From the EVO drivetrain for dummies thread:
https://www.evolutionm.net/forums/sh...d.php?t=149737

So it seems we are all confused...

 

The correct answer here is "depending on conditions, yes, but not necessarily" .

 

Don't necessarily believe everything you read:

Incorrect. 50/50 torque split occurs when the ACD is open/disengaged.


Correct. I believe this is why the car exhibits FWD tendancies, just like my '99 Dodge Ram 2x4 exhibits "left wheel drive" tendancies. Maybe it could be called a 49:51 split ??


Incorrect. If you have the front wheels on jack stands and the rear wheels on dyno rollers, when the ACD is engaged 100% of the torque is tranmitted to the ground by the rear wheels up to the mechanical limits of the ACD clamping force.
The opposite is also true. . . so the ACD equipped center diff in the EVO can put down torque to the ground anywhere from 100:0 to 50:50 to 0:100 (up to the mechanical limits of the clamping force of the ACD clutches)

EVOlutionary

 

This is turning out to be a very interesting discussion... perhaps one of the mods could transfer it to the tech/drivetrain section?

Regarding this last paragraph of yours; I find this statement very very intriguing. Do you happen to have a link to any whitepapers discussing this? It seems everything I've read (here on the forums, and on the web) contradict what you've just written. At this point, I think I'm more confused about this subject than when I first started reading.

How does Subaru's DCCD function in comparison? Is it fixed at 41:59, whereas your claim leads me to believe that 100 percent of the torque could be either shifted front or back depending on situation?

P.S. Some really interesting reading here. Haven't gone through all of it yet, and not quite sure how accurate the material is, but entertaining nonetheless: http://www.autozine.org/technical_sc...action_4wd.htm

 

I don't know if I'm right from a technical standpoint, but the way I work it out with logical reasoning that is the way it seems to work.

 

Well hopefully more people can chime in, as this is a topic I'd really like to clear up and understand. As it stands, it seems most here aren't 100% sure how the ACD handles torque bias, and I believe it would be beneficial to the EVO community to get a definitive answer.

 

I second Evolutionary... from a logical reasoning standpoint.

 

First off DCCD Reference:
http://wallace.as.arizona.edu/~cgrop.../DCCD_FAQ.html

How i understand it:
Now the gearing of subaru's DCCD system is 35/65 front/back. This means while open if the engine was producing 100ft/lbs of torque 35lbs obviously would go to the front wheels and 65lbs would end up in the rear.

Now the DCCD system uses a LSD to control the transfer of torque from the rear to back using complex adjustable clutches. Its Important to understand that LSD can't transfer torque without a speed difference. Furthermore the differential will only be able to transfer the torque split between the two shafts even at 100% lock.

If you were to say lock the DCCD at 100% you could get a maximum torque split of 20/80 , thats because its transferring the available 15% difference to the rear or in the case of the rear slipping a max of 50/50. Of course the Evo doesn't work quite in this manner

The Evo:
If the front wheels are spinning faster than the rear wheels, then the ACD begins to lock the clutches up. If the amount of force exerted by the front wheels is less than the amount the clutches resist, then the slip is stopped and both front and rear spin at the same speed. However, if the force the clutches can resist is less than the force exerted by the slipping set of wheels, then the wheels will slip, but only by how much remaining force the slipping wheels overpowered the clutches with.

This is of course exactly as evolutionary put it, I believe people are just having trouble with how an LSD really works and thus I'm going to post a link to howstuffworks.com

http://auto.howstuffworks.com/differential4.htm

Edit:
Hope that clears up some of the confusion here

 

^^^ OK, so it's starting to make sense now, but still one thing about ACD seems iffy to me. As EVOlution stated in his previous post, if the front end of the car for example where to be on jackstands, and the rear wheels where on dyno rollers and throttle was applied, then theoretically the ACD would transfer 100% of the torque to the rear wheels? I would then assume that the front wheels would be stationary, or at the very least spinning a lot slower than the rear.

If you were to reverse the position of the jack stands, and the front wheels are now on the rollers whereas the rear are elevated, then the ACD would transfer 100% of the power to the front wheels. Then finally, once both wheels are on the surface, depending on slippage the ACD can vary the amount of torque sent to each axle up to a 50/50 ratio.

I suppose then DCCD is different in function, as the torque split is physically limited to 35:65 or 49:51 (depending on MY), and the driver can lock it to a full 50:50 split by use of the interior control. The DCCD would not variably split torque 100-0-100 as in the case of Mitsubishi's ACD. I would assume then that the ACD system is superior in design to Subaru's DCCD offering?

Am I correct in my understanding?

 

No thats not how it works.

The lock up of the diff would create a solid axle forcing both axles to move at the same speed. This would happen at loads that do not exceed the clamping force of the center differential once you mechanically exceed the pressure plates the axles would then spin at different rates.

I think thats how people get confused about this torque bias thing.

The DCCD has a mechanical advantage to transfering the torque to the rear wheels so in that primary case the DCCD's system would be advantageous. While we sit at the mercy of how much are diffs locking plates can take . Simply changing our CD's gearing to the 35/65 system would essentially put us on level playing fields.

 

I think a major source of confusion comes from people trying to think of the system as "sending" torque to either the front or the rear wheels. If you look at it as an issue of relative wheel speed and the center diff is trying to keep both the front and rear wheels spinning at the same speed it gets a little easier. If the front wheels have 0 traction, then the center differential will attempt to lock and all of the torque will go to the rear wheels. If the rear wheels have 0 traction, the opposite will happen. The torque split is essentially controlled by available traction. The only limit to this is the amount of torque the center differential can support, and beyond that limit the differential slips causing the wheels with limited traction to spin.

-Paul

 

Thank you everyone for helping me understand this. I guess the folks over at Car and Driver do have it wrong then by claiming a front bias. It's misleading on their part. It honestly lead me to believe at first that the EVO was essentially a FWD vehicle, then only sending power to the rear when needed. I have noticed a little bit of torque steer, but not what I would normally associate with a true FWD vehicle. Good to know.

Regards
James

 

Just FYI, I had my car set up last year with a big front splitter and stock rear wing. It made so much downforce and traction in the front that in corners my car acted more like a RWD car. When accellerating through a corner the rear would lose traction first and oversteer, rather push like most EVOs.

While differentials and mechanicals can make a car handle a certain way, so can suspension tuning. . . just something else to keep in mind. . .