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#1 (permalink) Old 01-01-2013, 07:59 PM
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4WD sytem newbie question

Hi all from the Newbie.

I have decided to trade my '02 2WD for a '12 V6 4WD. Only where and how much remains the question. I hope to have this done by weeks end. (I love dealing with car salesmen) I am doing it now because I love the rear door and V6 that are going away.

BTW/OT, '12 V6 LTD w/premium + value pack., rear camera, Tow prep pack. and pearl paint for $28,000 (CASH) +tax and reg. OK or not?

My question is if about the transfer case and 3rd members. Maybe someone has a link to technical doc. or? The salesperson tried to tell me that the system had locking differentials on the axles. I find that hard to believe and suspect a locking transfer case/transmission then brake modulation at the wheels. In any case, I don't think it is able to get all 4 wheels spinning as claimed, right? I have had 4WD trucks with air lockers at the axles and know first hand this can be hard to control.

Does this system use a viscous coupling method when not locked?

TIA,

Now on to decided on accessory list/priority Whee!


'02 RAV4 Base 2WD (leaving soon)
'12 RAV4 LTD V6 4WD (coming soon)
'08 Dodge RAM 4X4 MegaCab
'08 Cadillac XLR-V
'03 Honda Valkyrie (modded w/NO2)

Last edited by Phrede; 01-01-2013 at 08:06 PM. Reason: add signature
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#2 (permalink) Old 01-01-2013, 08:22 PM
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Surprise: the salesman is in error about locking differentials.

http://rav4world.com/pdf/2006/2006_4wdsystem.pdf

There is an electromechanical coupling in the drive shaft. The rest is done with brakes. However you could maybe spin 4 wheels in low traction condition as a front brake slows the fast wheel in the front, transferring torque to the other front, and the same could happen in the rear.

2009 V6 4wd Base
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#3 (permalink) Old 01-01-2013, 08:37 PM
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Quote:
Originally Posted by Carbon View Post
Surprise: the salesman is in error about locking differentials.

http://rav4world.com/pdf/2006/2006_4wdsystem.pdf

There is an electromechanical coupling in the drive shaft. The rest is done with brakes. However you could maybe spin 4 wheels in low traction condition as a front brake slows the fast wheel in the front, transferring torque to the other front, and the same could happen in the rear.
Thanks for the info. I suppose all 4 wheels could spin as the brake control modulates the low traction side. I don't think this would happen in a real world situation.

The engineer in me wants to know more about the em clutch design. I assume it to be a wet clutch of some kind. I also wonder how much torque is able to be sent to the rear axle. I know it says maximum, but max for what? Is that the max engine torque or limited? Yea, I know, I ask too many questions when the real answer is that it works.
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#4 (permalink) Old 01-01-2013, 09:34 PM
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The engineer in me wants to know more about the em clutch design. I assume it to be a wet clutch of some kind.
Ditto here. There is another pdf that describes it in detail - enough that I didn't fully assimilate it. Decided some really smart MEs designed it and this EE could accept that 0 to 3A can take it from off to full lock linearly. Maybe Carbon can find the pdf as I'm not home where I have all the info.

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I also wonder how much torque is able to be sent to the rear axle. I know it says maximum, but max for what? Is that the max engine torque or limited? Yea, I know, I ask too many questions when the real answer is that it works.
My conclusion is the whole system including the 4WD ECU and its sensors is quite sophisticated. The amount of transfer is under electronic control not mechanical limitation.

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#5 (permalink) Old 01-01-2013, 09:57 PM
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I'm a retired Automation Engineer that designed automated manufacturing equipment ie. robots, machine tools and material handling systems. So hopefully I will not be overwhelmed.

I did find a link that claims that 45% is the max torque to the rear axle. I'm still searching. Somewhere there has to be something before I reach the end of the internet.

Not knowing the system... I would suspect the torque control output would be PWM in nature. Analog outputs are pretty rare anymore. It effectively would appear as a variable output since it happens quicker than the clutch can react. Again, just a guess right now.
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#6 (permalink) Old 01-01-2013, 09:59 PM
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What would passing "45% of the max torque to the rear axle" mean to you? 45% of what? If the front wheels are on glare ice, very little torque will go to the front wheels. As far as I can tell, talking of passing xx% of torque to the rear wheels would imply a no-slip condition.

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#7 (permalink) Old 01-01-2013, 10:15 PM
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What would passing "45% of the max torque to the rear axle" mean to you? 45% of what? If the front wheels are on glare ice, very little torque will go to the front wheels.
To me that means that the torque output of the engine is able to be split 55/45 front/rear. The actual amount of torque delivered to either axle depends on the available tire traction and the amount of brake applied. The amount of torque delivered to any axle must be independant of what is going to the other axle or the system would be useless. So even if the front wheels were off the ground 45% of the engines output (at the time) would be transmitted to the rear axle and drive the car. Of course the front wheels would turn at the same speed as the rear ones.
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#8 (permalink) Old 01-01-2013, 10:23 PM
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Quote:
Originally Posted by Carbon View Post
What would passing "45% of the max torque to the rear axle" mean to you? 45% of what? If the front wheels are on glare ice, very little torque will go to the front wheels. As far as I can tell, talking of passing xx% of torque to the rear wheels would imply a no-slip condition.
I just reread this 2010 thread on the same subject. A lot of theory was discussed, but I'm not sure we ever came to a conclusion.I'm hoping that Phrede, with his background, might be able to offer his theory as well.

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#9 (permalink) Old 01-01-2013, 11:55 PM
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To me that means that the torque output of the engine is able to be split 55/45 front/rear. The actual amount of torque delivered to either axle depends on the available tire traction and the amount of brake applied. The amount of torque delivered to any axle must be independant of what is going to the other axle or the system would be useless. So even if the front wheels were off the ground 45% of the engines output (at the time) would be transmitted to the rear axle and drive the car. Of course the front wheels would turn at the same speed as the rear ones.
Take your case where the front wheels are off of the ground. So the front axles have near zero torque on them. Suppose the engine is delivering 100 units of torque. The rear drive shaft gets 45 units of torque? Where did the 65 units of torque difference go?

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#10 (permalink) Old 01-02-2013, 01:15 AM
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I'm still learning the system and since that post have learned that there is no center differential. This is an entirely different situation. It seems the driveline is directly geared to the drive system and turns 100% of the time in sync with the front axle (at 3.08 times the speed). The limiting factor is an electromagnetic clutch that slips when the torque is exceeded. The amount of allowable torque that can be transmitted thru the coupling is controlled by the ECU.

It seems to me that the the spec means that up to 45% of the AVAILABLE torque can be transmitted thru the coupling before it slips. Remember, the engine never produces more torque than required by the load. In case of the front wheels being off the ground and a light total load then it is probably closer to 15% front 85% rear of the required torque. 15% being lost in inefficiency of the gear train itself. But the 85% being used by the rear does not exceed 45% of the available torque but is enough to move the car. Again, the amount of torque that "could" be transmitted thru the coupling is controlled but the ECU and the design of the coupling. As long as the car moves (the required torque is less than the available torque) there will not be any slippage in the coupling and the net rear axle speed will equal the net front axle speed. Without any slippage in the clutch the amount of torque it could transmit doesn't really matter.

The system knows if there is any slippage only when there is wheel speed difference greater than can be attributed to cornering. I expect it takes into account engine speed and load and maybe it takes into account the yaw or other sensors to get a better idea of wheel slippage and when to disengage the clutch from freewheeling 2WD mode. It then increases the torque that could be transmitted thru the clutch until 45% of the available torque/engine load is reached. This could be a much higher percentage of the torque required to move the car. This example of having zero traction at the front wheels is extreme and is more likely much more balanced front to rear in the real world.

At this point I'm making a lot of assumptions and I'm sure I've oversimplified it. Certainly the algorithm used to control the clutch took a lot of development time.

HTH
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