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AWD burnouts FTW.
 

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burnout? uhm.....donut? :confused: and uh it doesnt really take alot of power to do a donut ;)

cool nevertheless, were they on rcomps? :lol:
 

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it doesnt take alot of power to do donuts... u can do it with stock STI, because they have DCCD controller which u can put most of the power to the rear wheels
 

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Discussion Starter #5
lol, I guess your right, that ain't no burnout!
 

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bonecracker said:
it doesnt take alot of power to do donuts... u can do it with stock STI, because they have DCCD controller which u can put most of the power to the rear wheels
^ just to clarify, this is BAD INFO.... not true... the DCCD adjusts the lock of the center dif, not the transfer of power...

u basically can either make your car AWD or 4WD like a truck for offroad (closer to lock position)

Thats why when you try and turn the car slowly with the DCCD set to one of the positions close to lock you can feel the car bind up. Its like taking a 4wd truck, putting it in 4wd and making a turn on pavement, it starts to jump becuase both wheels are turning at the same rate... aka 4wd. On an awd car, it allows the outside wheel to slip to keep the car from jumping...

do a little more studying on the DCCD my friend ;)
 

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all4bspinnin said:
^ just to clarify, this is BAD INFO.... not true... the DCCD adjusts the lock of the center dif, not the transfer of power...

u basically can either make your car AWD or 4WD like a truck for offroad (closer to lock position)

Thats why when you try and turn the car slowly with the DCCD set to one of the positions close to lock you can feel the car bind up. Its like taking a 4wd truck, putting it in 4wd and making a turn on pavement, it starts to jump becuase both wheels are turning at the same rate... aka 4wd. On an awd car, it allows the outside wheel to slip to keep the car from jumping...

do a little more studying on the DCCD my friend ;)
That was a nice explanation in layman's terms. Thx.:tup:
 

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This document is intended to show how the Subaru STi Driver Controlled Center Differential (DCCD) works. This will be a three-part document. Part 1 consists of a theoretical view of the DCCD as a simple planetary differential with a simple clutch, and explains how torque gets from the engine to the front and rear wheels. Part 2 consists of a description of the actual operation of the DCCD, referencing the Subaru tech manuals that explain the nitty-gritty mechanical details of the DCCD, and Part 3 is practical; how does auto mode work and suggests when it might be desirable to use manual mode. The DCCD is a complicated mechanism, and it is only one part of an AWD car's drivetrain, which is also very complex. This complexity can get in the way of understanding the basic physics of how the engine makes the car move. This text will strive to simplify as much as possible the mechanics of the car to aid in understanding, without ignoring any important concepts in the process. Much of the confusion regarding the DCCD operation has come from sales literature that (in the interest of not confusing potential buyers) oversimplifies the operation of the DCCD, and LSDs in general.... i found this which proves u wrong so whos right? it says here that it transfers the TORQUE to the front or rear wheels... so i think u shud get ur facts straight
 

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this is the rest of it have fun...

The information in this article comes from several sources, including Subaru published materials, websites and various individuals. A list of links and contributors including NASIOC members is provided at the end of this document. A great deal of information is from the website howstuffworks.com, the perfect name for an excellent site. Portions of this text have been lifted directly from these sources sometimes without proper credit, for which apologies are made in advance.

Part 1

Before discussing the DCCD, it is necessary to define torque, which will be mentioned over and over in this document. Torque has been discussed a lot on NASIOC, but the definition is worth reiterating since "moving torque around" is what the DCCD is designed to do, and we need to be clear on just what it is that is being moved. From howstuffworks.com comes this definition of torque:

Torque is a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. The unit of torque we will use is the lb-ft. As you can see, the lb-ft contains a unit of force (the pound) and of distance (the foot), which is what torque is: force, applied at a distance, in a manner to rotate or twist something. The engine in a car generates this twisting force, and the drivetrain - including the transmission, center, front and rear differentials, and axles - moves, multiplies and divides this force to get it to the wheels. By twisting the wheels, the car forces the tires' contact patches to push against the ground. The ground pushes back, and the car moves.


The DCCD is a just a special type of limited slip differential (LSD), so to understand how it works, one needs to first understand how basic differentials work, and then move on to what the "limited slip" part does, and then finally examine the special feature of the DCCD, the fact that the amount of "slip" in the "limited slip" portion can be adjusted, either by onboard computer or by the driver.


The Open Differential

The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed. The differential has a special property in that the torque from the engine is divided between the output shafts in a fixed ratio, regardless of the rotational speeds of the output shafts. In the conventional differentials found in the rear and front of cars, the torque split ratio is 50:50. The DCCD is a center differential and happens to be a planetary type. In this design it is easy to select the ratio in which the engine torque is divided by varying the different sizes of the gears involved. In the STi, the Subaru designers selected the gear sizes in the DCCD to split the input torque in a ratio of 65% Rear: 35 % Front. The WRX and Mitsubishi EVO use 50:50, the BMW 3-Series AWD models and the Porsche Cayenne use 62R:38F. The term "Open Differential" is used to describe this kind of differential, where there is no limited slip component (or where that component is deactivated). Figure 2 shows this case for the STi.

[Note: It is difficult to show why a differential has this property, so it will remain beyond the scope of this article.]



Open Differential Benefit:

The benefit of this property (the one in bold print) is that it helps when the car goes through a turn. In the case of a turn, the front and rear driveshafts must be allowed to rotate at different speeds, because the turn causes the rear wheels to track inside the front wheels, and so the rear driveshaft will turn at a slower speed than the front driveshaft. The center differential allows this to happen while still keeping the same torque split between front and rear shafts.

Open Differential Drawback:

To understand what the limitation of an open differential is, one needs to understand some things about traction. Isaac Newton's third law of motion says "For every action there is an equal and opposite reaction", or to paraphrase: "You can't push against something that won't push back". This concept becomes important when talking about traction. Again, borrowing from howstuffworks.com:

“There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; but in a low traction situation, such as when driving on ice, the amount of torque transferred to the ground is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If more throttle is applied after the wheels start to slip, the wheels will just spin faster.” It takes very little torque to rev the engine and drivetrain without the load of the pavement pushing back, so the engine produces very little additional torque before it gets to redline. It’s not possible to push against something that won't push back. So with all four wheels on ice, the amount of torque (or push) the engine can produce and transfer to the ground is severely limited. This is because the only things that are pushing back are the traction the ice gives the car and the inertia and friction of the engine and drivetrain parts, which are both negligible.

Now what happens if just the front wheels of the STi are on ice? Here is the problem with the open differential. The differential must preserve the 65:35 ratio. It is understood that the front wheels can accept very little torque without breaking traction. So the torque the engine puts out becomes limited by that fact. The total engine torque being applied is 2.8 times the amount of torque that goes to the front wheels. (1/0.35) If the maximum torque the front wheels can make use of is a very small number then the max the engine can usefully supply is 2.8* a very small number which is still very small. 65% of this total torque goes to the rear wheels, but it is probably not enough to move the car.

Applying some numbers to that example (for simplicity in all examples assume a transmission gear ratio of 1:1):


Open differential and front wheels on ice: Assume the torque on the front driveshaft that will cause front wheels to spin is 10 ft-lbs. So the engine can put out a maximum useful torque of 10*2.8=28 ft-lbs, and when it does, the rear driveshaft sees a torque of 18 ft-lbs. 18 ft-lbs may not be enough to turn the rear wheels, but that does not matter, the engine (through the differential) is still twisting the rear shaft whether or not it is actually turning. Remember the open differential allows the shafts to move at different speeds and still keeps the same torque split. In this case, no matter how much the engine is revved, the torque it produces is limited to 28ft-lbs (to be precise we must add the amount it takes to accelerate the engine and front wheels when we rev it, and overcome internal frictions etc.).

Here is another case of the open differential that illustrates the "twisting" vs "turning."

Open differential with the handbrake pulled: Here the rear wheels are prevented from turning by the rear brake calipers, but the car is still capable of motion. Assume that the rear tires are on gravel and front tires are on pavement. If the torque required to turn the front wheels while dragging the rear wheels is 100 ft-lbs, then the engine is required to put out 2.8*100 = 280 ft-lbs. The rest of the torque (180ft-lbs) goes to twisting the rear driveshaft. This torque ends up "pushing" against the brake pads, which must "push back" to keep the rear wheels from turning.


Note that in both these cases all the engine power is going to the front wheels, but the torque is still split 65:35. (And also note that the law of conservation of energy dictates that in both of these cases the front driveshaft must be spinning 2.8x faster than the engine, and the design of the center diff ensures that it does.)

Now in the second case, the open diff is a benefit. If one wishes to do a "handbrake turn" and still move the car with the front wheels, this is possible. (This is why the DCCD unlocks when the handbrake is pulled.) But in the first case, there is a problem, because the fact that the front wheels are on ice limits the torque output of the motor and prevents the rear wheels from getting the car off the ice.
 

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The DCCD does transfer power. In full rear it is split 35F 65R in the 04/05s. They changed it later on. That means it wont compensate for when there is slippage. In auto, when it detects slippage it will transfer power where it is needed.
 

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520hpSTi said:
The DCCD does transfer power. In full rear it is split 35F 65R in the 04/05s. They changed it later on. That means it wont compensate for when there is slippage. In auto, when it detects slippage it will transfer power where it is needed.
but thats not manually controlled

bonecracker said:
Open Differential Benefit:

The benefit of this property (the one in bold print) is that it helps when the car goes through a turn. In the case of a turn, the front and rear driveshafts must be allowed to rotate at different speeds, because the turn causes the rear wheels to track inside the front wheels, and so the rear driveshaft will turn at a slower speed than the front driveshaft. The center differential allows this to happen while still keeping the same torque split between front and rear shafts.
^taken from your post... see i am right cheif ;)
 

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all4bspinnin said:
but thats not manually controlled


^taken from your post... see i am right cheif ;)
It is manually controlled because you are manually putting it into full rear aka 35/65.
 
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