I Know that the forester have the traction in 4 weels, and is permanent, 50% in front and 50% in rear AL THE TIME. But in the owners manual of legacy, i read that the AWD isn´t permanent, normally is FWD, and when is neccesary it converts to AWD.

 

I think all subarus have the AWD permanet, and this is the reason for the good results, compare with volvo cross country and similar. (the Outback is better than volvo in hard conditions...)

 

And outback? is like forester or like normal legacy?

 

it doesn't differ so much from Forester to Outback or Legacy, but from automatic transmissions to 5-speeds. Automatics have computer controlled clutch packs in the center diff that will send more power to the rear wheels when slippage is detected. Normally it's split 90-10. The transfer is very quick, though, and not really noticable. I can't tell when it does it, unlike, say the delay in GM trucks in 4x4 auto mode. Also, in an automatic, when the selector is in 1st power is split 50-50.

 

5-speeds have a viscous coupling, so it's sort of split 50-50 at all times. When there's a difference is speed from front to back, the fluid shears, heats up, becomes more viscous, and forces both ends to turn at the same speed. I've heard it doesn't react as quickly as the auto.

 

The 6-speed in the STi is different, and I think the automatic WRX has a 45/55 rearward bias or something. There are also two traction/stability control things available. Something like VTD or VDC or VTC. Anyway, one of them will brake individual wheels to keep traction, the other will brake individual wheels and vary the front/rear torque split. IIRC.

 

The Subaru Global website has a bunch of really cool stuff. Like this. Also check out endwrench. They have cool technical articles.

 

Okay my bad partially not right

VC is plates that are almost in contact, the oil expands as it heats up and forces them together.

 

Split is 50/50, going to a theoretical 0/100 if need be.

"from the wheels that slip, to the wheels that grip"

The transfer is very quick, though, and not really noticable. I can't tell when it does it, unlike, say the delay in GM trucks in 4x4 auto mode. Also, in an automatic, when the selector is in 1st power is split 50-50.

 

Interesting...

 

After reversing out of a parking spot and coming to a complete stop, I have noticed a small "surge/shudder" after I put the car in first and move a couple of feet forward. I assume it is the extra 40% going to the rear?

 

(05LW)

Split is 50/50, going to a theoretical 0/100 if need be.

Well, not really. In a 5-speed the front diff is driven directly by the output shaft, and the back of it is attached to the viscous coupling (which is attached to the transfer gears which drive the prop shaft). So, the car is sort of FWD until there is a speed difference, at which point the VC splits power 50-50. Of course, the vc won't ever allow the car to be only fwd, which is why I said "sort of."

 

 

Autos are pretty much the same way, but instead of a viscous coupling there is a set of computer controlled clutches, which can vary the transfer from 90-10 to 50-50 (or 45-55).

 

For the 45/55 thing, the transfer gears are different sizes and the rear diff ratio is different, giving greater mechanical advantage to the rear prop shaft, and then the diff keeps the rears turning the same speed.

 

Interesting...

 

After reversing out of a parking spot and coming to a complete stop, I have noticed a small "surge/shudder" after I put the car in first and move a couple of feet forward. I assume it is the extra 40% going to the rear?

 

(05LW)

 

Kind of. If you had turned a little and then put it in first, there would probably be a little bit of rotational difference between the front and rear, and then the shudder would be the slack being taken up when the center diff forces the rears to turn at the same speed (although some slippage is allowed so we can drive on dry surfaces). I guess.

Kind of. If you had turned a little and then put it in first, there would probably be a little bit of rotational difference between the front and rear, and then the shudder would be the slack being taken up when the center diff forces the rears to turn at the same speed (although some slippage is allowed so we can drive on dry surfaces). I guess.

 

So if I keep turning in this manner, are the rear tires going to continue to skip? I have read this behavior is a 4WD problem for certain designs, but I thought Subaru was above all of that?

So if I keep turning in this manner, are the rear tires going to continue to skip? I have read this behavior is a 4WD problem for certain designs, but I thought Subaru was above all of that?

 

You still have diffs on each axle.

When something is broken or wrong, you can get weird behavior. Subaru's sensitivity is why, when things are normal, the system works so well. But it can't 'know' the difference between a slipping wheel, or one that is rotating faster because it is undersized. And if something is broken - can't expect it to compensate for that.

So if I keep turning in this manner, are the rear tires going to continue to skip? I have read this behavior is a 4WD problem for certain designs, but I thought Subaru was above all of that?

 

In a corner, the front and rear wheels are turning at different speeds, which is what the center differential tries to prevent. It slips a bit to compensate for slow tight turns on dry ground, but a little bit of shudder (or wheel spin, if you try) is normal.

 

If it's really bad and is happening on regular corners, it could be torque bind, which has been discussed pretty thouroughly on every Subaru message board if you want to seach.

Very cool, that makes a lot of sense. I'm starting to see the big picture.

 

So how does the car know to supress this "hyper correcting" when the car is traveling faster?

 

And thanks for indulging my first steps into understanding my sube more...

hyper correcting?

 

 

nipper

Zephyr - you and others reading this might enjoy these 2 links;

 

 

http://www.archive.org/details/Aroundth1937

 

http://auto.howstuffworks.com/differential.htm

 

fyi

hyper correcting?

 

 

nipper

 

Sorry- new to this stuff.

 

The diff is so sensitive that it will try to correct the non existant slippage when I am turning the wheel.

Zephyr - you and others reading this might enjoy these 2 links;

 

 

http://www.archive.org/details/Aroundth1937

 

http://auto.howstuffworks.com/differential.htm

 

fyi

 

wow, those are great links.

 

Thanks Lucky.

Sorry- new to this stuff.

 

The diff is so sensitive that it will try to correct the non existant slippage when I am turning the wheel.

 

The CENTER diff is only going to attempt to transfer torque when there is slip detected on one AXLE versus the other. There probably istn't enough difference when turning to do much to activate the center diff. Again, this is under normal conditions. (and others here may be able to fill in some details) .In other words, when you turn right, the left front and the left rear wheels turn faster than the right side wheels (the front and rear diffs take care of this so there is no scrubbing or dragging) BUT the front drive shaft and the rear drive shaft are turning about the same rate so there is no slippage detected to cause the CENTER diff to shift any torque. Again, under ideal conditions.

 

(my head hurts)

Just to clear up some misinformation about the operation of the Subaru AWD systems…

 

The 4EAT is directly geared to the front axle and power is transferred to the rear through an electronically controlled (TCU) clutch pack. Torque balance can vary from 100% front to 50/50 split, though under TCU control (no “FWD” fuse) the max imbalance is designed to be 90F/10R.

 

The 5MT has a true VLSD center diff, not just a viscous coupling connected to the rear shaft. The front shaft is NOT connected directly to the trans output, there is a gear diff with the VC between the front and rear outputs. There are no clutch plates in the center VC, its purely fluid shear, as the fluid heats from the differential movement it gets thicker and resists further movement. The VC in the center diff is designed to transfer a given level of torque and can be over powered, torque beyond the rated value will cause wheel spin.

Gary