...and no one who "read it on the forums somewhere".

A friend of mine and I have a bet. To determine the winner, we need to know how the NON-STi AWD bias is set up. I hear it depends on AT, MT, and the biases are 45/55, 50/50, 90/10, etc.

Now, let's say we have an MT with a 50/50...WHAT DOES THIS MEAN? Is this the MAX rear distribution? Is it 100% FWD when not slipping and only up to 50% can be transferred to the rear? Or is is ALL TIME 50/50? If all time 50/50, how does that work? That would make it more like a DSM and not variable at all. And how do they do all time 50/50 and not break rear halfshafts? They're like 18mm in diameter! Hmmmmm...

So...is the 50/50 a.) MAX bias or b.) ALL TIME bias.

Thank you for your help, and again, please answer ONLY if you know the answer AND you can Paypal me $500 if you answer incorrectly ;)

The way a manual wrx works is:

50/50 with optimal traction
You can have 50f/50rr/0lr in a situation where one rear wheel has no traction
You can less than 50 in the rear but no more
You can have 100f/0r if the both rears are slipping
You can even have 100fr or fl if 3 wheels are slipping.

The whole subaru slogan about sending power to the wheels that grip instead of the wheels that slip is a farce. More like it takes away power from the wheels that slip if it is the rears. But it will always send power to the fronts and send even more power to the front that slips (open diff)

There are three distinct AWD systems subaru uses, minus the DCCD, which I don't know much about.

On MT equipped cars they use a center differential with a viscous coupling. The "normal" power distribution is 50/50. The center differential is an open differential. As you may know with any open differential, when you have one side that spins faster (i.e. slipping in this case) all the power goes there. What the viscous coupling does however is limit that slip, and transfer power back to the side that isn't slipping. It does this by the nature of the fluid in the coupling. When slip occurs inter-twined plates between the front and rear drive shafts that reside in the coupling rotate at different speeds. This creates heat and changes the properties of the fluid....which in turn provides a "locking" effect of the front and rear drive shafts. How much locking effect (i.e. torque distribution) will depend on several different variables.

The bottom line is that the numbers given on torque distribution are nice happy numbers the public can understand. There is no definable way to measure it, IMO. Whenever slip occurs on the front or rear drive shafts, the above process happens, and essentually creates a "semi-locked to locked" center differential, depending on the situation. The MT's inherit design of the AWD system will never allow you to fully take power away from any set of wheels, just apply power to the wheels that aren't "slipping."

Here's a FSM scan of the MT AWD system
http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWDMT_description.jpg

For AT equipped cars the most common, or used AWD system up to the WRX has been the MPT or Multi Plate Transfer system. It's essentially a standard automatic transmission that has a rear extension housing on it with a set of clutch plates. Line pressure is fed to these plates from the main transmission. It is controlled by a duty solenoid which limits/adjusts the amount of torque transfer. It then uses various inputs from the computer to control torque transfer. The primary means of control is a front and rear speed sensor. When a difference in speed is detected the computer alters the duty cylce of the solenoid to provide more torque to the rear wheels. (I say provide more torque, because with these transmissions normal torque distribution is 90/10....and to have the rear wheels spinning with only 10% torque is not too likely) This system has its advantages over the MT system because it is electronic and does not need time for the fluid in the coupling to heat up. Again, the max torque distribution is 50/50, which is due to the inherit design of the system.

Here are FSM scans for the MPT AWD system
http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWD_transfer_system1.jpg
http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWD_transfer_system2.jpg
http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWD_transfer_system3.jpg

The third kind of AT AWD system is the VTD (Variable Torque Distribution) It's normal distribution is 45/55. It actually does have a center differential, and is more advanced then its predecessor. I'm not as up to speed on this system, so I'm going to leave it at that. If you want more info, try searching, or maybe someone else can input some information.

Josh

Thanks guys, that all made perfect sense. Boy am I glad I have an STi :D

There are three distinct AWD systems subaru uses, minus the DCCD, which I don't know much about.

<snip>

The third kind of AT AWD system is the VTD (Variable Torque Distribution) It's normal distribution is 45/55. It actually does have a center differential, and is more advanced then its predecessor. I'm not as up to speed on this system, so I'm going to leave it at that. If you want more info, try searching, or maybe someone else can input some information.

Josh

The most recent issue of the Canadian distributed Subaru "Six Star" magazine had a write-up on the 45/55 split center differential, along with a pretty clear exploded view. Anyone have this in electronic form? If not, I can try scanning in the page. The exploded view shows very clearly that the 45/55 distribution comes from the gearing ratios used within VTD center diff.

-- Dendrobium

Hi Guys

I've got to complicate this..

It is a common mis-conception that with an open diff the torque is transfered to the path of least resistance ..

If you have a 50/50 open diff, you will ALWAYS (ignoring imperfections in the diff, etc) get 50/50 torque split, even if you have one wheel on ice.

This means that each shaft will receive the same amount of torque.. and that amount of torque will be the maximum that the least resistive shaft can transmit.

Cheers

Simon

adding VTD post to this post

http://forums.nasioc.com/forums/showthread.php?t=607771&highlight=vtd

I was told by a mechanic that you could adjust the viscous coupling on the WRX to make it more rear biased? is this true or was he misinformed?

this was way back in 2002, long before the STi was released

I too was told a bit different. I was actually told the front/rear distribution is electronically controlled. I was also told it could be adjusted from 20/80 to 80/20 depending on what was needed. Default was 50/50 and would start there initially. This was on the manual transmission. He explained the automatic differently. Unfortunately, I have no factual data on this, but the individual was a long time Subaru owner and talked(wrote) like he knew his stuff. He also did explain that it varied depending on if it was an older model or a more current version as well as if it was the manual transmission or an automatic. The older models were described in the manner you stated, a simple viscous coupling. However, on the newer models, he said it was electronically controlled. An interesting example is the newer Impreza/Forester models. Apparently, you can turn them into fwd cars by taking out some fuse or relay...don't know, would have to find where I saw that...was looking at info about fuses at the time.

A qoute:
"The viscous type on your forester does drive the wheels 50/50 initially, which is why you probably get crappy gas mileage. The 5MT will vary power up to 80/20 or 20/80 depending on conditions and slippage. The 4EAT (Electronic Automatic Transmission) are 80/20 initial split and can go to 60/40 under slippage (with pre-96 models having a 90/10 initial split). Post-2000 models used all sorts of computer sensors to split power for optimum traction using clutch-packs, and also raised atf and clutch fluid pressure to maximum to give a faster reaction time."

This was from a discussion on drifting techniques with an awd system. I was inquiring as to how my system worked specifically, and this was his reply. There was also discussion about fooling the computer using the handbrake where locking it up repeatedly could simulate the gripping wheels if the computer is slow enough. This would make the computer send more power to the rear. This is a bit off topic, but this is where I got my info from...and I leave further discussion to you. (you being everyone else that sees this, lol)

^
should be true... the Porsche Carrera 4S has a viscous center, but an unequal torque bias... so they claim. I'd assume it has something to do with some of the plates being partially locked from the get go? :shrug:

^
should be true... the Porsche Carrera 4S has a viscous center, but an unequal torque bias... so they claim. I'd assume it has something to do with some of the plates being partially locked from the get go? :shrug:

Nope. The torque split of a differential is a function of the gearing and cannot be changed. Obviously, the front and rear differentials have to have a 50/50 split so you can drive it ;) . The center differential can have any geared split the mfgr want - 50/50/ 35/65, whatever.

As mentioned, an open diff always transfers power in that ratio and is governed by the whel (or axle in the case of a center diff) that can accept the least torque.

In order to "shift the torque split", you need a locking diff. Various factors enter into how much torque can be redirected - traction at each wheel, weight distribution, locking diff type and amount of lock possible.

There is a great write up on the web:

http://wallace.as.arizona.edu/~cgroppi/DCCD_FAQ/DCCD_FAQ.html

HTH....