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Old 05-06-2017, 10:35   #61
plip1953
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I don't profess to understand much at all about the mechanical side of how ACD and AYC work, but, at a very basic level, if you're going along and then apply the handbrake that locks the rear wheels, doesn't drive that was to those rear wheels have to go somewhere else? Does it all get directed to the front or does something else happen?
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Old 05-06-2017, 10:54   #62
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Originally Posted by plip1953 View Post
I don't profess to understand much at all about the mechanical side of how ACD and AYC work, but, at a very basic level, if you're going along and then apply the handbrake that locks the rear wheels, doesn't drive that was to those rear wheels have to go somewhere else? Does it all get directed to the front or does something else happen?
With an open centre diff you'd get a 50:50 split with a lot of torque going front and rear. The rear can handle a lot of torque as the wheel is locked so the fronts get that amount of torque too. The front wheels will break traction due to torque overload and now the fronts can't take much torque as they are slipping so that reduced amount of torque is what is now sent to the rear as well (both front and rear will see the same amount of reduced torque, the split is always 50:50). As the rears are locked still the result is massive strain all round.

Last edited by Aidy; 05-06-2017 at 20:42..
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Old 05-06-2017, 11:39   #63
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Originally Posted by stu8 View Post
I cant see any real difference to the earlier evo 7 to 9 setup. Would be helpful if someone with technical knowledge could confirm


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no real difference mechanically... I guess there is some difference in the control ecu, since the ESP is integrated in the system...
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Old 05-06-2017, 11:41   #64
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Originally Posted by plip1953 View Post
I don't profess to understand much at all about the mechanical side of how ACD and AYC work, but, at a very basic level, if you're going along and then apply the handbrake that locks the rear wheels, doesn't drive that was to those rear wheels have to go somewhere else? Does it all get directed to the front or does something else happen?
due to the way an open diff works(and when you apply the handbrake the ACD opens), when you apply the handbrake the front wheels will actually try to spin at 2x the speed they did before the handbrake.. One of the reasons it is relatively hard to lock the rears on an evo with the handbrake is that you always have 50% of the tq trying to turn the rear axle..
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Old 05-06-2017, 12:38   #65
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So with handbrake applied ACD doesn't close it to send all the power to the front axle?
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Old 05-06-2017, 14:53   #66
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you are mistaking the "sending power" part.

ACD clutch just controlls the speed difference between the front and rear axle. The front and rear are ALWAYS connected togeather and always are getting TQ/POWER. Now, imagine the ACD is open all the time, lets say no hydraulic pressure... Both axles will still get 50%. However, if one axle looses grip those set of wheels will start to spin. Then both axles will get less power, because one pair of wheels will have no traction and those wheels will just spin out.. This is similar to a FWD car with an open diff... Lift inside wheel and whole car stops accelerating..
Now, what ACD does is limit the slip between the axles. Journalists like to talk about torque split, but it is really a wrong way of talking about it.. Imagine again the rear axle loosing traction... ACD locks and then it slows down the rear axle to match the front axle speed, so two things happen.. Front wheels start getting power again an pulling car, because all the "power" is not being spun out trough the rear wheels,... also, rear wheels start getting traction, because locking the ACD will lower the speed of the rear axle and equalize it with the front.


This is not like the old GTR system when you always had rear wheel drive and would turn on the front wheel drive when rears loose traction.. mitsubishi has 4wd on all the time, and with the ACD it controlls the relative slip between the axles.
So, we agree that for max traction we need the acd locked.. ok? But, why not drive like that all the time... Well, in corners, front and rear axle do not have the same speeds.. because all 4 wheels are not running on the same circle.. In fact, all 4 have different speeds.. Inside wheels being slower than the outside ones, and fronts having different slip angle than the rears... So, if you lock the ACD in the corner, esp on the corner entry, you will get understeer, because a 4wd car with all diffs locked will tend to understeer on corner entry with no power on. It is really interesting to watch it work, on the logs, as it tends to react to steering input, and especially on steering input speed really fast...

So, then the ecu, when you turn in, opens the ACD, and lets the car turn in in the best way, and then regulates the locking of the diff according to the way you apply power, steering, and the way the car behaves (how much it slips around)...

To answer the handbrake question.. if the ACD would lock when you pull the handbrake, you couldnt sto the rear wheels turning.. that is why the acd opens when the handbrake switch is activated..

Last edited by kikiturbo; 05-06-2017 at 14:55..
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Old 06-06-2017, 16:13   #67
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That is an excellent explanation! Thank you for your time writing it, at least for me it is extremely helpful. What I need to get my head around now is how ALL parts of the system interact with each other creating this amazing cornering performance. I personally don't get excited about straight line speed. For me it is all about cornering speed and braking performance. Many a time I run circles around cars with much more power but driven by people who could only use the loud pedal and were completely lost in braking zones and corners .
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Old 06-06-2017, 16:40   #68
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my experience with the evo platform was that its 4wd system (especially the ACD/AYC versions) was not done just to increase the overall grip, but to enable better adjustability and easier change of direction at the limits of the grip. In other words, an audi s3 will give you loads of grip but when you reach the limit and try to turn in more it will just plow on with understeer... Evo is more like a unbalanced machine, always having a degree of rear end slip, changing direction easily..
It is most evident on snow... I am not a superb driver, but when we have our winter fun drives on snow covered roads I tend to be faster than all the rally dricing boys in their imprezas.. What I have found out is that while most of them have to setup the car before the corner and then go trough it fully commited, I can usually change my line and angle of attack mid corner, with much more ease..
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