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Old 12-06-2005, 11:35 PM   #1
volkl23
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Default DCCD setting in winter.. specifically 50:50 Lock

I read somewhere where when it's really slick, to use the DCCD setting into the lock position (50-50 split).

What is gained by locking the differentials compared to setting in Auto under extreme slick winter conditions.
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Old 12-07-2005, 12:05 AM   #2
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Locking the center diff would make the car understeer alot more than in automode. Viseversa , having it locked will make it exit a slippery corner alot faster and make the oversteer more controllable/predictable.
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Old 12-07-2005, 10:25 AM   #3
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I leave it auto. 50:50 helps if you get into an odd situation like where your car will only step sideways in the snow.

In general I think the car feels better on the slick stuff when the ECU has the control. 50:50 breaks everything loose much more easilly. But it might be a driving style thing. When it snows, find a big empty parking lot and play with the settings.
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Old 12-07-2005, 10:46 AM   #4
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try taking a corner (90 deg) in auto and then in dccd.
I found it more controllabe and less sliding in full lock

My tires as all season. YMMV
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Old 12-07-2005, 05:48 PM   #5
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Thanks for the replies.. although you guys have given me a lot of examples where lock would be better, but mechanically speaking... why is it better than auto?

I would assume under slick situations, it would be better to allocate more torque/power to the wheels that have more traction. In a 50:50 split, this can't be done. It just seems counterintuitive to not use auto and let the computer give traction where it's needed as opposed to just a 50:50 lock.

Strangeness!
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Old 12-07-2005, 08:44 PM   #6
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Quote:
Originally Posted by volkl23
I would assume under slick situations, it would be better to allocate more torque/power to the wheels that have more traction. In a 50:50 split, this can't be done.
But in the case of the locked DCCD, this is exactly what's done. 50:50 split is only a theoretical state of lock which takes place when both front and rear axles are loaded equally, a condition which prevents actually locking the DCCD. The diff won't actually begin to lock until the front and rear axles are beginning to rotate at different rates (thus how could they heve been loaded equally?).

Any attempted differential rotation of the two driveshafts while "lock" is selected causes the DCCD to begin to compress its clutch pack, and as this progresses through a very small amount of differential rotation the two driveshafts will become effectively locked together.

Once locked, the DCCD will provide most of the available torque to the loaded axle so long as the total amount of torque applied to the DCCD does not exceed the capacity of its clutch, and the loading of the two axles remains sufficiently unbalanced so as to keep the clutch pack compressed.

I have not found any spec for the maximum clutch torque value, but I suspect (from examining the '04 MY technical update on the DCCD) that it is plenty large enough.
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Old 12-08-2005, 05:51 PM   #7
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Dude, your making it sound way to confusing than it actually is.
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Old 12-08-2005, 06:55 PM   #8
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Better confusing and accurate than more simple and erroneous posts claiming that an STi center is ever 50/50.

- Jtoby
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Old 12-08-2005, 07:14 PM   #9
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Quote:
Originally Posted by Ver.III
Dude, your making it sound way to confusing than it actually is.
I can only interpret your comment in one of two ways:

1) You did not understand some part of my post.
2) You are confident that there is an easier way to explain some part of my post than that which I attempted.

If the former, how can I help clarify it?

If the latter, please elaborate - I would really appreciate hearing a simpler approach.
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Old 12-08-2005, 10:45 PM   #10
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Quote:
Originally Posted by STI_FFY
But in the case of the locked DCCD, this is exactly what's done. 50:50 split is only a theoretical state of lock which takes place when both front and rear axles are loaded equally, a condition which prevents actually locking the DCCD. The diff won't actually begin to lock until the front and rear axles are beginning to rotate at different rates (thus how could they heve been loaded equally?).

Any attempted differential rotation of the two driveshafts while "lock" is selected causes the DCCD to begin to compress its clutch pack, and as this progresses through a very small amount of differential rotation the two driveshafts will become effectively locked together.

Once locked, the DCCD will provide most of the available torque to the loaded axle so long as the total amount of torque applied to the DCCD does not exceed the capacity of its clutch, and the loading of the two axles remains sufficiently unbalanced so as to keep the clutch pack compressed.

I have not found any spec for the maximum clutch torque value, but I suspect (from examining the '04 MY technical update on the DCCD) that it is plenty large enough.
Your reply is very detailed... I'm still trying to digest all that was said. So at the lock setting and you're going really slow in a circle, what exactly is causing that binding sound/action? In that case, is it really locking up the differentials?
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Old 12-08-2005, 11:10 PM   #11
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Quote:
Originally Posted by volkl23
Your reply is very detailed... I'm still trying to digest all that was said. So at the lock setting and you're going really slow in a circle, what exactly is causing that binding sound/action? In that case, is it really locking up the differentials?
My advice would be DONT DO THIS ON DRY PAVEMENT!

When in lock position and driving slowly in a circle the rear wheels travel in a smaller radius than the front, so the front drive shaft must rotate more rapidly than the rear. This will cause the cams to compress the clutch pack and lock the two driveshafts together in an effort to disallow this relative motion.

This will put one hell of a lot of stress on all the components until the weakest link gives. This could be the clutch in the DCCD if the torque wrap in the driveline exceeds the clutches ability to withstand. The overpowering of the clutch could cause a clunking noise as it snaps back to a lower stress position.

Given that no one knows or is saying how much torque that clutch can withstand, it is possible the value might be high enough to actually break a driveline component (like a front or rear axle).
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Old 12-08-2005, 11:13 PM   #12
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Quote:
Originally Posted by volkl23
Your reply is very detailed... I'm still trying to digest all that was said. So at the lock setting and you're going really slow in a circle, what exactly is causing that binding sound/action? In that case, is it really locking up the differentials?
There is a mechanism that has two plates, with balls between them. The plates have cups for the balls, and are being forced together by the solenoid system. This is where the initial slip in the system occurs, and the balls push the plates apart, which exerts pressure on the clutch system that resists additional slip. The ball system can be forced far enough apart that they will jump to the next indent, which is what you are hearing. It is a built in torque release/overload system-and since one plate is constrained by solenoid action, rather than a mechanical system, it _probably_ doesn't do any real damage, but I sure try and avoid it.
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Old 12-08-2005, 11:22 PM   #13
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Quote:
Originally Posted by STI_FFY
My advice would be DONT DO THIS ON DRY PAVEMENT!

When in lock position and driving slowly in a circle the rear wheels travel in a smaller radius than the front, so the front drive shaft must rotate more rapidly than the rear. This will cause the cams to compress the clutch pack and lock the two driveshafts together in an effort to disallow this relative motion.

This will put one hell of a lot of stress on all the components until the weakest link gives. This could be the clutch in the DCCD if the torque wrap in the driveline exceeds the clutches ability to withstand. The overpowering of the clutch could cause a clunking noise as it snaps back to a lower stress position.

Given that no one knows or is saying how much torque that clutch can withstand, it is possible the value might be high enough to actually break a driveline component (like a front or rear axle).
See, my hunch is that in this situation what happens is the ball/plate system just 'pops' to the next detent on the plate. This type of system is used very often as a torque limiting system in industrial apps, and in this case I'd think enough differential torque might overload the solenoid pushing the ball/plate assembly together.
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Old 12-08-2005, 11:44 PM   #14
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Quote:
Originally Posted by gregsachs
See, my hunch is that in this situation what happens is the ball/plate system just 'pops' to the next detent on the plate. This type of system is used very often as a torque limiting system in industrial apps, and in this case I'd think enough differential torque might overload the solenoid pushing the ball/plate assembly together.
That actually makes a lot of sense as a fail safe mechanism, and I'm embarrassed to say that it didn't occur to me. I had been thinking that there was no mechanism to retain the balls if they ever climbed completely out of those ramps, but - duh - referring to t photos again the planet carrier cams are nestled within the pilot clutch plates - so there's nowhere for the balls to wander off to.

Still the amount of torque required to cause this "popping" action might be very high, and repeatedly doing so would wear the clutches (they have to slip under maximum pressure to permit this motion) and the cam ramps and balls. ...and I'd think there's no worse feeling than the knowledge that your balls are worn out.

I'm almost old enough to know this feeling.

Thanks, Greg for putting another piece of this puzzle in place for me.
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Old 12-10-2005, 04:44 PM   #15
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I was stuck in the snow last year no matter at what mode until i clear the snow.

Anyway, I found two things. Despite slipping, the auto mode just never go to 50/50. When I am stuck and setting at auto, I felt that the rear is spinning muhc faster than the front. When I set to 50/50, I now felt that the front is spinning also.

So the whole idea about auto mode setting 50/50 when the front is slipping is questionable based on my experience.

- Charles
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Old 12-10-2005, 05:09 PM   #16
jtmcinder
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Given that you have a front-heavy car, even if it were 50/50 (and incompletely locked), you would not get equal wheel-spin.

- Jtoby
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Old 12-10-2005, 05:47 PM   #17
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Quote:
Originally Posted by chaddeus
I was stuck in the snow last year no matter at what mode until i clear the snow.

Anyway, I found two things. Despite slipping, the auto mode just never go to 50/50. When I am stuck and setting at auto, I felt that the rear is spinning muhc faster than the front. When I set to 50/50, I now felt that the front is spinning also.

So the whole idea about auto mode setting 50/50 when the front is slipping is questionable based on my experience.

- Charles
It is certainly possible to get stuck in the snow. There are lots of ways to do it. I would definitely use the "lock" setting if this happened. That way the lion's share of the torque will always go to the axle with the most traction. If both axles have no traction, then it sucks to be you.
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