LANCER EVOLUTION X
Mitsubishi's Active Center Differential
The ACD is a bevel-gear type center differential with a front/rear torque distribution of 50:50. The differential is controlled by a hydraulic 'wet' multi-plate clutch, where the clutches are made from high strength steel. The maximum limited-slip torque of the multi-plate clutch is about three times that of a conventional VCU (viscous coupling) center differential: IE: 3 times the grip can be transferred to the wheels that need torque. Although some drag racers still prefer VC for its simplicity, ACD is technically far superior to the preceding Evo models (and certain USDM models) VCU. When the car is accelerating or decelerating rapidly, ACD calculates what the optimum locking amount would be and then engages the limited-slip locking mechanism by applying pressure to the plates via hydraulic ACD pump and solenoid. This ranges between full lock (16bar pressure), full open (0bar pressure), and all points in between. The harder you accelerate or decelerate, the more it locks - for maximum stability and traction. If the front wheels are spinning faster than the rear wheels, then the ACD begins to lock the clutches up. The pressures that can be generated or removed by the ACD system occur at surprisingly high speeds!
If the amount of force exerted by the front wheels is less than the amount the clutches resist, then the slip is stopped and both front and rear spin at the same speed. However, if the force the clutches can resist is less than the force exerted by the slipping set of wheels, then the wheels will slip, but only by how much remaining force the slipping wheels overpowered the clutches. When turning, the wheels in the front have to travel a further distance than the wheels in the rear. This is because the wheels in the front travel using a larger turning radius than the rear wheels, so in order for smooth turning, the center differential needs to become open to allow for the front set to turn at a different speed. If you were to leave the center differential locked, the tires would skip and chirp on asphalt. ACD effectively allows the differential to operate in more of a free state when steering movements are made. However the preset ACD maps that come with the car are designed to be forgiving for average drivers in stock lancer evolutions, not true motorsport setups with high power engines in modified chassis and suspension.
Mitsubishi's Active Center Differential
How ACD Works in an Evo X:
Mitsubishi's ACD system works by dynamically adjusting the limited slip locking state of the center differential clutchpack. When we talk about "lock" on the center diff, it should not be interpreted as a distribution of torque from front to rear, rather it should be thought of only as a clamping force - which locks the Front and Rear outputs together, and prevents the center differential from behaving like an open differential. This is not the same as Nissan's ATTESA-ETS or Subaru's DCCD (both of which adjust torque split) -- ACD is fixed at 50:50 FWD to RWD, but instead ACD alters the clamping force on the center diff's clutchpack, see diagrams below:The ACD is a bevel-gear type center differential with a front/rear torque distribution of 50:50. The differential is controlled by a hydraulic 'wet' multi-plate clutch, where the clutches are made from high strength steel. The maximum limited-slip torque of the multi-plate clutch is about three times that of a conventional VCU (viscous coupling) center differential: IE: 3 times the grip can be transferred to the wheels that need torque. Although some drag racers still prefer VC for its simplicity, ACD is technically far superior to the preceding Evo models (and certain USDM models) VCU. When the car is accelerating or decelerating rapidly, ACD calculates what the optimum locking amount would be and then engages the limited-slip locking mechanism by applying pressure to the plates via hydraulic ACD pump and solenoid. This ranges between full lock (16bar pressure), full open (0bar pressure), and all points in between. The harder you accelerate or decelerate, the more it locks - for maximum stability and traction. If the front wheels are spinning faster than the rear wheels, then the ACD begins to lock the clutches up. The pressures that can be generated or removed by the ACD system occur at surprisingly high speeds!
If the amount of force exerted by the front wheels is less than the amount the clutches resist, then the slip is stopped and both front and rear spin at the same speed. However, if the force the clutches can resist is less than the force exerted by the slipping set of wheels, then the wheels will slip, but only by how much remaining force the slipping wheels overpowered the clutches. When turning, the wheels in the front have to travel a further distance than the wheels in the rear. This is because the wheels in the front travel using a larger turning radius than the rear wheels, so in order for smooth turning, the center differential needs to become open to allow for the front set to turn at a different speed. If you were to leave the center differential locked, the tires would skip and chirp on asphalt. ACD effectively allows the differential to operate in more of a free state when steering movements are made. However the preset ACD maps that come with the car are designed to be forgiving for average drivers in stock lancer evolutions, not true motorsport setups with high power engines in modified chassis and suspension.
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