Quattro vs. Symmetrical

[AWD_Rules3830]

Will This Thread Ever Die!!!!!

 

SUBARU = AUDI in AWD prowess ... Enough already people.

[erito]

And I still do not know how my AWD system work because i have a Subaru Outback 2006 2.5i VDC 4EAT which it seem like not beeing an option i the US.

 

Very tricky since no one knows... maby I have to make a phonecall to Japan...

And maby i'm off-topic with that question...

[NSFW]

NSFW; what is unsmooth about the fluid is that it does take a fraction of a second to react. VC are based on shear but shear also creates heat in the fluid. The heat puts more pressure on the fluid because it expands and you basically have a higher "preload". Just like what you experienced on the hot days in your old car. Likewise, there is a point where the fluid overheats and breaks the housing. Then the fluid leaks out and you can't build any pressure between the plates so the unit behave like an open diif. That is why I asked BOY about toasting a VC and it becoming "locked". Mechanically it goes against the design.

 

The VC transfers torque as the fluid heats (or as shear increases, depending who you ask), as power is transferred gradually - smoothly.

 

I've never heard of a transfer case actually breaking that way. Such a failure would for sure be abrupt, but this would be the first time I've heard of suck a thing. In my case (pun intended) the diff was sticky for a while but returned to normal within hours. Otherwise I never felt it kicking in.

 

Torsen system always if transfering torque whether it is needed or not. There is no reaction time required.

 

This is true for VC diffs as well. By default you get a 50/50 split (usually) so all wheels have torque whether they need it or not. Should one wheel lose traction, the VCs keep torque on the other wheels. Sure there might be some lag between the onset of wheelspin and the onset of VC binding, but again, I never noticed it happening. It's a smooth process.

[b4wantab]

The VC unit is basically an insert that takes the place of the spider gears in the diff itself. It shouldn't really be looked at as a transfer case.

 

I agree it is smooth but it is not as smooth as a Torsen, nor as linear in lock-up. My BH has two, center and rear, and they are great for my needs. I don't road-race or autoX. I am not digging on VC they are very good units for everyday use. I also feel they are much better than the computer reactive systems out there. That includes Subbie and Audi auto trans offerings.

 

Peace,

 

Greg

[erito]

So you say Subaru 4EAT with VDC is not as good as Subaru manual trans without VDC? Ofcourse the 5EAT VDC cars is (IMHO) the best solution, but i thought the 4EAT with VDC was second best installation of Symmetrical AWD...

 

And Quattro automatic trans vs manual trans have big differences?

[BOY]

The VC transfers torque as the fluid heats (or as shear increases, depending who you ask), as power is transferred gradually - smoothly.

 

I've never heard of a transfer case actually breaking that way. Such a failure would for sure be abrupt, but this would be the first time I've heard of suck a thing. In my case (pun intended) the diff was sticky for a while but returned to normal within hours. Otherwise I never felt it kicking in.

 

 

 

This is true for VC diffs as well. By default you get a 50/50 split (usually) so all wheels have torque whether they need it or not. Should one wheel lose traction, the VCs keep torque on the other wheels. Sure there might be some lag between the onset of wheelspin and the onset of VC binding, but again, I never noticed it happening. It's a smooth process.

 

I can't speak on all the details as I didn't do a tear down of the faulty diff but from what I understand the Subie VC LSD is set to 50/50 static and the VC allows progressive lockup to decrease the slip when temps rise. Remember, this is a LSD, not a contant rate transfer case. What I figure happened is that the transfer plates got "bent" or warped thus causing the lock. Trust me when i say locked, 50/50 all the time especially noticable at low speeds.

 

Whatever caused the lockup was strictly in the diff. The reduction gearset was fine, as were all the splines in/out of the diff.

[Vimy101]

Anedotal evidence:

 

I attempted to see how fast the center diff reacted to slip. I put the front tires on some snow covered ice with the rear wheels on dry concrete and accelerated briskly (just a little punch to see if I could break traction). No slip from the front wheels at all. Not scientific by any means and I was not even sure what I expected to result or even look for but the torque transfer seemed immediate.

 

As I understand it, it's the shear force not the temperature of the viscous fluid that is the key factor.

[BOY]

As I understand it, it's the shear force not the temperature of the viscous fluid that is the key factor.

 

yup... but one gets you the other.

[Vimy101]

^ Silly Putty might be used as an example. Leave it alone and it will slowly flow like a liquid. Hit it with a hammer and it'll shatter into a million pieces.

[DukeTrout]

But the shear causes the temp increase, not the other way around, so if the viscous fluid reacts to shear, the response should be much faster.

 

And good example with the Silly Putty. I hadn't thought of it as a positive shear reactive viscous fluid before!

[b4wantab]

erito: I like the manual subbie system better than the automatics. Don't know which is better for you or you prefer. Subbie basically has two auto systems now. Do you know which one you have? VTD or VDC.

 

VTD (variable torque distribution) uses a electronic clutch pack in the center diff to transfer torque front to rear. It also now has a LSD in the rear but I do not remember when that became standard. Similar to the manual system but the center diff is computer controlled. This is on the 4eat here in the states.

 

VDC (vehicle dynamic control) has the same components as the VTD but has software to individually brake the wheels and decrease the trottle. VDC is also paired with TCS (traction control system) which is a different program depending on vehicle speed. Again, lock a wheel or two to get out of a ditch, not highway speed stuff. This is on the 5eat here in the states.

 

Hope that helps.

 

Peace,

 

Greg

[Vimy101]

But the shear causes the temp increase, not the other way around, so if the viscous fluid reacts to shear, the response should be much faster.

 

And good example with the Silly Putty. I hadn't thought of it as a positive shear reactive viscous fluid before!

 

I believe Silly Putty is also silcon based like the fluid found in vicous LSDs.

[Brady]

Yeah, Silly Putty is a great comparison! I thought about that last night after I posted. Which reminds me, I should get some again. Good times!

[BOY]

erito: I like the manual subbie system better than the automatics. Don't know which is better for you or you prefer. Subbie basically has two auto systems now. Do you know which one you have? VTD or VDC.

 

 

Not quite. The 2 automatic AWD setups are Active and VTD. Active uses the clutch pack that varries from 55/45 f/r up to 90/10 for max fuel economy. Active AWD is on the 4EAT N/A 2.5 and FXT-L.

 

VTD AWD uses a planetary center diff/clutch pack setup w/ static split 45/55 f/r and can vary +/- like the VC BUT VTD can actually accellerate torque transfer via the planetary. VTD has 3 variants: VTD (LGT 5EAT), VTD w/ VDC (LGT-Navi, OBXT 5EAT (all), Sports FXT, WRX 4EAT, Tribeca), and VTD w/ DCCD (STI).

 

VTD w/ VDC is really cool in that unlike normal ESP's it allows f/r torque transfer before the nannies get involved. On my OBXT, for example, it'll allow some front wheel spin if the rears are grabbing (i.e. hard launch) but will apply brakes/pull throttle on ice. Also, VDC allows a little slip angle when cornering before it gets involved too! I over cooked a cloverleaf in the wet w/ the OBXT and it just slipped out the rear perfectly before I was able to get back on the power... my old RS-T would've just understeered right off the corner in the same conditions (or snap oversteered... but that's another story).

[NSFW]

Not quite. The 2 automatic AWD setups are Active and VTD. Active uses the clutch pack that varries from 55/45 f/r up to 90/10 for max fuel economy. Active AWD is on the 4EAT N/A 2.5 and FXT-L.

 

So... It's 90/10 when the clutch is disengaged, and 55/45 when the clutch engages to connect the output shafts? Please explain...

 

(I am suspicious again...)

[IwannaSportSedan]

Not clutch as in manual transmission.... These systems are in automatic transmissions.

 

There is a series if friction rings, and a series of shim rings, and a mechnaism that adds or removes pressure from the friction rings onto the shim rings. It is more of a continuous variance clutch than an on/off clutch that one thinks of in a manual transmission.

 

When the clutch pack is loose, most of the torque is transmitted to the front, and only 10% to the rear. increasing slip acts to tighten the clutch pack in the differential, and transfers up to 45% rearward for more traction.

 

The output shafts are always connected, but the amount of torque applied to them varies with the slippage alowed by the clutch pack. Less slippage, more rearward torque.

 

VTD and DCCD work similarly, but sensors adds input from around the vehicle to an on-board computer that can electronically interact to manipulate the differential before and during it's operation, and affect or even lock the torque bias on the center differential. Even on automatic management mode (VDC is permanent automatic) hey tend to bias rearward for performance, rather than forward for fuel economy and front wheel traction in the dry.

[NSFW]

I realize how/where the clutch works; perhaps "couples" would have been a better word than "connects." So you're saying that when "tightened" (to use your term) then if the front wheels have no traction and the rear wheels have great traction, the clutch won't transfer more than 45% of the torque to the rear wheels - the rest of the torque just makes the front wheels spin. That does make sense.

 

But the 'torque split for fuel economy claim' is still suspicous - regardless of how the torque split works out, the engine still has to generate the same force to turn all of the same driveshafts to move the car forward. It can turn the rear shafts by sending torque out from the transfer case, or it can turn the rear shafts by driving the front wheels forward and dragging the rears along behind it.

 

(Mind you, I'm not claiming that you're making stuff up - I'm just of the opinion that auto manufacturers and journalists make a lot of BS claims when talking about AWD systems so I like to drill into those claims and see what's really going on.)

[erito]

Not quite. The 2 automatic AWD setups are Active and VTD. Active uses the clutch pack that varries from 55/45 f/r up to 90/10 for max fuel economy. Active AWD is on the 4EAT N/A 2.5 and FXT-L.

 

VTD AWD uses a planetary center diff/clutch pack setup w/ static split 45/55 f/r and can vary +/- like the VC BUT VTD can actually accellerate torque transfer via the planetary. VTD has 3 variants: VTD (LGT 5EAT), VTD w/ VDC (LGT-Navi, OBXT 5EAT (all), Sports FXT, WRX 4EAT, Tribeca), and VTD w/ DCCD (STI).

 

VTD w/ VDC is really cool in that unlike normal ESP's it allows f/r torque transfer before the nannies get involved. On my OBXT, for example, it'll allow some front wheel spin if the rears are grabbing (i.e. hard launch) but will apply brakes/pull throttle on ice. Also, VDC allows a little slip angle when cornering before it gets involved too! I over cooked a cloverleaf in the wet w/ the OBXT and it just slipped out the rear perfectly before I was able to get back on the power... my old RS-T would've just understeered right off the corner in the same conditions (or snap oversteered... but that's another story).

 

 

Okay maby this is still going to be a mystery, but at least i got some of you really digging into this Lovely.

 

I have a JDM 2.5i Outback 2006 WITH VDC and 4EAT aut. tranny. Default split is 60% front and 40% rear. This i know for 100% sure.

 

According to http://www.subaruoutback.org/ forums, my car is 50-50 ratio when driving really slow, full-throttle or when i reverse. Otherwise it is 60-40 with the option to transfer all torque to the wheel with traction. Not only xx % of the torque. Ofcourse some power is lost in the drivetrain.

VDC breaks the spinning wheel and the torque is transferred to the other wheels.

 

And sadly i was deep in a ditch. Deep snow and slush. Never ever thought i would be able to get back up on the road, but i did!! The VDC really worked hard that time, too bad i didn't get it on tape

 

My car is always at least three wheel drive.

[b4wantab]

I was leaving off the DCCD system because I am pretty sure none of us have it. However, I thought that since VDC was the top of the line they were not marketing VDC w/VTD. I thought they were just marketing the system as VDC?

 

NSFW even though you are moving the same mass there are physics involved with effective force angles. It is more efficient to pull then push at times. It has been awhile but I can explain it more if you are interested. Anyway, varying to 90% front helps on the highway.

 

Peace,

 

Greg

[BOY]

I was leaving off the DCCD system because I am pretty sure none of us have it. However, I thought that since VDC was the top of the line they were not marketing VDC w/VTD. I thought they were just marketing the system as VDC?

 

NSFW even though you are moving the same mass there are physics involved with effective force angles. It is more efficient to pull then push at times. It has been awhile but I can explain it more if you are interested. Anyway, varying to 90% front helps on the highway.

 

Peace,

 

Greg

 

Just to piggyback your comment (and my original), the Active AWD only goes 90/10 in 3rd and 4th gears (i.e. cruising gears) and is always varrying between the 90/10 and the 55/45 split depending on throttle input and wheel speed sensors (slippage). 1st and 2nd gears maintain the static 55/45 (used to be 50/50 in the older active setup). For the 4 speed guys out there, you are roughly 50/50 in auto-x gears

[gallax]

Do you have some references for what you say? I have an 06 OB 2.5i 4EAT and I was always wondering how the gear affected torque split between F/R...

There used to be some documentation on the subaru site about the different types of awd (now it's not there anymore), but for the active AWD all they said is "it varies from 90/10 to at most 50-50 depending of throttle/breaking, cornering and wheel slippage).

Btw, even with this "not so great" awd, I was looking for troubles (meaning mountain roads (not public ones) during summer or mountain road during winter using the Crapenzas and I managed to get stuck (as in the car wasn't moving forward but I could back away) only once, and there were special circumstances (1 ft of ice with 5 inches of snow and then some rain over the whole mess).

[rporter]

There used to be some documentation on the subaru site about the different types of awd (not it's not anymore).

 

Yes, the former site had a very nice write-up on the various types of AWD they offer, but the new site is higher on flash and lower on good tech info.

[BOY]

Yes, the former site had a very nice write-up on the various types of AWD they offer, but the new site is higher on flash and lower on good tech info.

 

I've got it all over my showroom and office

 

Seriously, I think Subaru figured it was too complicated for mere mortals to figure out.

[outahere]

Do any of the Subaru models have an AWD system equivalent to the Haldex XWD (4th generation Haldex)? This system has a computer controlled clutch type rear differential and center differential. They are claiming 3 tires can be on a low friction surface (2 fronts and one rear), and 80% of available torque can still be transferred to the 4th tire. The system will be available first in early 2008, on the Saab 9-3.

 

http://www.haldex-xwd.com/

[Impulse]

Do any of the Subaru models have an AWD system equivalent to the Haldex XWD (4th generation Haldex)? This system has a computer controlled clutch type rear differential and center differential. They are claiming 3 tires can be on a low friction surface (2 fronts and one rear), and 80% of available torque can still be transferred to the 4th tire. The system will be available first in early 2008, on the Saab 9-3.

 

http://www.haldex-xwd.com/

 

theoretically, your 5mt can transfer 99.9999% of torque available under the same conditions. center transfers from front to rear, rear lsd transfers that and the opposite slipping wheel to the gripping one, and its alot more predictable-- mechanical transfer (laws of physics) vs electronic (programmed software laws)