Thread: Dccd Faq
View Single Post
Old 06-08-2007, 04:09 PM   #6
Scooby Guru
Member#: 7327
Join Date: Jun 2001
Chapter/Region: NWIC
Location: Seattle, WA
2008 Mazdaspeed3
2006 Wrangler Sport


here are some of the posts I and another guru have made on the topic in the past:

Originally Posted by nhluhr View Post
OK i'm going to say this once and only once:

DCCD varies the slip limiting, which in turn varies the amount of torque that can be redistributed via the center diff. The native torque bias of the planetary gearset in the diff is 35f/65r (04 and 05) and 41f/59r (06+), and the only time you can really be sure exactly what the torque bias is would be when the DCCD is set to full OPEN, meaning no slip limiting and no torque transfer (note: the 06+ has an additional mechanical LSD in the center diff that cannot be turned off).

Dialing in more DCCD lock does not direectly 'vary' the torque bias. What it does is prevent the front and rear from going different speeds and if there IS a speed difference, torque is transferred through the slip plates to the slow side to bring it up to the same speed as the faster side. The amount of torque transferred is dependent on two things:
-the difference in traction(and speed) between front and rear
-the amount of lock dialed in with the DCCD dial (or the amount commanded by the automatic mode).

It is possible, under extreme conditions to have all the torque transferred to the front or the rear, but this does NOT mean what most might take it as. It means that if one side of the car has 0 traction, the other side of the car gets ALL the torque transferred to it. This is exactly how you want AWD to work and represents the state of the art in performance AWD systems.

Since this question has been rehashed so many times and almost always gets answered incorrectly (as above), I am ending the thread here. For more information on this topic, check:
Originally Posted by nhluhr View Post
1) The DCCD manual control ONLY adjust the lockup percentage of the center diff. Contrary to ALL of subaru's marketing and popular belief, it does NOT directly adjust power or torque ratio. The transfer of torque through a partially or fully locked differential happens only when there is wheel slippage and even then it is highly variable based on slip ratio and lockup ratio. MOST drivers will benefit most from leaving it in the AUTO mode, which takes advantage of computer control based on yaw, g force, brake, and throttle inputs. In special conditions such as gravel or snow, you may find that putting it to MANUAL with a high lockup ratio will provide you with a more stable and predictable drive.
Originally Posted by nhluhr View Post
ok unabomber, i'm gonna ramble for a while here....

If you're trying to get somebody to understand why a particular diff is better or how a diff works, it is important to first define some of the terms that are tossed around. First and foremost, it is important to realize that the marketing snobs who write the brochures for Subaru are absolutely clueless when it comes to this topic and their claims are complete bull**** as a result.

The first and biggest myth is that the STI DCCD has a user-controllable torque bias. This is NOT the case. Torque is the twisting force applied on a rotating shaft. To help debunk this myth, lets introduce perhaps the single most useful analogy. Imagine a 4-way lugnut wrench. There is a side facing you, a side on the lugnut, and two more sides perpendicular to the first two that you grab onto with your hands to turn the wrench. Now imagine the torque wrench is your center diff. The end that's on the lugnut is the end of the car with traction and the end that is facing you is the end of the car that's on ice or has 0 traction. The two ends you hold with your hands represent the torque input from the engine. This would be a good analogy of a fully locked differential. Subaru would have us believe that this locked differential provides a perfect 50/50 torque split. If you are putting 80 ft-lbs into it and that is all being resisted by the lugnut, then how much torque is being resisted by the end of the wrench that's facing you? 0. In this case the torque split is 100/0. As you can see, the moment you lock a differential, the torque split becomes 100***37; traction dependent and very far from the set 50/50 that is touted in magazines or advetisments.

Ok enough of the analogies, how about the real thing... The 04/05 STI center diff in reality has gearing that causes the front to receive 35% of the torque and the rear to receive 65%. This gearing is the native torque split of the diff and cannot be changed with any electronic dial. What you CAN change is the lockup ratio of the diff which will have the effect of transferring torque from the wheel with less traction to the wheel with more traction. The amount of lockup ratio (what DCCD really controls) will affect the amount of total torque transfer. The more lockup ratio you can dial in, the more total torque can be transferred in a slipping condition.

Important to note that if no slippage is occurring, no torque is transferred and the bias remains, as always, at 35/65. What the STI's sales literature should say is that the center diff can go from 35/65 to wildly variable at full lockup. This doesn't look nearly as clean to folks who are reading because most folks won't separate the concept of torque and the wheel slipping. They think that 50/50 torque means the wheels are all moving at the same speed. Let me ask you, if one wheel is on pavement and receiving 50 ft-lbs of torque, and the other wheel is on ice and also receiving 50 ft-lbs of torque, which one is gonna spin first?

eh... i'm rambling. I'll stop for now.
Originally Posted by nhluhr View Post
You have to separate "torque" and "power" from each other in your head. They are NOT the same. You can apply torque without applying power.

Differentials don't really worry about distributing "power". They worry about distributing torque. In the 04/05 Center Diff, they native torque split (which is governed by gearing inside the diff), is 35f/65r. This never changes. The DCCD has no ability whatsoever to directly control the torque split. What the DCCD does do directly is control the lockup of the pilot clutch. Again, this does NOT change the torque ratio directly, despite what the subaru marketing literature tells you. When you read the service manual and technicians reference bulletins, there is no mention of "50/50". That is pure marketing hype. The truth of it, nay, the truth of ANY limited slip differential is that the torque split becomes wildly variable, subject to ANY ratio between 0/100 and 100/0.

Torque can only be applied if there is something to resist it. If a tire is in the air, the only torque that can be applied is a brief couple lb-ft that it takes to spin the wheel. With an open differential which makes sure the torque split is ALWAYS the same regardless of wheelspeed differences, the fronts being in the air and only able to make 5lb-ft would mean the rears can only get 5*65/35 = 9.3lb-ft. This might not be enough to move the car very quickly, if at all. Result: front wheel spins fast, rear wheel barely moves.

Now if you have a limited slip differential, the gearing is still splitting the torque 35f/65r BUT now you also have a clutchpack that will re-transfer torque when there is a wheelspeed difference. In this case, the amount of torque that can be transferred is limited only by the lockup and the traction of the other wheels. If we have our fictional case of the car with the front wheel in the air only able to take 5lb-ft, and the rear wheel on pavement able to take 300lb-ft, then the rear wheel will get 300lb-ft and the front will get 5. Now, what's that torque split? It's a damn sight far away from 50/50! Result: rear wheels push car very fast, front wheels accelerate at the same speed as the rears.
Originally Posted by STI_FFY View Post
This subject keeps coming up and the discussion continues to show that this is one of the most misunderstood technologies in the STi.

1) When the dccd is open the torque split front/rear is dictated by the planetary gear ratio in the DCCD. So as stated in the 2006 documentation this would be 41/59 (as opposed to 35/65 in the 2004).

2) When the dccd is truly "locked", the torque split is dictated only by the maximum traction of the main clutch in the dccd. To my knowledge no one has ever measured this nor does anyone know the theoretical limitation. This clutch, when fully engaged, locks the elements of the planetary gearset to one another - effectively eliminating their function.

The open state is easily achieved by selecting it.

The truly "locked" state is not so easily achieved. It requires not only selecting "lock" but then there must be attempted differential rotation of the front/rear drive shafts by a small amount (something less than ~1/6 of a relative revolution).

When "locked" as above, and assuming that the main clutch in the DCCD can withstand it, the torque split can be thought of as 50/50, but in fact nearly 100% of the engine's available torque will be effectively routed to the axle with traction if the other axle has zero traction.
Originally Posted by nhluhr
Originally Posted by Master2192
Now, with the open differentials, Torque is always split 50/50 IF traction is equal. If a tire loses traction power will go out the direction with least resistance.
This is partially correct, but mostly just misleading. Open differentials ALWAYS apply the same torque split to both sides, regardless of traction. The correct part of the quote is that more power goes to the slipping wheel, but that is only because power is the product of torque and angular velocity and if one wheel is spinning and the other is not, then one has higher power, but that doesn't really mean anything. The force of torque is what moves the vehicle and both sides of an open differential are seeing the same torque. If the front wheels are on ice and can only handle 2lb-ft of torque before slipping, while the rear wheels are on pavement and can handle as much torque as the engine can supply, the front wheels get 2lb-ft and the rear wheels get 2lb-ft. Now, that's a total of 4 lbft. If that's not enough to move the car, the car doesn't move. This is why sometimes you hear people suggest holding the brake a little bit when trying to get unstuck - because this resists the slipping and allows the differential to send higher amounts of torque to the wheels with traction.

LSDs keep torque split at 50/50, when traction is not equal it will try to split the torque towards the tire with the most traction.
This is incorrect. The whole point of a limited slip differential is to allow the transfer of torque to the wheels with more traction. If the torque split is always 50/50, you are not transferring it. With a limited slip differential, the amount of torque that can be trasferred is limited only by the ability of the LSD mechanism to "lock up".

In the case of the viscous coupling on a WRX, the center diff is just a 50/50 open differential with a viscous coupling backing it up to provide torque transfer. Since it's a viscous coupling, the amount of torque it can transfer is limited and is also a function of axle speed difference (i.e. if both front and rear are going the same speed, 0 torque is transferred from the normal 50/50 distribution). In the case of the STi, the center diff is a 35f/65r torque bias plus an electro-hydrailically actuated clutchpack that is capable of providing a large amount of lockup.

Now, what is the actual torque split with LSD engaged? That is a very complex question but the short answer is that it can vary basically from 0:100 all the way to 100:0. Lets look at a couple examples:

*Locked differential and front wheels on ice: The torque on the front driveshaft that will cause the wheels to spin is 10ft-lbs. That is the max amount of torque that can be applied to the front driveshaft. Why? Because the ice will only "push back" with 10 ft-lbs, so the front wheels can only "push" with 10 ft-lbs. This is very important to understand. You can't push against something if it won't push back. But since the driveshafts are locked together, the engine is free to apply more than 10 ft-lbs to its connection at front driveshaft. In fact, think of it as just one large driveshaft, with the engine twisting the shaft at two places (side by side), one with 35% of its effort, the other 65%. If the engine applies 100 ft-lbs, what happens? 35 ft-lbs on the "front twist", 65 on the "back twist", but remember the front axle can only accept 10 ft-lbs. If the rear can take 90 ft-lbs before slip, then rest of the engine torque (90 ft-lbs) goes to the rear axle, which is (let us assume) enough to move the car. In this case the torque split is 10%F/90%R

*Locked differential and front wheels on ice and rear wheels on tarmac: Say in this case it will take 3 ft-lbs to spin the front wheels and 200 ft-lbs to spin the rear wheels, and these numbers take into account both the available traction and load distribution. Initially we will view the locked diff as creating one long driveshaft. Assume that it takes 100 ft-lbs to slip the clutch in the DCCD at 100% full lock. What this means is the maximum torque the engine can twist the front driveshaft with is 3 ft-lbs plus up to a max of 100 ft-lbs. Any more torque on the front driveshaft will cause the clutch to slip and the front wheels to spin. Working these numbers out shows the following: If the engine is supplying 294 ft-lbs of torque, then the front driveshaft "twist" is 35% or 103 ft-lbs, and the rear is 65% or 191 ft-lbs. At this point any more torque applied by the engine will overcome both the ice friction and the limited slip clutch friction, and the front tires will start to spin, and any more effort to increase engine torque after that will just cause the front driveshaft to spin faster and rev the engine. The split at the axles at that point will be 3 ft-lbs F/291 ft-lbs R, a torque split of roughly 1f:99r

So as you see, the actual torque split is going to be all over the place based on wheel speed differences, torque applied, traction at the wheels, etc.
And here are some clips showing how the system functions:



* Registered users of the site do not see these ads.
nhluhr is offline   Reply With Quote