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#1
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Too old for kindergarten?
OK. - I give up.
I'm trying to understand how the centre differential in VTD equipped cars works, and it's got me stumped. The only explanation I can find is here http://www.subaru-global.com/about/parts/06.html. But the explanation falls a little short of being useful. Quote:
Because the planetary gears (and the meshed ring and sun gears) are different sizes, the torque split between the front and rear sun gears is going to be unequal (65% rear/35% front in the SVX). This would mean that the natural output shaft speeds would be different too. But because the front and rear wheels will be coupled, via the roadwheels to the road surface, the output shafts will normally be forced to rotate at exactly the same speed. This discrepancy is taken up by allowing the ring gear to freely rotate. Have I got it right so far? If I have, then here's the kicker! When the two speed sensors for the front and rear output shafts differ by some unknown percentage, (let's say 5% for argument's sake) then the Transmission Control Unit presumably applies a brake via a brake band to the ring gear to progressively lock the front and rear driveshaft's rotational speeds together. You can imagine that the greater the discrepancy in the output shaft speeds is, then the harder the brake would be applied. BUT, surely this means that the transmission is now forcing the front and rear roadwheels to rotate at grossly different speeds, and therefore forcing the brake to be applied even harder!!! This cannot be right can it????? I've obviously misunderstood something somewhere. Has anybody found a better explanation somewhere, or can explain where (or if) I'm going wrong? I suspect I am too old for kindergarten. Last edited by ItsPeteReally; 08-20-2006 at 10:49 AM. |
#2
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Howzet work?
Not easy to understand Eric. This is taken from a few of the posts ithat I have put in different threads.
Torqueing splits. Chike said it “ Subaru designed and built a great trans for our cars!” The only problem is they did not put it in the US model. The Varied Torque Diff, was to be the one of the main features, that the SVX was created with. There was nothing like this AWD being used anywhere in the auto world. Unfortunately the US auto industry was not up to pace, to be able to use it. The default Transfer clutch system was resorted to. This was to allow the car to be run on a two wheel dyno for emissions testing, as there were not enough AWD dynos there. The transfer clutch system is a front wheel drive, with rear wheel assist. The front wheels are constantly driven, if there is wheel slip, the clutch engages the rear wheels to assist the front. The Varied Torque Diff does just that, sending 36% of the torque to the front wheels, and 64% to the rear wheels. This is done through the varied leverage, of the different size gears, that drive the front and back wheels. This ratio does not change, if wheel grip is lost, either in the front or rear wheels, the Limited Slip Clutch is applied, to lock the two halves together, to prevent wheel spin. Just like a locker diff. The main difference between the two types is how they work in corners. The transfer system cannot apply a lot of drive to the rear wheels, as the same clutch has to allow the difference in rotation, necessary to prevent binding. The tighter the turn the less it can drive the rear wheels. This would be most noticeable in Auto Cross, where tight turns plus hard acceleration are needed. This is why the C solenoid is modulated to limit the amount of grip it gives, and still allow the difference in front/rear wheel rotation. A duty cycle voltage operates the C solenoid. This means that the solenoid is turned on and off at a constant rate, that is chosen to suit the armature and return spring characteristics This type of operation allows the valve to be held in any position between full on and full off, depending on the level of the applied current, that is sent during the duty cycle ON time. This is the same type of operation that works the A and B solenoids also, to vary the level of oil flow. There can be no doubt what the VTD is the best system, especially if the engine has large amounts of torque added. The VTD can deliver it to the wheels, under any conditions, no risk. The Transfer system will always have trouble catching the front wheel spin in time, and the more torque that is added, the harder the clutch will suffer. These are a couple of drawings that I did, to show the difference in the two types of AWD. The Transfer type is a front wheel drive box, with a TCU operated clutch to drive the rear wheels. The VTD uses a compound planetary gear set, that uses different size sun and planet gears, to divide the torque, in the ratio set by the gear sizes. It uses a TCU operated clutch as a Limited Slip unit, that prevents either front or rear wheel spin. The drive comes in the front sun gear, to drive the compound planet gears. The planet gears are mounted on the carrier that drives the front wheels through the transfer gears. The rear sun gear is driven by the other half of the compound planet gears, to drive the rear wheels. The Limited Slip clutch is connected between the carrier and the rear sun gear. Any difference in wheel speed over 20%, is detected as wheel spin, so the TCU applies the clutch to move more torque to the other wheels. Under normal driving conditions, the whole gear set revolves together, as a solid unit, the gears don't rotate against each other. The only time that they rotate is when there is wheel spin. Now the hard part. Although the gears don't rotate, with each other, they still divide the torque. A set of gears, are only a set of revolving levers, so even though they don't revolve, they still divide the torque by the differential leverage, of their diameter. Hope this is a help Eric. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#3
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The following statement statement from the above has been consistently posted over a considerable period. It is not correct and members are being wrongly informed.
Quote --- A duty cycle voltage operates the C solenoid. This means that the solenoid is turned on and off at a constant rate, that is chosen to suit the armature and return spring characteristics This type of operation allows the valve to be held in any position between full on and full off, depending on the level of the applied current, that is sent during the duty cycle ON time. This is the same type of operation that works the A and B solenoids also, to vary the level of oil flow." --- N.B. Solenoid "B" is in fact a sraight on/off device and is not modulated by means of a duty cycle. N.B. Solenoids "A" and "C" are not "held in any position between full and open", as is claimed. The correct method of operation is as follows :- DUTY SOLENOID VALVES "A" and "C". These are pulse width modulated duty solenoid valves, ( Sometimes known as a pulsoid). The device is incorporated in the SVX transmission control system in order to adjust pressures in the following manner :- SOLENOID "A". The fluid line is provided with a bleed or bypass via an on/off device, in the form of an electrically operated valve. This solenoid valve is opened and closed repeatedly, in a rhythmical manner by a control current which is turned on and off by the transmission control unit (TCU) at a very fast rate. The valve is a normally closed device, and remains closed in the event of the loss off a control current. After passing through this modulated solenoid valve, the continually interrupted pressure is in the form of a pulsed flow. When the peaks level off with the troughs, there is a resulting overall steady reduced pressure. The level of this pressure is adjusted by varying the on/off intervals. Most often the length of the on time is adjusted and the number of on/off pulses per second is kept constant. The usual rate is around 50 cycles per second. The resulting adjusted output pressure is therefore delivered as a rapidly fluctuating stream. The system incorporates an expansion chamber as a smoothing element, which works as a sort of cushion. This device is usually in the form of a cylinder and piston or diaphragm, backed by a coil spring. In the SVX system the component is described as a Pressure Modifier Accumulator. The high pressure peaks in the stream press the piston outwards and become rounded off, while the low pressure troughs are filled in as a result of the piston moving inwards under spring pressure. The end result is a smoother level of pressure, such that controlled devices are not materially affected. An increase in the volume of fluid controlled, is achieved by transferring the solenoid regulated pressure, to a pressure modifier valve and a regulator valve. It should be clear that by “chopping” the fluid supply in an adjustable way, pressure control is achieved economically using a simple poppet type solenoid valve, with few mechanical or electrical complications. N.B. --- The valve remains in a continuous cycling mode, which imposes rather arduous mechanical stresses. SOLENOID "C". Also a pulse width modulated solenoid valve, this device is arranged to adjust control pressure to the centre differential, or alternatively clutch actuating mechanism, according to model. Solenoid C operates exactly along the same lines as solenoid A and cycles continuously. However the duty is in no way as arduous as that involving constant control of line pressure.
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#4
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Quote:
The A,B,C, solenoids ALL operate by a Duty Cycle to control the oil flow through them. Not only them, but also the EGR, Idle Air valve, Power steering, Carbon canister purge valve, Air Conditioning wobble plate solenoids, are ALL controlled by a Duty Cycle to smoothly, alter the leakage through them. You go to great lengths to describe, due to your theory on the solenoids operation, how the Chopping action of the A solenoid is smoothed, But how does the C or B solenoid's chopped oil flow get smoothed.????????? Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#5
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How wrong can you be?
So it seems that I got everything wrong!
Many thanks for the explanation. Pete. |
#6
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Quote:
The devices you list are in no way comparable and as I have stated solenoid B is not modulated and is simply an an on/off device. You have now extended the scope of your errors. All is as clearly shown in the manuals. I am prepared to cover the exact method of controlling each device if you will raise the matter in a separate thread. Meantime please stay with the current topic. What would you say to a small wager of say US$1,000.00? On the basis that the SVX A and C solenoids do not remain held stationery at variable positions in order to control fluid flow as you continue to state, but are continually in motion as I have stated. My winnings I would undertake to pass over to Chris for the maintenance of the facilities he so kindly provides. You would be free to keep yours. In view of the confidence you have expressed in respect of your knowledge and ability, a spontaneous reply can surely be expected. However you will be able to make time available to undertake further research and the anticipated ploy will be evasion. Your reaction will be indicative.
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#7
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Not again.
This is the second time you have tried this.
I am just not interested in any thing you have to say on this subject. It has all been said before, in a heap of threads. You have your description, I have mine. The readers can decide what to believe, as with all posts here. I don't condone gambling in this thread. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#8
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Quote:
Surely if you know that you are correct, I have not in effect asked you to gamble. I have simply asked you to demonstrate faith in that which you put forward to others here as the truth. You should be positive in the beliefs that you foster on others as the truth, to the extent of benefitting the network. Is the cause not cut and dried? In anticipation of another side step, I point out that your "description" of your belief is absolutely specific and has been repeated. The is no contingency in respect the "descriptions" that you or I have stated. There is no misunderstanding as you try to suggest.
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#9
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Quote:
Quote:
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R.I.P Cool Signature. You're gone but not forgotten. I mean, sure, I kindof don't remember some of the finer details..but I remember you were funny, and at one point you said spaghetti in Dutch....but definantly not forgotten |
#10
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Quote:
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#11
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Quote:
http://www.subaru-svx.net/forum/showthread.php?t=35942 Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#12
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Quote:
Further information can be found in this thread:- http://www.subaru-svx.net/forum/show...306#post442306
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#13
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Harvey,
The following is an extract from the opening post in the thread you have suggested by way ot a reference. Do you continue to claim that what you state is correct? Quote ---”When full throttle is used, the solenoid stays as an open drain, and the pressure is held high. As the throttle is closed the Throttle Duty Cycle current, is increased to reduce the solenoids drain on the Pilot pressure that acts on the Regulator Pressure Modifier valve and Line pressure is reduced.”
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
#14
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Quote:
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! |
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