AWD system function

Well my it's been raining here a lot. Last night I was parked on my lawn and went to pull out. Sure enough because of some mud I had to rock it to try and get it out. Someone pointed out that "my AWD system is broken" because not all four wheels were spinning. *Insert chuckles here,* this was a car novice. I tried to explain to her how AWD/limited slip differentials work, and that got me thinking, do I know what I'm talking about.

Out back, we have an LSD whether it be viscous (2.5i 5mt/GT) or Torsen(Spec.B). Once one of the back wheels is spinning, this LSD transfers power from the wheel with least resistance (spinning), to the wheel with potential grip out back, correct?

The center diff, viscous on all of our cars, is responsible for power front to back. Once there is slippage out back/front it transfers power where it's needed? How does it determine where/how much power to transfer.

We have an open differential upfront so wouldn't this mean the wheel with the least resistance will always spin? Basically only one front wheel will ever have power?

Do I have AWD/differentials understood correctly? This of course got me thinking, if I bought the STI I was looking at, which uses three Torsen differential, how much easier would it have been to get out of this mud.

That is basically correct; on manual transmission ars the front/back split is based on which set is spinning SLOWER, on autos it is electronically controlled.

This will get ugly.

This should answer your questions.

http://www.legacygt.com/forums/showthread.php?t=48112

The viscous LSDs are speed-based, they are measured in amount of torque per RPM speed difference between axles. There's a set number based on the characteristics of the unit. Stockers are fairly loose, I believe aftermarket companies offer units with greater torque transfer per RPM.

Front will always be 50/50 measured to the ground, in the sense that if one wheel slips when 10 lbs-ft of torque is applied to it, if you measured the other side, it would only be receiving 10 lbs-ft of torque as well.

Rao gets his information from www.bobistheawdguy.com

I actually clicked on that, you rat.

2.5i's do not have LSD in the back. I wish.

The 2.5i Outbacks do.

In for the learning experience and to see how long rao can last before he gives up on this thread.

He's switching to safer oil for his cars and warming them up fully before he drives.

He's switching to safer oil for his cars and warming them up fully before he drives.

I thought he quit driving for fear that something would break.

The 2.5i Outbacks do.

Yes they do. The Legacy does not.

Here's a good article about 4x4 systems from a Jeep forum I am a part of:

(substitute "full time 4x4" for AWD)

http://www.jeepforum.com/forum/f27/4x4-torque-answers-130169/#post870821

Same thing happened to me in the snow 2 days ago. Had to be pulled out with chains by a truck. Then I had to hear everyones favorite line " I thought you drive a subaruuuu". I'm wondering if maybe the rear lsd is not locking up?

I wasn't happy. This is the second time this winter I overestimated the AWD on my car. Last winter she was a champ, so maybe I'm just expecting too much now that I've been spoiled by AWD.

I wonder how differently things would have gone if I had an STI with three Torsen LSDs.... Could have, would have, should have...

I got stuck this past weekend out at Lake Tahoe. Made it through the chain checkpoints and got up to the driveway of the rental cabin. The snowplows had made a nice wall in front and after shoveling a path, drove up. At least I tried. Snow/ice + AWD + summer tires = fail. I'm surprised they let me through the chain checkpoint. After struggling for probably 15 minutes, I just threw the chains on and up the steep driveway she went in one go. I wonder how I would have fared with all seasons, or even real snow tires.

On flat ground, I was still able to plow through at high RPM spinning all 4 wheels.

And also, VDC on ice is useless, that thing will bog you down and stall you out.

Still had a great trip, I'll have to get dedicated winter tires now.

The manual transmission uses a viscous coupler center diff. Same thing I had in my 86 VW Vanagon that was all wheel drive. The more speeds differ (front to back) the more the oil heats up, and the more it "locks up". However it will not lock up like a transfer case, and you can still be left with the back spinning and the front standing still, or close to it.

The front diff is open, the torque takes the path of least resistance meaning the tire with the least traction.

The rear is a limited slip, not a locker. The torque still takes the easiest path, but with more resistance. If one rear tire was in the air and the other stuck, the tire in the air would sit and spin. This is particularly true with a clutch based limited slip that also wears out over time.

In essence, the Subaru could become one wheel drive under the right situation.

I'm not sure how the autos work, or VDC but that is how a manual trans LGT works.

I know in most clutch based limited slips, they can be rebuilt with stronger springs and varying the clutch plates for a tighter limited slip.

A helical gear type limited slip (torsen, detroit truetrac) has a higher bias and can get closer to lockup than a clutch limited slip. Especially if you use brakes to slow down the faster spinning wheel. Definitely better than a clutch based diff.

The manual transmission uses a viscous coupler center diff. Same thing I had in my 86 VW Vanagon that was all wheel drive. The more speeds differ (front to back) the more the oil heats up, and the more it "locks up". However it will not lock up like a transfer case, and you can still be left with the back spinning and the front standing still, or close to it.

The front diff is open, the torque takes the path of least resistance meaning the tire with the least traction.

The rear is a limited slip, not a locker. The torque still takes the easiest path, but with more resistance. If one rear tire was in the air and the other stuck, the tire in the air would sit and spin. This is particularly true with a clutch based limited slip that also wears out over time.

In essence, the Subaru could become one wheel drive under the right situation.

I'm not sure how the autos work, or VDC but that is how a manual trans LGT works.

 

 

I know in most clutch based limited slips, they can be rebuilt with stronger springs and varying the clutch plates for a tighter limited slip.

A helical gear type limited slip (torsen, detroit truetrac) has a higher bias and can get closer to lockup than a clutch limited slip. Especially if you use brakes to slow down the faster spinning wheel. Definitely better than a clutch based diff.

This is a good explanation. Something to add is that Limited slips work on a percentage. When Choppedair refers to a tighter L/S, that means a higher percentage. So, the wheel with the least resistance gets the power then the L/S transfers a percentage, depending on the style and setup of the L/S that will very. So say its a 25% L/S it will transfer 25% to the wheel not slipping. But like said above if you have a wheel that is in a very low traction situation like being in the air, you are transfering 25% of 0% traction aka 0% over to the other wheel. Which is why lockers and spools are so awesome, just not for the road. (I have a spooled rear, locked front Truck, great in the dirt not so nice on the road.)

The transfer is a rough figure, and the technology for the LSD matters how much that is really transferred. Some clutch LSD:s and viscous LSD:s use the difference in RPM between the outgoing axles to start engaging and transfer the power to the non-slipping side.

Any LSD is better than no LSD.

We don't have a viscous center diff. We have a viscous center limited slip. 50/50 all the time manual is sent through actual gears, unlike the old VW and Porsche systems and others that just had a viscous coupler.

Key difference.

And the viscous LSDs work of RPM difference. IE, here's aftermarket versus stock:

"Increases LSD action of the center differential from 4kgf-m/100rpm stock setting to 20kgf-m/100rpm. "

Out back, we have an LSD whether it be viscous (2.5i 5mt/GT) or Torsen(Spec.B). Once one of the back wheels is spinning, this LSD transfers power from the wheel with least resistance (spinning), to the wheel with potential grip out back, correct?

Almost... in fact, the LSD will only try to make both wheel spin at the same speed.

I really love this PDF : http://stuff.urbanghetto.net/Subaru_AWD.pdf

We don't have a viscous center diff. We have a viscous center limited slip. 50/50 all the time manual is sent through actual gears, unlike the old VW and Porsche systems and others that just had a viscous coupler.

 

Key difference.

 

And the viscous LSDs work of RPM difference. IE, here's aftermarket versus stock:

 

"Increases LSD action of the center differential from 4kgf-m/100rpm stock setting to 20kgf-m/100rpm. "

Yeah you're right. I had to think back a bit to remember how my VW worked. The VW Vanagons were generally rear engine, rear wheel drive. I believe the AWD Vanagon was still been mechanical rear drive, with an additional output shaft with a viscous coupler on the front driveshaft.

That setup would actually be better in some low traction scenarios having atleast one axle mechanically driven. Although if the coupler wore out on the loose side you could be stuck with rear wheel drive almost. Normally they got tighter as time/miles went on though.

With a viscous limited slip center diff the torque will generally be distributed 50/50, however the torque will tend to take the path of least resistance. As the speed difference increases between front and rear, the fluid heats up and it gets more of a "lock-up".

Yeah the whole wheels that slip to wheels that grip is kinda a misnomer.

A solid axle does the same thing, a la truck part-time system in 4WD. Lift one set of wheels off ground, other set will measure nearly 100% of the torque. Swap ends and the other set will measure nearly 100%. Can get everything in-between as well. OMG a full 0/100 to 100/0 system, hehe. No magic diffs or electronics required.

I count systems that can actually reduce torque to one axle and send it to the other to actually be able to do the wheels that slip to the wheels that grip, like the new diffs from Acura/Audi/etc, with clutch packs on each axle that can reduce torque being sent to a wheel slipping or not and send more to another.

Viscous couplers are kinda cool, kinda like the predecessors to Haldex systems.

Speaking of Haldex a beefed of version is kinda neat, given that they can now partially lock based on sensor input (steering, throttle, feedback from ABS and stability control) as well as when fully locked, there's actually a physical connection which theoretically can be much better in real low-traction situations, at least better than just a loose viscous limited slip.

I got stuck this past weekend out at Lake Tahoe. Made it through the chain checkpoints and got up to the driveway of the rental cabin. The snowplows had made a nice wall in front and after shoveling a path, drove up. At least I tried. Snow/ice + AWD + summer tires = fail. I'm surprised they let me through the chain checkpoint. After struggling for probably 15 minutes, I just threw the chains on and up the steep driveway she went in one go. I wonder how I would have fared with all seasons, or even real snow tires.

 

On flat ground, I was still able to plow through at high RPM spinning all 4 wheels.

 

And also, VDC on ice is useless, that thing will bog you down and stall you out.

 

Still had a great trip, I'll have to get dedicated winter tires now.

This is the issue, if you had some nice snow tire on the car you might have had a better time. I ran hankook winter iPikes on my 04 STI and I hit hills that were unplowed just to see the quality and they got me right up the hill. You cannot expect to do good in a good amount of snow if your on all season's or summer tire. It is a FAIL.