.

MiddleAgeSubie said:
Do you know how much can the system on the current CVT models transfer? Unlike the older 2.5 models and like the later 2005-9 generation cars it splits torque 60:40 by default. Apparently, this is also the system that will replace the VTD on the H6 Subarus. Is that capable of transferring the vast majority of the power to the back?
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As far as I know the MPT system on the CVT models is identical to that on the later 4EATs in that it has a fixed coupling to the front differential and a variable clutch coupling on the rear output, controlled by the transmission computer. Certainly it can also split all the way from 0:100 to 100:0, again if you had either front or rear wheels with no traction. And also like the coupling in the VTD, the MPT would probably still have a modest torque limit and may still slip under load when fully engaged.
 
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Kevin said:
I believe the ratios changed over the years where 60/40 was from 2008/2009. With a quick look I could find no reference to model year in the article.

From Forester Technical Description Page 19 for the second generation Forester “Automatic Transmission AWD” Subaru Australia publication P-FTB03:

“This system can infinitely vary the distribution between 95% front and 5% rear wheel drive to a 60/40 ratio with static weight distribution”

It's a PDF so I'm happy to email it to anyone who is interested
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hey Kevin do you still have this PDF? If so PM me please
 
[MENTION=15671]johnsonmiles[/MENTION] PDF emailed
 
subalex said:
1) I put the auto in 1st, making it 50:50 front rear split.
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I think this is a myth. There is no firm 50/50 split on the autos, definitely not on the 5 EATs I have had. It is all "soft" split which looses up the moment things get tough.

I think the MTs used to have something like an actual 50/50, but not the autos.
 
I'm going by the best of the sources I can find, direct from Subaru. Shifting my 4EAT tiptronic (can't comment on your 5EAT SH with flappy paddles if that's what you have) into 1 or 2nd automatically puts it into a 50:50 split. However, this cannot be the case all the time, otherwise I'd have trouble turning hard into a parking spot in 1st gear. So, when this is locked and unlocked I have no idea.

You're right about the compromise of either suffer the downsides or get back into the Patrol. It's up for sale, so trips like this are important for me to make my mind up before it's gone for good. :-)
 
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If you can, keep both!

They are complimentary...an incredible pair of horses that cover pretty much all the courses....

Back to 50/50, it puts it in a 50/50, ok, but it cannot hold it in 50/50 as it is a "soft" split. This is my observational, non-pro take on the matter.... I am not an engineer and don't need to know the details!
 
The SH Forester has a different centre diff than other Subaru models except the the Outback. They call it "Active Torque Split" and it delivers varying torque to the front/rear axles depending on the level of traction available. Under normal driving conditions where all 4 wheels have traction, torque is distributed 60% front and 40% rear. When wheel slip is detected, the centre diff alters the distribution which can be up to 100% either front or rear. -
[ame="https://www.youtube.com/watch?v=mryGnENTsoI"]Live demonstration of Subaru Forester and Outback's 4x4 symmetrical all wheel drive system - YouTube[/ame]
https://www.awdwiki.com/en/subaru
 
Ha! Not really.

The "active torque split" is the modern version of the antiquated one I had in the 95 Legacy. It does not have a 60/40 split. You can track it via a phone application and see that it constantly varies the split generally between 80/20 and 60/40. Many have done this and provided the info on the forums.

True, I have not had that system. The three VTD Subarus I had since the Legacy have all been, indeed VTD, with an actual normal split of 45/55 and a much narrower band of variation when accelerating etc.

But none of that matters with respect to what Alex experienced.

THERE IS NO FIXED SPLIT THAT A NON-STI AT SUBARU CAN HOLD.

Whatever the clutch packs are doing fails under challenging off-road conditions, resulting in crazy wheelspin of offloaded tires. As I think I said, I have had veritable front wheel burnouts with the car failing to move on challenging inclines with rear wheels on decent enough surface. This just cannot happen if there is any actual 50/50 split for more than an instant.

You see this also on Alex's videos. The crazy wheelspin results from the inability of the AWD to overcome the severe challenge posed by soft stuff over rock. It just cannot. It is working with three open differentials and as torque goes all over the place, the car struggles. As we all know, it is not really designed or made for this :)

There are far less capable vehicles overall that actually can lock the center differential. The enormous majority of Subarus cannot.
 
Sorry Middle Aged Subie but the info at https://www.awdwiki.com/en/subaru says the Forester has a nominal 60/40 split. Why do you say it doesn't? I don't deny that it looked pretty random on Alex's car but you can't see what all 4 wheels are doing.
 
Because I have wasted way too many evenings on 4 Subaru forums and I have much better overall grasp of the aspects of Subaru ownership that interest(ed) me than the info found on a random link :lol: Nominal, yes. Subaru says so. Owners monitoring their ECMs have found otherwise. Maybe it is 60/40 when cruising at a set speed on the highway.
 
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Well, that shuts me down. I'll keep my random links to myself from now on and leave it to you.
 
According to the interwebs, The SVX torque split seems to be either 60:40 RR:FR or 40:60 RR:FR It really depends on the websites you go to, about half of them say the one, the other half say the other.
e.g: this site says it's a 36:64 FR:RR split.
To make it more confusing, in the USA, some sites say it's 90:10 to the front.
I'd really like to find out what it actually is (I'd love it to be biased to the rear, much more fun in the dirt!).
Does anyone have any official information on this?

edlt: I should check the owners manual again, but I'm think it says that it's 60 Rear, 40 front.
 
Current info from here: https://www.subaru-global.com/technology/performance/drivetrain_awd.html

"Variable Torque Distribution AWD

An AWD system designed exclusively for 5-speed automatic transmission, E-5AT (with manual mode) models to provide aggressive, sporty driving with increased turning abilities all while retaining the fundamental driving stability offered by the Symmetrical AWD. An uneven 45:55 front and rear torque distribution has been set for the front and rear tyres. Sending greater torque to the rear wheels can reduce understeer, a trait that plagues 4WD vehicles, when cornering for smooth and agile handling. Torque distribution is controlled automatically up to an equal 50:50 front and rear to suit road surface conditions for both sporty driving and stability.

Viscous Centre Differential AWD

An AWD system for manual transmissions with a simple layout consisting of a centre differential and viscous LSD. A 50:50 front and rear torque distribution is set under normal conditions. This maximises traction and provides driving backed with stability. When the front or rear wheels slip and traction is lost, or the wheel speed differs between the front and rear axles, the viscous LSD maintains optimum torque distribution between the front and rear and distributes power to the wheels with the most traction. It links the flexible gear shifting of the manual transmission to deliver sporty driving that always maximises the available traction.

Multi-mode DCCD

The Driver’s Control Centre Differential (DCCD) allows the centre differential locking of the WRX STI to be dialed in and adjust its handling. It adjusts both an electromagnetic multi-plate transfer clutch and mechanical locking of a Limited Slip Differential (LSD) to distribute torque to the front and rear wheels. Auto Mode identifies the ideal power distribution for changing conditions, whilst Auto [+] keeps more power up front for increased traction. Auto [-] transfers more power to the rear for tighter cornering, and the six-step Manual Mode locks in the driver’s favourite handling characteristics."
 
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