Discussion on 4eat awd system....

[Ross]

OK, after looking in to how the AWD system on hte 4EAT works, it strikes me that there is a problem with simply using a clutch to transfer power rearward, instead of using a differential.

 

The four tires mounted on the car will never be EXACTLY the same diameter. Even if the tires are all exactly identical, small variations in pressure will mean that their efective rolling diameter changes.

 

If the average size of the two rear tires happens to be slightly larger than the average size of the two front tires, this is fine - the PT clutch will slip some relatively insignificant amount, but power will be transferred to the rear as it should be.

 

But what if the average size of the rear tires is smaller than the average of the fronts? Even if this is by a tiny amount, surely the clutch will be slipping backwards, thus providing a BRAKING torque to the rear....

 

This could easily happen if one were to fail to adjust tire pressures when a load was put in the rear of the car.

 

Am I missing something, or is this really the case?

[CNY_Dave]

OK, after looking in to how the AWD system on hte 4EAT works, it strikes me that there is a problem with simply using a clutch to transfer power rearward, instead of using a differential.

 

The four tires mounted on the car will never be EXACTLY the same diameter. Even if the tires are all exactly identical, small variations in pressure will mean that their efective rolling diameter changes.

 

If the average size of the two rear tires happens to be slightly larger than the average size of the two front tires, this is fine - the PT clutch will slip some relatively insignificant amount, but power will be transferred to the rear as it should be.

 

But what if the average size of the rear tires is smaller than the average of the fronts? Even if this is by a tiny amount, surely the clutch will be slipping backwards, thus providing a BRAKING torque to the rear....

 

This could easily happen if one were to fail to adjust tire pressures when a load was put in the rear of the car.

 

Am I missing something, or is this really the case?

 

That's why until you need more from the rear, it slips so much (90 front 10 rear).

 

I don't think which way the clutch slips makes much difference to the efficiency of the system.

 

 

Dave

[Ross]

Yes, but no matter how much it is slipping, if the rears are trying to go faster than the fornts, they will always be trying to slow the car down - giving a torque split more like -10/110.....

[idosubaru]

that is interesting from a technical perspective, never thought of that before. the theoretical tire scenario is probably more often true than not since people when neglecting tire maintenance, the fronts are typically warn more.

 

but the order of magnitude is too small. if it mattered it would matter either way, not just "one way" so to speak. if it was significant enough to cause relevant issues it would likely be significant enough to damage a differential or some other 4WD mechanism.

 

many things add friction to the system. might need a drum brake conversion on the front and rear since disc brakes are less efficient. the transmission cooler might not be necessary in the winter but adds hydraulic requirements to the trans pump, reducing efficiency, so you could remove that in the winter time....et.al. "sorry you can't ride with me today, i'm trying to be efficient".

 

technically the shifting resistor is less efficient and causes more wear on parts because it "allows" a smoother transition between gear shifts, giving a more comfortable ride. i unplug them in favor of a firmer shift. i "could" argue that increases efficiency and reduces wear since it's less gradual and not like "riding" the clutch so to speak. but i don't really *believe* it makes any difference.

 

this kind of thinking could be extrapolated in all sorts of ways.

[Ross]

I'm not talking about the effect this has on efficiency - as you say, it is insignificant and occurs whether the front or rear tyres are larger.

 

I'm getting at how this effects the power split. i.e. it is impossible for this system to give a 90/10 split if the rears are smaller than the fronts, since the output of the PT cluctch will always be trying to go FASTER than the input. The rears will therefore always be driving in the opposite direction to the fronts.....

 

I'm not saying that this has any real detrimental effect, since as soon as you enter a corner, or start losing traction, the tiny difference in wheel diameter will become negligable. But it is interesting in that you will likely be affecting your straight line torque split by putting a load of stuff in the back of your car.... Could you actually feel/notice it? Probably not.

[idosubaru]

right on. clutches allow one surface to turn another but provide some allowance for slipping. they are there to allow slippage during turning and probably for other variables. that's what's happening and i don't know that such small revolution differences really amount to anything in terms of power transfer and traction.

 

would be neat to install a speed sensor or RPM gauge on each "side" of the clutch so to speak and compare them.

 

just install a duty C switch and control traction yourself, that's what i do and it's awesome!

[keltik]

Looks like the latest 4eat got rid of this problem with the VTD center diff setup...

 

But yes from an engineering standpoint it would create some drag having the rear tires slightly larger. I thought the TCU was programmed to open the center diff if the rear wheels were spinning faster

[idosubaru]

I thought the TCU was programmed to open the center diff if the rear wheels were spinning faster

4EAT doesn't have a center diff.

[keltik]

4EAT doesn't have a center diff.

 

Well the clutch packy thing.....you know what i mean....well you do now

[subaruplatt]

Yes, but no matter how much it is slipping, if the rears are trying to go faster than the fornts, they will always be trying to slow the car down - giving a torque split more like -10/110.....

 

First off on recent subaru 4eat models the computer maintains a friction coefficient necessary to send 60 percent forwards and 40 percent rearwards under normal condition. not 90/10

This is full time 4 wheel drive system pushing with 2/3's of the forward power rewards, not an on demand system.

 

Secondly even with the factory tire pressure of 33/30 the front tires are still slightly more compressed. Therefore off the line the fronts will spin first as they will have more immediate torque.

 

Then the computer will give 50/50 torque until all wheels travel at the same speed then it will back off to the 60/40 and try and maintain that.

 

Then again how awd in the 4eat really works seems to be a mater of conjecture strung together by mostly hearsay.

[subaruplatt]

Yes, but no matter how much it is slipping, if the rears are trying to go faster than the fornts, they will always be trying to slow the car down - giving a torque split more like -10/110.....

 

First off on recent subaru 4eat models the computer maintains a friction coefficient necessary to send 60 percent forwards and 40 percent rearwards under normal condition. not 90/10

This is full time 4 wheel drive system pushing with 2/3's of the forward power rewards, not an on demand system.

 

Secondly even with the factory tire pressure of 33/30 the front tires are still slightly more compressed. Therefore off the line the fronts will spin first as they will have more immediate torque.

 

Then the computer will give 50/50 torque until all wheels travel at the same speed then it will back off to the 60/40 and try and maintain that.

 

Then again how awd in the 4eat really works seems to be a mater of conjecture strung together by mostly hearsay.

 

Subaru says this.

Automatic transmission. Available on Impreza 2.5i models, the 4-speed Electronic Direct Control Automatic Transmission with SPORTSHIFT TM incorporates an advanced multi-plate transfer clutch centre differential for smooth and consistent acceleration, even under low traction conditions. Under normal circumstances, this active torque split system maintains a 60/40 front/rear split for more predictable handling and moves towards a 50/50 split when conditions warrant or under hard acceleration.

Edited December 1, 2008 by subaruplatt
More info

[keltik]

Well after having my wagon on a AWD dyno i can tell you for 100% fact the rear wheels began turning before the fronts did and once the tester backed off the throttle the front wheels slowed down faster than the rears. In fact as he touched the brakes the front wheels slowly came to a stop while the rears were spinning much faster

 

Ive also seen an older Forester on a AWD dyno and all wheels began rotating at once. I read in a service manual the TCU is supposed to hold a 50/50 torque split when the trans is in 1st gear to prevent wheelspin off the line.

[98obster]

In D (drive) the 4eat will go from 90/10 (normal/cruising) to 50/50 (full throttle/acceleration) etc.. Of course, if you interupt power to the Duty C solenoid you will have 50/50 all the time-mmm, torque bind

[subaruplatt]

In D (drive) the 4eat will go from 90/10 (normal/cruising) to 50/50 (full throttle/acceleration) etc.. Of course, if you interupt power to the Duty C solenoid you will have 50/50 all the time-mmm, torque bind

 

Please someone show me an official source of information for this 90/10 thing.

All I can find are private sites and people that really can't stand automatics.

I used to be one of them.

[subaruplatt]

Well after having my wagon on a AWD dyno i can tell you for 100% fact the rear wheels began turning before the fronts did and once the tester backed off the throttle the front wheels slowed down faster than the rears. In fact as he touched the brakes the front wheels slowly came to a stop while the rears were spinning much faster

 

Ive also seen an older Forester on a AWD dyno and all wheels began rotating at once. I read in a service manual the TCU is supposed to hold a 50/50 torque split when the trans is in 1st gear to prevent wheelspin off the line.

 

Yes the car is essentially a front wheel drive with a clutch attached rear wheel drive.

 

The 4eat car is always going to have front wheel drive.

The front wheels will get 100 % engine braking while the rears being loosely attached will get less.

[keltik]

Please someone show me an official source of information for this 90/10 thing.

All I can find are private sites and people that really can't stand automatics.

I used to be one of them.

My wagon has the VTD box so thats why the rears start turning first....as its advertised as having a 60/40 split to the rear - i think this proves the sales documents correct.

 

Sales stuff for the earlier transmissions state a 90/10 split in normal cruising which then builds to a 50/50 depending on throttle position etc.

 

Thats all i know for sure - the rest is as you say pure specualtion from forums and hearsay

 

Anyone wanna get an oscilloscope and prove some stuff for us?