[edit]

 

I'll correct your last section. The first scenario is correct.

locked ACD = supressed open diff function = in equivalent like having no diff
torque = wherever the grip is, i.e., this is now totally variable. Your car could be FWD or RWD or ANY percentage combination of the two depending on grip conditions at each end of the car. The only sure thing is that no matter what the front and rear wheels will be spinning at the same speed

Very simplistic explanation and we could add more variables but that is the gist.

You guys seem to be confused by the fact that all four wheels spinning at the same rate because of a locked diff does NOT mean torque is getting split equally to all 4 corners of the car.

 

so in the scenario of the locked ACD (effectively suppressed diff) what is responsible for distributing the torque F & R?

In the case of a IX:
FrontRight and FL is done by the helical LSD
RR and RL is done by the AYC

 

The Laws of Mechanics. Locked diff = torque goes where the grip is.

You ask about front/rear then cite an example about torque split left to right. I think you need to understand how a basic differential works before you add two more differentials into the equation and want to know where torque goes in an AWD car. Focus on a single diff first.

Maybe EVOlutionary can explain this better. I am running out of explanations that don't involve a high school physics book.

 

The Active Center Differential.

In the VIII it was the Viscous Coupling Unit attached to the center differential.

 

Are there 2 variables here?? that's where I am confused.

I understand how a simple diff works. I've looked it up and re-read it 4 times to make sure I wasnt missing something.


Shadowpriest - can you explain why if the ACD is locked (effectively NO diff - like a rigid connected axle) why the torque can go front or back?? Isn't it a rigid pole and how can the torque be spread if it's all rigid?

"Weld the center diff together so you have a solid axle between the front and rear, ie, there is no center diff OR look at it like the clutch packs have locked together (both scenarios are the same thing). Where is the torque going to go if the front is on tarmac and the rear is on ice? All of it will go to the front, because THAT IS WHERE THE GRIP IS. If the situation is reversed, all the torque will go to the rear. While the front wheels and the rear wheels are spinning at the same speed in this scenario and it "LOOKS" like the end on ice is doing something, really the end with the grip is doing all the work. There is no computer moving the torque around. It's all mechanics/physics. Get it?"

 

ok i think i worked it out.... I think the above explanation was misleading me.


The ACD acts on the limited slip action of the centre diff.

When it's locked, the limited slip action is in effect and therefore when the grip is less at say the front and more at the rear, the limited slip action of the centre diff allows less torque to go to the front and more to the rear (like a LSD).
This also means that the front and rear wheels will spin at the same speed.


When the ACD is free, it allows the front and rear wheels to spin at different speeds (eg for cornering) and there is no limited slip action so as a consequence the torque split goes back to it's 'default' split of 50:50.

Is that right?

 

The reason torque can go front to back at varying torque percentages depending on grip conditions at each end of the car with a welded center diff is physics. I've been simplifying the scenario with a welded diff instead of using a limited slip diff in the example because a limited slip diff action is partially an open diff, partially not and you are having trouble understanding how a welded diff (or a solid axle - same thing) works which is the simplest scenario here.

Pretend there are NO differentials in the car, no center diff, no front, no rear diff. All wheels are tied together by a solid axle. They will all ALWAYS turn at the same speed because they are being driven by the same axle. Now, place the car in a situation where one tire is on tarmac and the other 3 are on ice. Where will the torque go? 100% of it will go to the wheel on tarmac. Why? Because that's where the grip is. There is no magic there. It's just the laws of mechanics.

When you push against a wall it pushes back exactly the same amount you are pushing it or the wall would fall over. That's simple physics. When the tires try to push on the ice they can't because there is no grip so there is no push back to create forward motion and torque. That is why no torque is applied at the 3 wheels on ice, there is no equal and opposite reaction. The wheel on tarmac is tied to the source of torque (the engine) in a direct coupling (solid axle) JUST like the other 3 wheels, but the other 3 wheels can't utilize it so the one that can (the one with equal and opposite reaction) sees all the torque.

 

aaaah ok that makes sense now

 

Tarmac is the setting to be used in dry, paved conditions. In this setting, the ACD will almost immediately allow the center differential to go into a free state upon detecting a steering input. Additionally, this mode provides the strongest limited-slip clamping force of the three modes. (7) (12)

So does this mean its now rear wheel drive or front wheel drive when its in a free state?

when you yank the ebrake it disconets the acd now I want to swing the *** around a turn will this throw the power to the front or rear wheels?

 

No.

 

As i was sleeping last night i got it i think.
the center diff when locked its like a 4x4 the front and rear wheels are locked togeather. unlocked they spin free of each other. so in effect if i have the car up in the air anf if the diff was unlocked i could stop the front or rear wheels from spinning.
Kind of like having a regular axel on a normal car if the set of wheels are spinning in the air i can go to each side of the car and stop one wheel independently of each other.

I had a traxxas Revo i installed a center diff in, when i was in loose dirt and i gassed it it threw power to which ever set of wheels would spinn faster as it took off the front wheels would spinn out and it wouldnt pop wheelies anymore but the thing would turn like none other.

 

you got it.

 

great write-up! Ive always wanted to understand the ACD more in depth!

 

Just got my car back from the dealer and all 3 lights are still on, service was closed so couldn't complain, but the paper work said improper wheel code....
How am I supposed to see which mode I am in if all 3 lights are on...Does that make sense a wheel code error could cause them... I have my winter tires on, they have been used for 3 years no with no problem

ankur