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  #41  
Old 21st March 2014, 05:39 AM
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Quote:
Originally Posted by Dedman View Post
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.
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!

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Originally Posted by MiddleAgeSubie View Post
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.
There are 4 types of front & rear diff:
*open: useless offroad
*vLSD: better than open but very limited (pardon the pun )
*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
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  #42  
Old 21st March 2014, 05:45 AM
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Quote:
Originally Posted by Dedman View Post
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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Originally Posted by NachaLuva View Post
I'll have to check this but you may have a shot centre diff.
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.
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  #43  
Old 22nd March 2014, 12:25 AM
MiddleAgeSubie MiddleAgeSubie is offline
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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?
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  #44  
Old 23rd March 2014, 08:22 AM
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Quote:
Originally Posted by NachaLuva View Post
I'll have to check this but you may have a shot centre diff.
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 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.
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  #45  
Old 23rd March 2014, 11:29 PM
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Quote:
Originally Posted by MiddleAgeSubie View Post
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?
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.
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  #46  
Old 24th March 2014, 02:05 AM
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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?
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  #47  
Old 24th March 2014, 11:33 PM
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Quote:
Originally Posted by MiddleAgeSubie View Post
Simon, great many thanks, I had not been able to find an actual number.

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

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

Quote:
Originally Posted by MiddleAgeSubie View Post
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?
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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Last edited by simxs; 24th March 2014 at 11:41 PM.
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  #48  
Old 27th March 2014, 03:19 AM
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Quote:
Originally Posted by pezimm View Post
Nice!

Walks a finer line than here, I guess.
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  #49  
Old 12th November 2017, 10:36 PM
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Quote:
Originally Posted by Kevin View Post
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
hey Kevin do you still have this PDF? If so PM me please
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  #50  
Old 12th November 2017, 10:47 PM
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@johnsonmiles PDF emailed
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