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Old 03-17-2013, 02:56 PM   #1
JMlegacy
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Default How is the AWD supposed to work on LGT's?

I Have a 2005 legacy GT. Going up a gravel hill the two front tires would spin and nothing from the rear.

Is this normal?

Is the center diff broken?
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Old 03-17-2013, 03:33 PM   #2
04stigeorge
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you didnt say if it was automatic or not...guessing it an automatic 5at??
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Old 03-17-2013, 03:34 PM   #3
JMlegacy
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Yes, auto
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Old 03-18-2013, 10:38 AM   #4
JMlegacy
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Update:
This morning while reversing i heard the rear wheels skidding, so i know power is going around there. Center diff ok i guess
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Old 03-18-2013, 06:37 PM   #5
vicious_fishes
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front diff is worse than rear diff. often the rear grips much better than the front. hence the understeer our cars have.

people often put the understeer down to the chassis or weight distribution. i disagree. put a plated/TB diff in the front and you'll see what i mean.
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Old 03-20-2013, 04:58 AM   #6
Subisam75
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My 06 Outback does the same thing. It may have something to do with the LSD rear. Not sure thou so don't quote me haha
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Old 03-21-2013, 02:13 PM   #7
atomicfire
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The non-STi (and non Spec B LGT / non-S model Forester models) use an open differential all around. On loose road surfaces like gravel, you will spin and it will be easy to get stuck.
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Old 03-21-2013, 06:10 PM   #8
BlackHole
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The LGTs do have a viscous rear LSD, with open center and front. I don't know about the other trim / models.
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Old 03-21-2013, 08:13 PM   #9
Counterfit
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I'm curious as to why people keep saying open center. I know of no instance where an open center is of any use at all.
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Old 03-22-2013, 12:10 AM   #10
Tsu
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Quote:
Originally Posted by Counterfit View Post
I'm curious as to why people keep saying open center. I know of no instance where an open center is of any use at all.
I don't know either. It very clearly says in the manual the center is a viscous LSD with a "hump-mode" where the plates couple if the center overheats and the diff, for all intents and purposes, locks. There's even an awesome torque coupling vs. heat (or load) graph!

And the rear for the 05-09 LGT is the standard viscous LSD, the front is open. I tested this a few weeks back in the snow storm and mine still works. O

Source: I DD an 06 LGT, spent four years of it in the snowy north.
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Old 03-22-2013, 12:13 AM   #11
JMlegacy
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thnx for the replies. Today going up the same gravel road i tried to keep my revs low and i did feel the back tires compensating when the fronts were slipping.

Now for the Viscous LSD and all other aforementioned jargons, I have no idea how they are supposed to behave feel free to educate me
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Old 03-22-2013, 03:46 PM   #12
atomicfire
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Viscous is just about as useless as an open differential. It's torque transfer very very low, so low it won't actually make any noticeable difference. It uses the heat from slip to increase the viscosity (thickness) of the fluid, which in theory the thicker fluid will force torque transfer. There are a few problems with this - you need to be spinning the wheel pretty aggressively to heat up the fluid and force the torque transfer - and once the fluid heats up, the amount of torque it can send is still very very low.
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Old 03-22-2013, 11:25 PM   #13
Tsu
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Except that you're missing the point where the VLSD locks up as the sheer forces increase temperature. The sheer thickening fluid transfer is what transfers the torque (heat independent), and the heat generated induces lockup. Infact, fluid only serves to provide smooth lockup, it's mostly the plates acting in a clutch-like fashion that do the torque transfer if needed. As subaru states, the hump phenomenon serves to eliminate the rotational difference between the housing and the hub, directly coupling the front and rear axles.




Further discussion on VLSD from a SAE paper and VLSD design evolution, from sheer only torque transfer to sheer-couple torque transfer:
http://forums.nasioc.com/forums/showthread.php?t=523781
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