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Old 02-12-2005, 01:25 PM   #1
Steven_RW
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Default How does a front to rear torque split diff work?

How does a variable torque split diff work. In the cossie its 66.666 rear 33.333 front. How can you control the amount of torque going to one diff but not induce slip at the same time?

If it was like a viscous coupling would you not just induce slip? I have never really sat down and thought how that works and would be interested to know. I expect that from the answer to this question I will be able to work out how an evo works when it varies the torque front to rear and side to side in the later models.

Any info would be appreciated

Thanks Steven RW
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Old 02-12-2005, 02:12 PM   #2
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To achieve a torque split you monkey with gear ratios.
On the Cosworth centre diff, there is what is known as an "epicyclic gearset".

Like this:


Click the image to open in full size.

The centre gear is called the "sun", the three gears "planets", and the outer ring the "annulus".

On the cossie, power is fed to the planet set, rear drive is taken by the annulus, front drive by the sun.

The viscous coupling links sun and annulus to promote an equal rotation speed.

The relationship between ratios (planet gear to sun and planet gear to annulus) decides your torque split.
There is obviously a big difference between planet to sun ratio, and planet to annulus ratio, and when you consider that every bit of gear reduction equals torque multiplication, you can see that there will be a greater torque multiplication to the annulus than to the sun.
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Old 02-12-2005, 02:17 PM   #3
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When a unit varies the split, it is done with clutch plates applied by hydraulic pressure (usually), with that pressure being electronically controlled to suit the driving situation. This clutch pack system typically replaces what would be the viscous pack as we know it on the Cosworth.
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Old 02-12-2005, 02:22 PM   #4
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Bernie,

Thanks for your explanation. I've read it about 4 times and will continue to do so till i feel I totally understand it. At which point I will try and work out how you can have a variable split front to rear considering the ratios of the gears are set due to being steel cogs.

So just to check.

Power is fed to the three planet gears from the regular gear box.

The annulus feeds the rear and the sun feeds the front.

The difference between the ratio between annulus and planet and the ratio between the planet and the sun make up the torque split.

Then after this the drive from the annulus and the sun are connected via a vicous coupling to ensure both rotate at the same speed (with obviously a small amount of slipage as the viscous coupling is not always locked)

Thanks very much for your explanation

Not sure if evos are your thing, if they are, how do you have a variable torque split front to rear without inducing slipage? Same question would apply to a subaru with a lever/nob that allows u to control the power split front to rear.

The diagram helped very much

Thanks

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Old 02-12-2005, 02:23 PM   #5
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You have already answered my question on the variable split using the clutch pack. I do struggle to see how that would not result in a varying of wheelspeeds from front to rear but I'll keep mulling that one over for a while!

Cheers

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Old 03-12-2005, 10:41 AM   #6
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Steve, I found this in the archives.
http://www.passionford.com/forum/vie...er=asc&start=0

HTH
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Old 03-12-2005, 10:48 AM   #7
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http://en.wikipedia.org/wiki/Planetary_gear
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Old 03-12-2005, 04:01 PM   #8
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Steve, I robbed (cut & paste ) this from a Mitsubishi site...

Active Yaw Control, first incorporated into the Evo IV, was developed by Mitsubishi to improve a vehicle's cornering and acceleration performance, and consequently its safety, under a wide range of operating conditions.
This is achieved by utilising a torque transfer differential which is controlled by various sensors and an electronic control unit (ECU) to enable a difference in torque to go to each of the rear wheels.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

As shown in diagram 1 (right hand bend), by increasing the level of torque to the left rear wheel and reducing the torque level to the right wheel, it is possible to change the yaw movement of the vehicle. This in turn will cause the vehicle to steer inwards and reduce the amount of slip on the front tyres, thus resulting in reduced under-steer. It also works if over-steer occurs, as shown in diagram 2, by reducing the torque level to the left hand rear wheel and increasing the torque level to the right hand wheel, again changing the yaw movement to reduce over-steer.

By controlling the amount of torque transmitted to the rear wheels when there is less traction, or a difference in grip on the road surface, AYC also works to improve acceleration and stability on slippery roads (as shown in diagram 3).

Active Centre Differential

The Active Centre Differential, first introduced in the Evo VII, is an electronically controlled hydraulic multi-plate clutch which distributes torque between the front and rear to improve traction under acceleration out of a corner. It works in conjunction with Active Yaw Control which enhances grip and steering response whilst driving through the bend itself. Using sensors, ACD regulates slippage in the 50:50 torque-split diff from free to lock-up according to speed and load. So under hard acceleration the ACD moves towards lock-up to put more torque down on the road for stronger traction, but with rapid steering inputs it operates virtually like an open differential to improve steering feel and response.

A choice of three setting - tarmac, gravel and snow - operated manually, gradually lock up the Active Centre Differential depending on road conditions.
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Old 03-12-2005, 04:20 PM   #9
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good topic Steve
Bernie,
do you know what the diff split is between the 3 different settings on the evo??Gravel on the wet back roads is very entertaining
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Old 03-12-2005, 04:27 PM   #10
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Good choice justin - thats what we had andys in in the rain yesterday - a nice setting for playing on the road lol
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Old 03-12-2005, 06:52 PM   #11
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Some info I got whilst looking at the EVO for competition...

AYC & ACD systems
The new ACD system is an extension of the technology used in the anti yaw control system employed in various previous Mitsubishi models. Recently introduced, the ACD system comprises an electric motor, driving an oil pump, which pressures an oil reservoir to a peak 16-bar pressure then the motor is turned off. If the reservoir pressure drops to 10 bar the cycle repeats. This system pressure is fed to the piston of the ACD plate pack via a modulation (proportion) valve. The modulation valve is software controlled by the ACD electronic control unit. The ACD electronic control unit is a completely separate item to the engine electronic control unit. The control inputs to the ACD electronic control unit are
• 4 wheel speeds
• G force both lateral and longitudinal
• Throttle position a variable value
• Braking state - on or off
• Steering angle neutral position (straight ahead)
• Steering angle rotation from straight ahead provided by 3 optical inputs generated by rotation of the steering wheel.
• Handbrake state on or off
• System pressure Hydraulic pressure state a returned voltage from the pump pressure switch in the 0-5V range
• Mode switch state to select Gravel, Snow or Asphalt software strategy
Owners should be aware that in order to change the steering wheel, provision must be made to drive the steering angle sensor. OMP has produced a unique steering wheel mounting boss to engage and drive the ACD steering angle sensor. The part is available from OMP dealers part number OD/1960M1290. The steering angle sensor ring has 3 offset square drive slots, which can easily be seen as a new detail to the rear of the standard steering wheel - just outside of the centre-mounting hole. The sensor drive slot alignment is essential for the correct functioning of the anti yaw control and ACD systems. One of the three is a master slot and is aligned at the 3 o clock position as viewed when seated in the car, in the straight ahead steering position. Correct assembly of the steering wheel and boss are therefore critical. The boss features a Top mark to aid correct assembly.
The anti yaw control and ACD systems have some sensitive components which are not particularly dirt tolerant. It is therefore important that dirt is not introduced to the system during routine oil changes or topping up of the AYC / ACD fluid reservoir. The reservoir neck and cap should be cleaned prior to removing the cap and topping up.
Three variants of ACD electronic control unit exist. The production version and two Ralliart variants - RA553831K1 being listed as a fast road unit and RA553831K2 a Rally unit.
BTR added a Motec® logging system to their Evo 7 RS test car for a 90 mile shakedown run in the Yorkshire Forest prior to the Network Q Rally GB. This produced some interesting data on the ACD system and a good understanding of how well the system functions. Of general interest to the average owner is
• That Gravel and Snow software strategies are similarly complex. Gravel has the highest ultimate locking value of the two. This is true of all the electronic control unit variants. The aggregate locking value in Gravel mode being 4 bar peaking at 10 bar on the production electronic control unit and 6 and 10 bar on the rally version of the ACD electronic control unit.
• In Asphalt mode, the centre differential is generally running low pressures (and therefore ultimate lock value) unless the car is braking, where the pressure increases on braking and reduces as steering angle increases during turn into the corner. This is the basic mode that most owners should use for normal day-to-day driving.
• The software strategy in the production ACD electronic control unit is relatively soft and looks to be biased towards safety and long component life. For performance and track day use the Ralliart fast road ACD electronic control unit will compliment the chassis better than the standard unit. The Rally unit has the most aggressive software strategy, working the pump assembly and differential hard. To get the best of both worlds, long system life and track performance, fitting the K2 Rally specification performance electronic control unit for track days only could be considered while fitting the road unit for off track use. If this is considered too much trouble do not fit more than the K1 electronic control unit. In all cases run Asphalt in normal use.
• The Gravel and Snow strategies do not appear to be sensible for track day use.
• Misalignment of the suspension resulting in the steering wheel being offset while driving in a straight line will screw up the functioning of AYC and ACD systems.
• The system is technically very good. The centre differential performing well and able to limit front to rear axle slip to a mean of around 2% with the production ACD ECU and 0.6% with the Rally ECU on a gravel surface.
• Pressure can be introduced and lost within the ACD system at close to WRC standard speeds, which has allowed Mitsubishi engineers to use a complex and sophisticated control strategy.
• To summarize - ACD is not a gizmo and is far superior to the Evo 4-6 VC centre differential control system. The system has more benefits on slippery surfaces than dry surfaces, but is better on all surfaces overall.
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Old 03-12-2005, 08:01 PM   #12
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Thanks for your time and effort Bernie.

Justin/Martin

Likewise I agree that the gravel mode seemed like the mosty "drifty" of the settings on Andys evo. It made for lots of fun

Cheers all!

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Old 03-12-2005, 08:01 PM
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