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Originally Posted by dustinr
Well I just received my Subaru Performance mag Version 1.2 and it has a good article about the DCCD and what it does etc. In an earlier post in this thread I said that I thought the car's Auto DCCD transferred torque forward during hard cornering so as to pull the car out of the corner and both Ban Suv's and Sperry said that it did not, that if anything it moved the torque backwards to full unlock 35/65. Not knowing for sure I didn't argue the point. Well in the article they explain that it's full unlocked for braking and that it gradually increase the torque forward thru a corner till it goes full lock 50/50 at corner exit so as to pull the car out of the turn and counter-act any oversteer or wheel-slip. The article even has a cool 3-D graph which shows what the DCCD does given later-G's, throttle position, and corner position. And they reiterate the fact that the '05 has a Yaw-Rate sensor.
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Are we still arguing this?
First, the DCCD *never*, and I mean *never*, changes the torque split of the center diff. It is always 35f/65r (I was under the impression initially that it was 50/50, but 35/65 seems to be the correct split). That split is defined by the physical characteristics of the differential. Unless the DCCD is somehow swapping out internal parts in the diff during a turn, the torque split is fixed.
What the DCCD changes is the locking bias. (Yes, I've explained this before):
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Originally Posted by sperry
The power/torque from the motor is fixed to the wheels (I thought it was 50/50 but I guess it's actually 35/65).
What's varied is the amount of speed difference is necessary before the center diff locks...
When the DCCD is set to full forward mode, the center diff is locked, i.e. the front and rear wheels have to turn at the same speed... so while the rears are getting 65% of the power, they are still locked with the fronts. If you jump on the gas you'll break all 4 loose.
When the DCCD is set to full rear, the center diff requires a much higher speed at the rears before they lock to the fronts. So those rear tires are getting 65% of the power and are free to spin wildly while the fronts grip and pull with their 35% of the power. At full rear, the car behaves more like a RWD car because jumping on the throttle will break the rears loose w/o losing the front traction.
Auto mode uses a lateral G sensor to decide where to set the split. I doubt it will ever go all the way forward, because a locket center diff is only usefull on gravel or snow or in the wet, otherwise the wheels will bind up. My guess is that the diff sits at middle ground normally, but will throw the bias rearward under high latteral G's because having the rears uncoupled lets throttle modulation turn the car... i.e. auto mode helps the car to turn more like a RWD car, but gives you additional traction when driving in a straight line.
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To reitterate, the whole point of the DCCD (in Auto Mode) is to blend the advantages of AWD with the control of RWD. If you've ever driven at the limit, you know that "throttle steering" is very useful to control the car. AWD inherrently counter-acts this ability. The DCCD is designed so that in the corner, the car throttle steers, but in a straight line (i.e. when
exiting the corner) the locking bias of the center diff moves towards 50/50 meaning that rear wheel spin translates into front wheel traction more easily, helping the car to pull out of the corner (yet limiting the car's ability to turn, which is less necessary on exit).
As far as the whole "shifting bias to the rear during cornering"... that *is* what's happening. According to the chart, when you brake and turn in the car has shifted to full rear bias. As you apply throttle for the 1st half of the corner, the bias stays rear-oriented to give the driver a RWD-like throttle steer ability. Once the latteral-G's begin to unload (signifying that the car is through the apex of the turn and is accelerating out to the exit), the bias is ramped forward, adding traction.
Getting back to the yaw sensor. I'm still missing what it's used for that's somehow different from the . It's only ever mentioned in conjunction with the latteral-G sensor. I guess that yaw sensor data combined w/ latteral-Gs might result in more accurate DCCD mdulation, but the addition of a yaw sensor doens't make any fundamental difference in the way the DCCD works. In fact, I would propose that a good driver that's skilled at managing their traction circle would gain no advantage from the addition of a yaw sensor, since the car's yaw and latteral-G's should always match because the driver's not tossing the car around and drifting, thereby making the latteral-G sensor alone sufficient.
I'm much more interested in hearing about the yaw control built into the ABS system. I've seen the tech articles and I understand how it's supposed to work, but is it actually in the '05 STis? I haven't seen any marketing literature that mentions the STi having "traction control" or "yaw control"... you'd think that stuff would get more press.