voidhawk

iTrader: (20)

Here's something interesting: I found this motec piece in a magazine ad. Now I thought the ACD only controls the duration when the center diff stays locked or open while the torque split is 50/50 or 100/0. How do they control torque split then? Slipping the clutch pack?? I checked the motec website but couldn't find any more info.

WWEVOIX

iTrader: (4)

yeah i saw a simliar ad in modified mag this month too.. i'm searching for some more info on it.. anyone can help?

SHOMSPEED

Okay I've notice alot of confusion surrounding the ACD controller from Motec.
So I'll give you my understanding of it, to see if it makes any sense to some of the members here. The ACD controller from my understanding, manipulates how much the center diff locks up, (no sh_t sherlock...lol). Before you had to use three pre-determined settings, snow,gravel,tarmac. So now, you can dial in your own personal setting. This is more geared towards to rally/track racers I believe. But here is the great thing about it, if you would like to have your evo "power oversteer" this is your ticket. Reason being, having the center diff fully open with ACD controller ( regardless of the EVO'S mechanically fixed torque split of 50:50) you will be able to push it into oversteer with enough power, or high enough cornering speeds. Because, you must remember, a conventional diff, will always send power to the wheel/pair of wheels, with the less amount of grip. Which in this case, would be your rear wheels with its 1.5 LSD out back. As the rear wheels would be locked together under acceleration with the 1.5 LSD, and would begin to slip at this point, as they have to turn the same speed (especially when going around a turn). But usually in the pass, this would'nt happen, because the center diff would be locked up, before you got a chance to overwelm the rear tires.
This is really very similar to the earlier JDM WRX 4WD system,(I have'nt really been following up on the new ones) were they came with a planetary center diff which by design, sends 65% to the rear. And this would be in its fully open position.
The driver would then be able to adjust from inside the car, the amount of locking action of the center diff. And I believe, 50:50 was the fully locked position, and was advised to be only used, in low traction situations like,snow,gravel, etc. To prevent the Drivetrain from binding up, as the two pair of wheels are trying their damn best to spin the same speed, similar to a 4WD pickup.
So maybe this explanation may make some sense, I can see already all the guys that are thinking about using this to go drift...lol. I dont think it would work as well as a RWD car, as their will always be some drive coming from the front wheels, pulling you out of the drift.

voidhawk

iTrader: (20)

maybe I'm being dense, but I thought a "fully open" center diff means no torque is going to the back at all and the car becomes FWD? Power oversteer means breaking the rear wheels loose by putting more torqie down than the tires have grip for - so you can't do this with an open center diff

It is my understanding that the Evo's ACD is always 50/50 (closed) or 100/0 (open). The only thing that the setting changes is the timing when the ACD is opened or closed, based on steering angle, throttle input, etc. "Tarmac" opens the quickest following turn-in, "Snow" opens the slowest and thus makes it easier to put the car sideways.

So the best the motec unit can do is alter the timing of when the ACD is open or closed, but it cannot alter the torque split from 50/50 or 100/0.

SHOMSPEED

Okay, It's impossible for the car to become fully FWD, unless you remove the transfercase. The back and front wheels are Mechanically connected through gears. The Motec ACD controller, is controlling the differential action of the center diff, it cannot alter torque bias, thats fixed. Its only altering the difference in speed, that the F/R pair of wheels are allowed to turn relative to each other, its controlling the "differential" action of the diff. The car will appear to be FWD if you dont have any ability to lock the center diff and you get, for example, the front pair of wheels stuck in some mud, while the back pair of wheels sit still on the tarmac. Then, not having any ability to lock the center diff, you would spin the hell out of the front wheels(or one wheel, if the front diff has no LSD action either), as a conventional diff will allow the wheel/wheels that have loss traction, to get all the drive, and you would basically wind up stuck in the mud. Thats why the LSD was born, for situation like this, or similar, so you would'nt get stuck.

voidhawk

iTrader: (20)

That is incorrect - the front and rear differentials are connected to each other through a hydraulically controlled clutch pack (the ACD). When the ACD fully opens, very little force is transmitted to the rear wheels, and thus the car is essentially FWD. This is exactly what happens when you pull the e-brake on an ACD equipped Evo (and stated in the manual that comes with the car).

It is also explained in the SAE paper published by mitsu "Development of Center Differential Control System for High-Performance Four-Wheel-Drive Vehicles".
The baseline torque split with the ACD fully closed is 50/50, and the differential limiting torque is 2000 Nm (the maximum toriqe that can be transfered to the rear wheels). As the ACD opens, the max torque that can be transfered decreases, and it is nearly zero when fully open.

Terry S

iTrader: (2)

It means that the front and rear set of wheels are allowed to spin at whatever speed they want independently of eachother. If the front has less traction, it will spin faster. If the rear has less traction, it will spin faster. And yes you can do this with an open diff, just put really really really crappy tires on the rear and some slicks on the front. The front will require more torque to lose traction so the rear will just spin away (as long as the diff is open).

The diff is mechanically set at 50/50 regardless of open or closed. Tarmac should be the easiest to get the car sideways because the differential is allowing the rear to do what it wants quicker, unlike gravel or snow which is limiting how fast the rear wheels can spin.

Correct. Also, Vishnu was looking into making a unit for this as well. They just need to know that there is a demand out there for this.

Terry S

Terry S

iTrader: (2)

Correct. This is called a transfercase or center differential.

Incorrect. It is mechanically set at 50:50 so when the ACD fully opens it acts just like an open differential. The power starts at 50:50 just like when it's locked, but the difference is that when a set of tires requires more torque, it allows the torque to goto whatever set needs more. (like in a turn when the front wheels turn faster than the rear the front wheels will take more. Or when the rear has no traction and the front does, then the rear takes more power)

When you pull on the ebrake, it intentionally deactivates, or opens the differential because if you didn't, then you would be directly fighting the ACD servo/unit with the ebrake handle. This would be very bad and put undue stress on the unit.

I just re-read that article and it mentions nowhere in there what you said in your last two sentences. It specifically says that the differential uses mechanically set gears at a 50:50 split and that the ACD clutches are there to maintain that split when required and up to a torque limit of 2000 Nm when engaged. Nowhere in that article does it say that "As the ACD opens, the max torque that can be transfered decreases, and it is nearly zero when fully open."

Also, just as a side note, the unit is not an on/off switch. It can engage at varying levels of strength. Tarmac/Gravel/Snow all have varying levels of where it "maxes out" the locking power.

Terry S

SHOMSPEED

I think this is one of those things that will be very hard for alot people to comprehend, unless the understand the basic function of a conventional diff, and LSD. And their place in AWD cars.

SHOMSPEED

And the Active Center Differential resides were?....in the Transfercase case.
So, wether you want the car to be FWD, AWD, RWD( like RMR drift EVO) it is all centered around manipulating the center diff. in the Transfercase.

SHOMSPEED

When you lift e-brake you are lessening/removing, the locking of the center diff. thus allowing the "differential" to do its job (allow each pair of wheels F/R, to rotate at different speeds,relative to each other.This particularly feature suits rally & AutoX drivers, to be able to rotate the car(oversteer), around turns, and cones in a parking lot. If it were to totally remove the drive from the rear axle as you are describing, making it FWD, then how would they be able to those cool Sliding turns with the car being FWD?

Terry S

iTrader: (2)

Hence why I made this Article.

Terry S

SHOMSPEED

Maybe You should do one called "Differential 101" or "Basic AWD 101"before you take on the more innovative AWD systems, like the one found in the current Evolution. So the members would have a understanding of the basics, before moving into more complex systems.

Terry S

iTrader: (2)

Funny you mention that, because I address that exact issue twice in the guide:



Terry S

sterdyevo

iTrader: (4)

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