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Old 08-15-2001, 03:19 AM   #1
fungo phil
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Question Isn't LSD on an AWD vehicle a bit redundant?

Doesn't a limited slip differential and an all wheel drive system do the same thing? If one of the wheels loses traction, power is shifted to the other wheel/s in either case, yes?
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Old 08-15-2001, 03:56 AM   #2
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No. Without LSD subaru's AWD spins tires, the center Diff ends up doing most of the work... (dealing with the 50/50 and stuff) The LSD prevents the tires with less traction from spinning... And in reality, you can have an even BETTER handling car with front AND rear LSD rather than just rear LSD. (The EVO and STi have both I believe)
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Old 08-15-2001, 05:43 AM   #3
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You're talking about the difference between marketing and reality. "..from the wheels that slip to the wheels that grip." nonsense.

I don't know how they got away with it (the marketing) until the LSD in 2000. From all accounts, it makes a BIG difference.
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Old 08-15-2001, 05:51 AM   #4
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Hmmm... Just a thought but if i remember correctly L.S.D. with an awd car could be well dangerous all that halusinating 7impaired judgement.
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Old 08-15-2001, 09:33 AM   #5
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"No. Without LSD subaru's AWD spins tires, the center Diff ends up doing most of the work... (dealing with the 50/50 and stuff) The LSD prevents the tires with less traction from spinning... And in reality, you can have an even BETTER handling car with front AND rear LSD rather than just rear LSD. (The EVO and STi have both I believe"


The 2001 RS has front and rear LSD, the 2002 only has a front LSD, drive one scooby hard with both front and rear LSD and then drive another with only the front, drive them hard and youll notice the difference.
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Old 08-15-2001, 10:00 AM   #6
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AWD refers to a 4WD system with a center LSD.
:P no, it's not redundant.
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Old 08-15-2001, 10:03 AM   #7
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Quote:
Originally posted by ITR&RSGUY
The 2001 RS has front and rear LSD, the 2002 only has a front LSD, drive one scooby hard with both front and rear LSD and then drive another with only the front, drive them hard and youll notice the difference.
Nope, sorry. The RS NEVER came with a front LSD (I don't know of any street Subaru to ever come from the factory with a front LSD). In 2000 a rear LSD was added to the RS and Forester S (not sure about Legacy GT). In 2002 the rear LSD was removed from the RS.

edit: oops, not Forester L

Last edited by Strepto; 08-15-2001 at 06:51 PM.
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Old 08-15-2001, 10:10 AM   #8
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Quote:
not sure about Legacy GT
My wife's GT LTD has a rear LSD. I'm not sure if the regular GT has one though; it probably does.
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Old 08-15-2001, 10:12 AM   #9
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so your Telling me than the 2002 RS is not an AWD car then if no subaru has a front LSD and the 2002 has no rear LSD its a 4X4 then. I don't think so.
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Old 08-15-2001, 10:26 AM   #10
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To go back to the original Q... the AWD system transfers power between the front and rear differentials, trying to keep both front and rear wheels spinning at the same speed. Without an LSD, though, all the front or rear power could be directed to the spinning wheels.

If I understand everything correctly (and please correct me if I'm wrong), if your right wheels are on ice and your left are on pavement, without an LSD your "AWD" car would be a "RWD" car. Not Rear Wheel Drive... Right Wheel Drive. All the power would be sent to the 2 spinning wheels and you'd go nowhere fast.

The LSD on the rear will at least turn it into a 3WD car... rear two and the front right, which is better, but not "AWD" imho...

And what's that magic blue stuff that mysteriously goes to the "wheels that grip" in the Subaru commercials? Can I get some of that?

-tim
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Old 08-15-2001, 10:28 AM   #11
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AWD cars have three diffs, front center and rear.

With a LSD in the center only (like 98-99, 02 RSs), you can still lose traction if any two DIAGONAL wheels lose grip. If you are high-centered diagonally, one front and one rear will spin freely.

With a LSD in the center and rear (like 00-01RSs, 02 WRX), you won't lose traction unless three wheels (both rears and one front) slip.

And yes, with three open diffs like an early Audi quattro, if a single wheel loses traction it will spin.
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Old 08-15-2001, 10:31 AM   #12
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Okay let's get this thing straight. Subarus have three differentials.

The front differential (an open diff) controls power delivery between the front wheels only.

The center differential (LSD- vicscous coupling on manual tranny, electro-mechanical clutch on auto) controls power delivery between the front and rear wheels.

The rear differential (open in most models, LSD in some) controls power delivery between the rear wheels only.

if the front wheels are slipping (it only takes one since it's an open diff), power is transferred by the center diff to the rear wheels and vice versa.

In the case where the front and rear differentials are both open, the car will lose traction if just one front and one rear wheel are both slipping.

In the case where there is a rear LSD, the car will only lose traction if one front wheel and both rear wheels have lost traction.

In the case where there is a front and rear LSD, the car will only lose traction if all 4 wheels are slipping.

4WD is like AWD but with a locked center diff, meaning that the power sent to the front wheels is always equal to the power sent to the rear wheels. Of course, Subaru used to call their AWD systems "4WD" until the early 90s, and I think the Brits still call it 4WD.

edit: Tim, Jon, you beat me to it. Jon, I don't think they need to be diagonal wheels for the car to slip. With the open diffs, any 1 front and any 1 rear will do it.
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Old 08-15-2001, 10:33 AM   #13
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Here is the simplest answer. The "AWD" system in all Subarus is realy talking about the viscous coupling center diff. This transfers power front to rear. The limited slip diff (LSD) in the rear of 2000 and higher 2.5RS's and WRX's transfers power left to right at the rear of the car. The front differential is an open type. Meaning that one tire can be stopped and the other tire can spin. But this would only happen in a few very special cases (like one tire jammed between two boulders and the other tire in the air). In normal use power is divided equally between all four wheels.
So yes in some cases your Subaru only has three wheel drive. Just like your '72 Duster that had an open diff in the rear would only spin one tire. But does that make the car one wheel drive? No, it just means that only one tire gets the vast majority of the power when traction is broken.

-edit- wow check out the times of the last four responses, we all started typing baically the same answer at the same time.

Last edited by Travis R; 08-15-2001 at 10:41 AM.
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Old 08-15-2001, 10:55 AM   #14
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Strepto is right... only the 2000 and 2001 RS got the rear LSD.
The way they market the AWD system (even for their cars without a rear LSD) avoids potential lawsuits from deceptive marketing tactics.
ie. If both front wheels are on ice while the rear ones are on pavement, the viscous center diff locks up and transfers most of the power to the rear wheels. Yet if half of the car (left side) is on ice and the other half is on pavement, without an LSD, the center diff never would lock up. All of the torque would be transfered to the tires spinning on ice since the open front and rear diffs naturally transfer torque to the tires that have no traction. To partially remedy this problem, Subaru equipped some of their models with a rear LSD to transfer at least some of the wasted torque to one of the wheels that has traction.

Yeah, it's not technically the best AWD system out there, but it is very light, very simple and still quite effective. It sure works well during the winter.
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Old 08-15-2001, 10:59 AM   #15
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Man, who needs LSD when you've got an AWD car. It's fun enough already, ok man?
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Old 08-15-2001, 11:10 AM   #16
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LOL woah! Everyone jumped on this thread in record time.
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Old 08-15-2001, 11:19 AM   #17
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Default Legacy B4 has a front LSD

Although not technically a factory car in the truest sense, the Blitzen does have a front LSD, and a damn good one at that. Having all three diffs limited-slip is a real work of art. It means your drivetrain is working its hardest to keep up with the traction. Some people say having all LSDs will cause more parts to fall off your car. I guess I'm not sure how strong an LSD is to an open diff.

Anyway, I've driven an RB5 with full LSD support on a snowy road with Hakkas on and I think the LSDs had the biggest role in keeping me out of the ditch. You could almost feel the torque tranferring to the outside tyres on a corner. In a way it almost makes the car whatever-wheel-drive it needs to be at the time. On acceration it moves it rear and on decel (especially with LFB) it moves power to the front. I think the Skyline did a better job of torque tranferrence but that was electronically-controlled and could always fail. Keep in mind all of this testing was on icy/snowy roads so slip was a little more pronounced than on pavement. I was lucky enough to be able to really realize how much fun these things can be.

~G
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Old 08-15-2001, 11:30 AM   #18
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Quote:
Originally posted by Strepto

Nope, sorry. The RS NEVER came with a front LSD (I don't know of any street Subaru to ever come from the factory with a front LSD). In 2000 a rear LSD was added to the RS and Forester L (not sure about Legacy GT). In 2002 the rear LSD was removed from the RS.
Not sure if it was optional in the L, but the 2000 Forester S (like my own) has a standard rear LSD.

Ed

edit: Strepto caught his "oops" so that makes my post redundant.

Last edited by lark6; 08-16-2001 at 01:24 PM.
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Old 08-15-2001, 11:38 AM   #19
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My question is this:

Does the AWD for the 4EAT work in a similar fashion? And if so, wouldn't it benefit the most from the rear LSD in the MY00/01s?
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Old 08-15-2001, 12:04 PM   #20
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Default tranny diferences

don't hold me to this, but IIRC the 4EAT would work the same way with the exception that the center diff is electronicly controlled. Power distribution from left to right would be the same as a manual, but front to back Should be more effecient I would hope. Isn't it basicly an electrical LSD rather than a mechanical LSD?

Edit: forgot about the question... yes, an auto would benefit just as much from a rear LSD as a manual. Would it benefit more.... I don't have a clue.
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Old 08-15-2001, 12:10 PM   #21
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The 4EAT split the power front/rear 90/10 until slip was detected. Then through electronically controlled clutch packs, it could vary the power up to 40/60 I *think*. Anywayz, don't quote me on the numbers, but under most conditions, the transmission behaved as FWD with the majority of the power being trasmitted to the front.

Boy, alot of newbies posted incorrect info initially on this post! Isn't this in a FAQ somewhere and shouldn't this be in the transmission section of the board?

LaterZ!
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Old 08-15-2001, 01:36 PM   #22
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If only that was true no US subaru has front LSD 2002 has an open front diff were are you getting this stuff


The 2001 RS has front and rear LSD, the 2002 only has a front LSD, drive one scooby hard with both front and rear LSD and then drive another with only the front, drive them hard and youll notice the difference. [/b][/quote]
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Old 08-15-2001, 01:46 PM   #23
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Quote:
Originally posted by ITR&RSGUY
so your Telling me than the 2002 RS is not an AWD car then if no subaru has a front LSD and the 2002 has no rear LSD its a 4X4 then. I don't think so.
What are you talking about if it has an transfer case its a 4x4 or an AWD there the same if power is feed to all the wheels it's AWD or a 4x4 an open diff sends power to the wheel with the lest resistance LSD is just that if a wheel is slipping then it sends more power to the wheel thats not are you just trying to have fun with us or what?
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Old 08-15-2001, 01:59 PM   #24
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I think the post just above mine is one long sentence, I couldn't read it all without making my head spin. People need to proofread their writing more often.

Anyway...

All RS's have 3 differentials. A LSD can transfer torque side to side, and open diff can't. That's the difference. For example, MY00-01 RS: open front, then center, and LSD in the back. MY98-99 and 02: open front and rear, and of course the "standard" center diff.

Also, the STi's come with front LSD's, don't they?
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Old 08-15-2001, 02:18 PM   #25
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I agre... I can't understand what milkman was trying to say...

What I can say is that no Subaru imported into America came with a Front LSD. Whoever told you that the MY01 did was mistaken

Eric
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